Biochemistry of hormones: ñlassification, mechanism of influence at
target cells. Biochemistry of thyroid and parathyroid glands hormones. Introduction To metabolism. General pathways of metabolism in the organism. Bioenergetics. Krebs cycle, biological oxidation, oxidative phosphorylation.
The survival of multicellular organisms
depends on their ability to adapt to a constantly changing environment.
Intercellular communication mechanisms are necessary requirements for this
adaptation. The nervous system and the endocrine system provide this
intercellular, organism- wide communication. The nervous system was originally
viewed as providing a fixed communication system, whereas the endocrine system
supplied hormones, which are mobile messages. In fact, there is a remarkable
convergence of these regulatory systems. For example, neural regulation of the
endocrine system is important in the production and secretion of some hormones;
many neurotransmitters resemble hormones in their synthesis, transport, and
mechanism of action; and many hormones are synthesized in the nervous system.
Endocrine vs. Nervous System
Nervous System |
Endocrine System |
Neurons release neurotransmitters |
Endocrine cells release hormones |
A neurotransmitter acts on specific cell
right next to it. |
Hormones travel to another nearby cell or act
on cell in another part of the body. |
Neurotransmitters have their effects within
milliseconds. |
Hormones take minutes or days to have their
effects. |
The effects of neurotransmitters are
short-lived. |
The effects of hormones can last hours, days,
or years. |
Performs short term crisis management |
Regulates long term ongoing metabolic
function |
Neurotransmitter acts on specific cell right
next to it. |
Hormone can travel to another nearby cell or
it can act on another part of the body. |
The word “hormone” is
derived from a Greek term that means to arouse to activity. As classically
defined, a hormone is a substance that is synthesized in one organ and
transported by the circulatory system to act on another tissue. They are secreted in response to
changes in the environment inside or outside the body. These are released into
the extracellular fluid, where they are diffused into the blood stream. The
latter carries them from the site of production to the site of action. They act
on specific organs called target organs.
The blood contains all the hormones but the cells of a target organ can pick up
the specific required hormone only and ignore all others. It has been found
that the target cell has on its surface or in its cytoplasm a specific protein
molecule, called a receptor, which can recognise and pick out the specific
hormone capable of action in that cell. The hormone delivers its message to the
target cell by changing the shape of the receptor cell and binds to it. The
receptors new shape sets up certain changes in the cell such as alteration in
permeability, enzyme activity or gene transcription.
http://www.youtube.com/watch?v=kIPYVV4aThM&feature=related
Hormones may stimulate or inhibit
specific biological processes in the target organs to modify their activities
thus acting as regulators. There is considerable co-ordination between nerves
and hormones. Nerves regulate synthesis and release of some hormones. Some
times hormones may also influence nerve activities. Thus, hormonal
co-ordination plays an important role in regulating body functions.
Calcitonin secreted by thyroid gland
regulates the concentration of calcium and phosphorus in the blood. When the
concentration of calcium rises in the blood, the secretion of calcitonin is
seen which lowers the concentration of calcium and phosphorus in the plasma by
decreasing the release for the bones.
Maintenance of
internal chemical environment of the body to a constant is called homeostasis.
Hormones play a major role in maintaining homeostasis by their intergrated
action and feed back controls.
Feedback control is mostlly negative,
rarely positive. In a negative feedback control, synthesis of a hormone slows
or halts when its level in the blood rises above normal. Some of examples of
feedback control is given below.
Rise of
testosterone level in the blood above normal inhibits ICSH secretion by the
anterior pituitary lobe. This negative feedback checks oversecretion of
testosterone
Hypothalamus in response to some
external stimulus, produces a thyrotrophin-releasing hormone for the secretion
of thyrotrophic hormone. The thyrotrophin-releasing hormone (TRH) stimulates
the anterior pituitary lobe to secrete thyrotrophic hormone. The latter in turn
stimulates the thyroid gland to produce thyroxine. If thyroxine is in excess,
it exerts an influence on the hypothalamus and anterior pituitary lobe, which
then secrete less releasing hormone and thyroid-stimulating hormone (TSH)
respectively. A rise in the TSH level in the blood may also exert negative feed
back effect on the hypothalmus and retard the secretion of TRH. This restores
the normal blood-thyroxine level.
Functions
of hormones.
Hormones regulate the following processes:
Growth and
differentiation of cells, tissues, and organs These
processes include cell proliferation, embryonic development, and sexual
differentiation— i. e., processes that require a prolonged time period and
involve proteins de novo synthesis. For this reason, mainly steroid hormones
which function via transcription regulation are active in this field
Metabolic
pathways Metabolic regulation requires rapidly acting
mechanisms. Many of the hormones involved therefore regulate interconversion
of enzymes. Themain processes subject to hormonal regulation are the uptake
and degradation of storage substances (glycogen, fat), metabolic pathways for
biosynthesis and degradation of central metabolites (glucose, fatty acids,
etc.), and the supply of metabolic energy.
Digestive processes Digestive processes
are usually regulated by locally acting peptides (paracrine), but mediators, biogenic
amines, and neuropeptides are also involved.
Maintenance
of ion concentrations (homeostasis) Concentrations
of Na+, K+, and Cl– in body fluids, and the physiological variables dependent
on these (e. g. blood pressure), are subject to strict regulation. The
principal site of action of the hormones involved is the kidneys, where
hormones increase or reduce the resorption of ions and recovery of water. The
concentrations of Ca2+ and phosphate, which form the mineral substance of bone
and teeth, are also precisely regulated. Many hormones influence the above
processes only indirectly by regulating the synthesis and release of other
hormones (hormonal hierarchy).
3. Endocrine, paracrine, and
autocrine hormone effects.
http://www.youtube.com/watch?v=3LW7TSBcFjE&feature=related
Hormones transfer signals by migrating from heir
site of synthesis to their site of action. They are usually transported in the
blood. In this case, they are said to have an endocrine effect (example:
insulin). By contrast, tissue hormones, the target cells for
which are in the immediate vicinity of the glandular cells that produce them,
are said to have a paracrine effect (example: gastrointestinal tract
hormones). When signal substances also pass effects back to the cells that
synthesize them, they are said to have an autocrine effect (example: prostaglandins).
Autocrine effects are often found in tumor cells, which stimulate their own
proliferation in this way. Insulin,which is formed in the B cells of the pancreas, has both
endocrine and paracrine effects. As a hormone with endocrine effects, it
regulates glucose and fat metabolism. Via a paracrinemechanism, it inhibits the
synthesis and release of glucagon from the neighboring A cells.
Endocrine glands
The endocrine system is made up of
glands that produce and secrete hormones, chemical substances produced in the
body that regulate the activity of cells or organs. These hormones regulate the
body's growth, metabolism (the physical and chemical processes of the body), and
sexual development and function. The hormones are released into the bloodstream
and may affect one or several organs throughout the body.
The major glands of the endocrine
system are the hypothalamus, pituitary, thyroid, parathyroids, adrenals, pineal body,
and the reproductive organs (ovaries and testes). The pancreas is also a part
of this system; it has a role in hormone production as well as in digestion.
The endocrine system is regulated by
feedback in much the same way that a thermostat regulates the temperature in a
room. For the hormones that are regulated by the pituitary gland, a signal is
sent from the hypothalamus to the pituitary gland in the form of a
"releasing hormone," which stimulates the pituitary to secrete a
"stimulating hormone" into the circulation.
The
stimulating hormone then signals the target gland to secrete its hormone. As
the level of this hormone rises in the circulation, the hypothalamus and the
pituitary gland shut down secretion of the releasing hormone and the stimulating
hormone, which in turn slows the secretion by the target gland. This system
results in stable blood concentrations of the hormones that are regulated by
the pituitary gland.
Classification of hormones.
The animal organism
contains more than 100 hormones and hormone-like substances, which can be
classified either according to their structure or according to their function.
In chemical terms, most hormones are:
Ø hormones of protein structure: all
hormones of anterior pituitary (except ACTH), insulin, parathyroid hormone;
Ø hormones of peptide structure: ACTH,
calcitonin, glucagon, hormones of posterior pituitary, factors of hypothalamus,
thymozin;
Ø
steroid
hormones: adrenal cortical steroids, sex hormones;
Ø
Ø hormones - derivatives of amino acid:
thyroid hormones, adrenal medulla hormones, epiphysis hormones;
Ø
Ø hormones derivatives of unsaturated fatty
acid: prostaglandins.
Pathways in biosynthesis of eicosanoids from arachidonic acid: there are parallel paths from
v Lipotrophic
v
Hydrophilic
http://www.youtube.com/watch?v=kIPYVV4aThM&feature=related
http://www.youtube.com/watch?v=8fh2HmdxQjQ&feature=related
Mechanism of
action
A.
Mechanism of action of lipophilic
hormones
Lipophilic
signaling substances include the steroid hormones, calcitriol, the iodothyronines
(T3 and T4), and retinoic acid. These hormones mainly act in the nucleus
of the target cells, where they regulate gene transcription in
collaboration with their receptors and with the support of additional proteins
(known as coactivators and mediators). There are several effects of steroid
hormones that are notmediated by transcription control. These alternative
pathways for steroid effects have not yet been fully explained. In the blood,
there are a number of transport proteins for lipophilic hormones. Only the free
hormone is able to penetrate the membrane and enter the cell. The hormone
encounters its receptor in the nucleus (and sometimes also in the cytoplasm).
The receptors for lipophilic hormones are rare proteins. They occur in
small numbers (103–104 molecules per cell) and show marked specificity and
high affinity for the hormone (Kd =
10–8–10–10 M). After binding to the hormone, the steroid receptors are able to
bind as homodimers or heterodimers to control elements in the promoters
of specific genes, from where they can influence the transcription of the
affected genes—i. e., they act as transcription factors. The
illustration shows the particularly well-investigated mechanism of action for cortisol,
which is unusual to the extent that the hormone–receptor complex already arises
in the cytoplasm. The free receptor is present in the cytoplasm as a monomer in
complex with the chaperone hsp90. Binding of cortisol to the complex
leads to an allosteric conformational change in the receptor, which is
then released from the hsp90 and becomes capable of DNA binding as a result of dimerization.
In the nucleus, the hormone–receptor complex binds to nucleotide sequences
known as hormone response elements (HREs).
The second mechanism involves steroid hormones, which pass through the
plasma membrane and act in a two step process. Steroid hormones bind, once
inside the cell, to the nuclear membrane receptors, producing an activated
hormone-receptor complex. The activated hormone-receptor complex binds to DNA
and activates specific genes, increasing production of proteins.
|
|
These are short palindromic DNA segments that
usually promote transcription as enhancer elements. The illustration shows the HRE
for glucocorticoids (GRE; “n” stands for any nucleotide). Each hormone receptor
only recognizes its “own” HRE and therefore only influences the transcription
of genes containing that HRE. Recognition between the receptor and HRE is based
on interaction between the amino acid residues in the DNA-binding domain (B)
and the relevant bases in the HRE (emphasized in color in the structure
illustrated). As discussed on p. 244, the hormone receptor does not interact
directly with the RNA polymerase, but rather—along with other transcription
factors—with a coactivator/mediator complex that processes all of the signals
and passes them on to the polymerase. In this way, hormonal effects lead within
a period of minutes to hours to altered levels ofmRNAs for key proteins in
cellular processes (“cellular response”).
B.
Mechanism of action of hydrophilic
hormones
The
hormones are released in very small quantities, yet they can cause widespread
dresponses in cells or tissues all over the body. These
responses in cells or tissues all over the body. These responses can be
quite specific and selective in different cells. All vertebrate hormones belong
to one of four chemical groups. Some hormones, such hormone, such as adrenaline
and thyroid hormone, are small molecules derived from the amino acid tyrosine,
others such as vasopressin and oxytocin, are short peptides, still other
hormones, like insulin and glucagons, are longer
polypeptide chains. Testosterone and estrogen are steroid hormones.
Catecholamines, peptide and protein hormones are not lipid-soluble, and so,
cannot enter their target cells through the bilipid layer of plasma membrane.
Instead, these water-soluble hormones interact with a surface receptor, usually
a glycoprotein, and thus, initiate a chain of events within it. The hormone
insulin provides a well-studied example of how this happens.
The membrane
bound receptors of insulin is a heterotetrameric protein consisting of four
subunits, two -subunits protrude out from surface of the ell and bind insulin,
and two -subunits that span the membrane and protrude into the cytoplasm.
Such
receptors range from fewer than
Binding of
insulin to the outer subunits of the receptor causes a conformational change in
the membrane spanning -subunits, which is also an enzyme, a tyrosine kinase.
The activated -subunits add phosphate groups of specific tyrosine residues
located in cytoplasmic domain of the receptor, as well as a variety of insulin
receptor substrates.
As a result of
-subunit activity, a transducer G protein activates enzyme phosphodiesterase.
This enzyme makes phosphatidylinositol 4,5-biphosphate
(PIP2) into a pair of mediators inositoltriphosphate (IP3)
and diacylglycerol (DG). In turn, IP3, which is water-soluble, and
so diffuses into cytoplasm triggers the release of
another messenger Ca2+ ions from intracellular endoplasmic reticulum
activating many calcium-mediated processes. While DG remains
in the membrane where it activates an enzyme called protein kinase C, which in
turn, activates many other enzymes, such as pyruvate dehydrogenase, and so
brings about the physiological effects.
Mediators
amplify the signal in an expanding cascade of response. A single -subunit of
insulin receptor, for example, activates many molecules of DG, and each protein
kinase C molecule activated by DG will, in turn, activate many other enzyme
molecules. DG and IP3 are examples of second messengers,
intermediary compounds that amplify a hormonal signal and so set into action a
variety of events within the affected cell. A variety of
events within the affected cell. A variety of hormones use another
second messenger, the cyclic form of adenosine monophosphate, (cAMP).
The enzyme adenylate cyclase converts adenosine triphosphate (ATP) into
cAMP. Because an enzyme can be used over and over again, a single molecule of
active adenylate cyclase can catalyse production of about 100 molecules of
cAMP. In muscle or liver cells, when hormones, such as, adrenaline bind receptors,
the receptors change shape and bind to G protein, causing it, in turn, to bind
the nucleotide guanosine triphosphate (GTP) and activate another protein
adenylate cyclase. The result of this complex cascade of interactions is the
production of large amounts of cAMP.
cAMP activates the enzyme protein
kinase A, which, in turn, activates the enzyme phosphorylate kinase. Each
molecule of protein kinase A activates roughly 100 molecules of enzyme,
phosphorylate kinase and so on. The net result is that a single molecule of
adrenaline may lead to release of as many as 100 million molecules of glucose
within only 1 or 2 minutes. No wonder only very small quantities of hormone are
needed.
Many cells
use more than one second messenger. In heart cells, cAMP serves as a second
messenger, speeding up muscle cell contraction in response to adrenaline, while
cyclic guanosine monophosphate (cGMP) serves as another second
messenger, slowing muscle contraction in response to acetylcholone. It is in
this way that the sympathetic and parasympathetic nervous systems achieve
antagonistic effect on heartbeat. Another example of antagonistic effect is
insulin, which lowers blood sugar level, and glucagons, which raises it.
Another type
of hormonal interaction is known as synergistic effect. Here, two or more
hormones complement each others actions and both are needed for full expression
of the hormone effects. For example, the production, secretion and ejection of
milk by mammary glands require the synergistic effects of estrogens,
progesterone, prolactin and oxytocin.
We have
discussed many dramatic effects of hormone, for instance, testosterone. Yet,
its concentration in the plasma of adult human male is only 30 to 100 ng per
ml. How can hormones in such tiny quantities have such widespread and selective
actions? Unlike catecholamine and peptide hormones, steroid and thyroid
hormones are lipid-soluble hormones and readily pass through the plasma membrane
of a target cell into the cytoplasm. There they bind to specific intracellular
receptor proteins, forming a complex that enters the nucleus and bind to
specific regulatory sites on chromosomes. The binding alters the pattern of
gene expression, initiating the transcription of some genes (DNA), while
repressing the transcription of others. This results in the production of
specific mRNA translation products, proteins and usually enzymes. The actions
of lipid-soluble hormones are slower and last longer than the actions of
water-soluble hormones. These cause physiological responses that are
characteristic of the steroid hormones.
http://www.youtube.com/watch?v=oOj04WsU9ko
Examples of
peptide hormones
Hormones of
hypothalamus (releasing and inhibitory factors), structure,
mechanism of action.
Hypothalamus
has the wide anatomic links with other parts of the brain. Therefore in
different mental disorders there is the change of secretion of hypothalamus
hormones.
http://www.youtube.com/watch?v=hLeBNyB1qKU&feature=related
Two
groups of hormones are produced by hypothalamus corresponding to the anterior
and posterior pituitary.
Hypothalamus and posterior pituitary. 3
peptides are synthesized in the hypothalamus that pass
to the posterior pituitary along axons where they are accumulated: oxytocin,
vasopressin (antidiuretic hormone) and neurophysin. The later binds the
oxytocin and vasopressin and promotes their transportation to the pituitary.
Hypothalamus and anterior pituitary.
Hypothalamus is connected with the anterior pituitary by the net of blood
capillaries, so called hypothalamic portal system. Hypothalamus produces very
active peptide compounds that pass via this portal system to anterior pituitary
and stimulate or oppress the secretion of tropic hormones. Compounds
stimulating the secretion are called releasing factors. 7 releasing factors are
known according to the amount of tropic hormones of anterior pituitary:
- corticotropin-releasing
factor
- thyrotropin-releasing
factors
- somatotropin-releasing
factors
- follicletropin-releasing
factor
- luteotropin-releasing
factor
- prolactotropin-releasing
factor
- melanotropin-releasing
factor.
Hypothalamus
also secretes substances called inhibitory factors or statins, which can
inhibit release of the some pituitary hormones. 3 inhibitory factors are known
today:
- somatostatin
- prolactostatin
- melanostatin.
Releasing
and inhibitory factors are produced in only minute amounts.
http://www.youtube.com/watch?v=-UaSfYKsFh0
Hormones of
pituitury, structure, mechanism of action.
Tropic
hormones are produced by the anterior pituitary. Usually tropic
hormones not directly regulate the metabolism but act on the peripheral
endocrine glands.
|
Somatotropic
hormone (STH, growth hormone)
Chemical
structure: simple protein
The
intensity of secretion is regulated by the relationship between the
somatotropic-releasing factor and somatostatin.
The main function of somatotropic hormone -
stimulation of growth. Hormone is necessary for the bone
tissue formation, for the muscle tissue growth, for the formation of
peculiarities of men and women body.
Somatotropic
hormone can act both directly on the metabolism and indirectly stimulating the
synthesis of somatomedines (specific protein growth factors which are
synthesized in liver).
The effect of somatotropic hormone
on:
- protein
metabolism: stimulates the passing of
amino acids into the cells;
activates the
synthesis of proteins, DNA, RNA.
- carbohydrate
metabolism: activates the insulinase of
liver;
inhibits the
conversion of lipids to carbohydrates;
activates the exit of
glucose from liver;
inhibits the entry of
glucose into the cells.
- lipid metabolism:
stimulates lipolisis;
stimulates the
oxidation of fatty acids.
The
deficiency of somatotropic hormone in children age causes nanism. Nanism -
proportional underdevelopment of all body.
The
deficiency of somatotropic hormone in adult persons hasn’t clinical symptoms.
The excess of somatotropic hormone in children age causes gigantism.
The
excess of somatotropic hormone in adult persons causes acromegalia (disproportional development of the separate body
parts).
http://www.youtube.com/watch?v=VX2wgM4kUfM
Thyrotropic
hormone (TTH).
Chemical
structure: glicoprotein.
This
hormone is necessary for the normal functions of thyroid glands.
Thyrotropic
hormone promotes:
- accumulation of iodine
in thyroid;
- including of iodine into the tyrosine;
- synthesis of thyroxine
and triiodothyronine.
Adrenocorticotropic
hormone (ACTH).
Chemical
structure: polipeptide.
This
hormone is necessary for the normal functions of adrenal cortex. It enhances
the formation of steroid hormones and their secretion into the blood.
ACTH
has also the melanocyte-stimulating activity.
Excessive
secretion of ACTH causes the Icenko-Kushing disease (symptoms of
hypercorticism, hyperpigmentation).
As
you know, Cushing’s is a rarely diagnosed endocrine disorder characterized by
hypercortisolism. Cortisol is a hormone produced by the adrenal glands and is
vital to regulate the body’s cardivoascular functions and metabolism, to boost
the immune system and to fight inflammation. But its
most important job is to help the body to respond to stress.
The
adrenal glands release cortisol in response to stress, so atheletes, women
experiencing pregnancy, and those suffering from alcoholism, panic disorders
and malnutrition naturally have higher-than-normal levels of cortisol.
People
with Cushing’s Syndrome live life with too much cortisol
for their bodies as a result of a hormone-secreting tumor. Mine is located in
the pituitary gland. Endogenous hypercortisolism leaves the body in a constant
state of “fight or flight,” which ravages the body and tears down the body’s
major systems including cardivascular, musculo-skeletal, endocrine, etc.
Symptoms
vary, but the most common symptoms include rapid, unexplained weight gain in
the upper body with increased fat around the neck and face (“moon facies”);
buffalo hump; facial flushing/plethora; muscle wasting in the arms and legs;
purplish striae (stretch marks) on the abdomen, thighs, buttocks, arms and
breasts; poor wound healing and bruising; severe fatigue; depression, anxiety
disorders and emotional lability; cognitive difficulties; sleep disorders due
to abnormally high nighttime cortisol production; high blood pressure and high
blood sugar/diabetes; edema; vision problems; premature osteoperosis; and, in
women, signs of hyperandrogenism such as menstrual irregularities, infertility,
hirsutism, male-patterned balding and steroid-induced acne.
Most people with Cushing’s long for the ability to do simple things,
like walk a flight of stairs without having to sit for half an hour afterwards,
or vacuum the house or even unload a dishwasher.
One
of the worst parts about this disease is the crushing fatigue and muscle
wasting/weakness, which accompanies hypercortisolism. Not only do we become
socially isolated because of the virilzing effects of an endocrine tumor, which
drastically alters our appearance, but we no longer feel like ourselves with
regard to energy. We would love to take a long bike ride, run three miles or go
shopping like we used to — activities, which we took for granted before the
disease struck. Those activities are sadly impossible at times for those with
advanced stages of the disease.
A patient
with Cushing syndrome showing signs of acne and hirsuitism
Moon face
in patienr with Cushing syndrome
Widened
purple striae in a patient with Cushing's
Gonadotropic
hormones.
Follicle
stimulating hormone (FSH).
Chemical
structure: glycoprotein.
Function:
stimulates the function of follicles (oogenesis) in women and spermatogenesis
in men.
FSH
(follicle stimulating hormone) regulates the development, growth, pubertal
maturation, and reproductive processes of the body
In both males and females, FSH stimulates the
maturation of germ cells.
In males, FSH induces sertoli cells to secrete inhibin and stimulates the
formation of sertoli-sertoli tight junctions (zonula occludens).
In females, FSH initiates follicular growth, specifically affecting granulosa cells.
With the concomitant rise in inhibin B, FSH levels then decline in the late
follicular phase. This seems to be critical in selecting only the most advanced
follicle to proceed to ovulation. At the end of the luteal phase, there is a
slight rise in FSH that seems to be of importance to start the next ovulatory
cycle.
Luteinizing
hormone (LH).
Chemical
structure: glycoprotein.
Function:
stimulates the formation of yellow body in women and testosterone secretion in
men.
In
both males and females, (LH) Luteinising hormone is essential for reproduction.
In females, at the time of menstruation, FSH initiates follicular growth,
specifically affecting granulosa cells. With the rise in estrogens, LH
receptors are also expressed on the maturing follicle that produces an
increasing amount of estradiol. Eventually at the time of the maturation of the
follicle, the estrogen rise leads via the 48 hour period.
This
'LH surge' triggers ovulation thereby not only releasing the egg, but also
initiating the conversion of the residual follicle into a corpus luteum that,
in turn, produces progesterone to prepare the endometrium for a possible
implantation. LH is necessary to maintain luteal function for the first two
weeks. In case of a pregnancy luteal function will be further maintained by the
action of hCG (a hormone very similar to LH) from the
newly established pregnancy. LH supports thecal cells in the ovary that provide
androgens and hormonal precursors for estradiol production.
In
the male, LH acts upon the Leydig cells of the testis and is responsible for
the production of testosterone, an androgen that exerts both endocrine activity
and intratesticular activity on spermatogenesis.
Prolactin (PRL).
Chemical
structure: protein.
Functions:
- stimulates lactation;
- stimulates function of yellow body (secretion
of progesterone);
- promotes formation of mother instinct;
- stimulates the formation of prostate glandular
tissue in men.
Lipotropic
hormone.
Chemical
structure: protein.
Functions:
- stimulates the mobilization of lipids from depot;
- decreases the Ca amount in blood;
- has the melanocyte-stimulating activity.
Melanocyte and
melanin. |
Posterior
pituitary.
Chemical
structure: peptide.
Functions:
- activates the hyaluronidase. This enzyme splits the hyaluronic acid. The
permeability of membranes is increased and reabsorption of water in kidneys is
increased too. As result the day diuresis is decreased;
-
narrows
arterioles and capillaries and increases the blood pressure.
-
-
AVP has two
principle sites of action: the kidney and blood vessels.
-
The primary function of AVP in the
body is to regulate extracellular fluid volume by affecting renal handling of
water, although it is also a vasoconstrictor and pressor agent (hence, the name
"vasopressin"). AVP acts on renal collecting ducts via V2
receptors to increase water permeability (cAMP-dependent mechanism), which
leads to decreased urine formation (hence, the antidiuretic action of
"antidiuretic hormone"). This increases blood volume, cardiac output and
arterial pressure.
-
A secondary function of AVP is
vasoconstriction. AVP binds to V1 receptors on vascular smooth
muscle to cause vasoconstriction via the IP3
signal transduction pathway, which increases arterial
pressure; however, the normal physiological concentrations of AVP are below its
vasoactive range. Studies have shown, nevertheless, that in severe hypovolemic
shock, when AVP release is very high, AVP does contribute to the compensatory
increase in systemic vascular resistance.
The
deficiency of vasopressin in organism causes diabetes insipidus. Clinical symptoms - poliuria, dehydration of the organism, low
density of the urine.
Diabetes
insipidus results in excessive thirst and urination. The reason is problems
with a particular hormone or its receptor. Diabetes insipidus increases the
risk for dehydration.
Diabetes
insipidus is caused by problems related to a hormone called antidiuretic
hormone or its receptor. Antidiuretic hormone (ADH) is produced in a part of
the brain called the hypothalamus. It's stored in the brain's pituitary gland.
Release of ADH causes the kidneys to hold onto water, which makes urine more
concentrated.
Normally, if
we are thirsty or slightly dehydrated, ADH levels rise. The kidneys reabsorb
more water and excrete concentrated urine. If, on the other hand, we chugged a
half-gallon of water (don't try this at home), ADH levels would fall. Clear, dilute
urine would pass. Diabetes insipidus can be caused by either of two problems
with ADH. One is too little ADH is produced. When that's the case, the
condition is called central diabetes insipidus.
The other is there's enough ADH produced, but the kidneys can't respond to it.
That condition is known as nephrogenic diabetes insipidus.
In
either form of diabetes insipidus, the result is the same. The kidneys can't do
their job of conserving water. Even when a person with diabetes insipidus is
dehydrated, the kidneys will excrete abundant, dilute urine. This inability of
the kidneys to conserve water leads to the symptoms of diabetes
insipidus:
In
some people, these symptoms can become extreme, causing dehydration.
Excessive
fluid losses can also cause electrolyte imbalances. Possible symptoms include:
But
why "insipidus?" People with diabetes insipidus
aren't insipid, but their urine is. Insipid can mean dull or lacking flavor.
Believe it or not, doctors long ago would taste urine to detect illness. Unlike
diabetes mellitus, which results in sweet tasting urine, diabetes insipidus
creates watery, flavor-free urine.
Oxytocin.
Chemical
structure: peptide.
Functions:
stimulates the contraction of smooth muscles, especially the muscles of uterus
and muscle fibres of alveoluses of mammas.
Oxytocin
is used for delivery stimulation, for stop of bleeding after delivery, for
stimulation of lactation.
Numerically
the largest group of signaling substances, these arise by protein biosynthesis.
The smallest peptide hormone, thyroliberin (362 Da), is a tripeptide.
Proteohormones can reach masses of more than 20 kDa—e. g., thyrotropin (28
kDa). Similarities in the primary structures of many peptide hormones and
proteohormones show that they are related to one another. They probably arose
from common predecessors in the course of evolution. Thyroliberin (thyrotropin-releasing
hormone, TRH) is one of the neurohormones of the hypothalamus. It stimulates
pituitary gland cells to secrete thyrotropin (TSH). TRH consists of three amino
acids, which are modified in characteristic ways.
Thyrotropin (thyroid-stimulating
hormone, TSH) and the related hormones lutropin (luteinizing hormone,
LH) and follitropin (follicle-stimulating hormone, FSH) originate in the
adenohypophysis. They are all dimeric glycoproteins with masses of around 28
kDa. Thyrotropin stimulates the synthesis and secretion of thyroxin by the
thyroid gland.
Hormones of pancreas,
structure, mechanism of action
Insulin
is produced and released by the B cells of the
pancreas and is released when the glucose level rises. Insulin reduces the
blood sugar level by promoting processes that consume glucose— e. g.,
glycolysis, glycogen synthesis, and conversion of glucose into fatty acids. By
contrast, it inhibits gluconeogenesis and glycogen degradation. Insulin causes
cells in the liver, skeletal muscles, and fat tissue to absorb glucose from the blood. In the liver and skeletal
muscles, glucose is stored as glycogen,
and in fat cells (adipocytes) it is
stored as triglycerides.
Insulin
stops the use of fat as an energy source by inhibiting the release of glucagon. With the
exception of the metabolic disorder diabetes mellitus
and metabolic syndrome,
insulin is provided within the body in a constant proportion to remove excess
glucose from the blood, which otherwise would be toxic. When blood glucose
levels fall below a certain level, the body begins to use stored sugar as an
energy source through glycogenolysis,
which breaks down the glycogen stored in the liver and muscles into glucose,
which can then be utilized as an energy source. As a central metabolic control
mechanism, its status is also used as a control signal to other body systems
(such as amino acid uptake by
body cells). In addition, it has several other anabolic effects
throughout the body.
When control of insulin levels fails, diabetes mellitus can result.
As a consequence, insulin is used medically to treat some forms of diabetes
mellitus. Patients with type 1
diabetes depend on external insulin (most commonly injected
subcutaneously) for their survival because the
hormone is no longer produced internally.[2] Patients
with type 2
diabetes are often insulin resistant
and, because of such resistance, may suffer from a "relative" insulin
deficiency. Some patients with type 2 diabetes may eventually require insulin
if other medications fail to control blood glucose levels adequately. Over 40%
of those with Type 2 diabetes require insulin as part of their diabetes
management plan.
The
human insulin protein is composed of 51 amino acids,
and has a molecular weight of 5808 Da. It is a dimer of an
A-chain and a B-chain, which are linked together by disulfide bonds.
http://www.youtube.com/watch?v=V1LjRi8Nvv4
http://www.youtube.com/watch?v=VLiTbb6MaEU&NR=1
http://www.youtube.com/watch?v=ZsTSoLhl3Y4&feature=related
http://www.youtube.com/watch?v=nBJN7DH83HA&feature=related
Glucagon,
a peptide of 29 amino acids, is a product of the A
cells of the pancreas. It is the antagonist of insulin and, like insulin,
mainly influences carbohydrate and lipid metabolism. Its effects are each
opposite to those of insulin. Glucagon mainly acts via the second messenger
Glucagon is a
linear peptide of 29 amino acids. Its primary sequence is almost perfectly
conserved among vertebrates, and it is structurally related to the secretin
family of peptide hormones. Glucagon is synthesized as proglucagon and
proteolytically processed to yield glucagon within alpha cells of the
pancreatic islets. Proglucagon is also expressed within the intestinal tract,
where it is processed not into glucagon, but to a family of glucagon-like
peptides (enteroglucagon).
The major
effect of glucagon is to stimulate an increase in blood concentration of
glucose. As discussed previously, the brain in particular has an absolute
dependence on glucose as a fuel, because neurons cannot utilize alternative
energy sources like fatty acids to any significant extent. When blood levels of
glucose begin to fall below the normal range, it is imperative to find and pump
additional glucose into blood. Glucagon exerts control over two pivotal
metabolic pathways within the liver, leading that organ to dispense glucose to
the rest of the body:
Glucagon also
appears to have a minor effect of enhancing lipolysis of triglyceride in
adipose tissue, which could be viewed as an addition means of conserving blood
glucose by providing fatty acid fuel to most cells.
Knowing that
glucagon's major effect is to increase blood glucose levels, it makes sense
that glucagon is secreted in response to hypoglycemia or low blood
concentrations of glucose.
Two other
conditions are known to trigger glucagon secretion:
In terms of
negative control, glucagon secretion is inhibited by high levels of blood
glucose. It is not clear whether this reflects a direct effect of glucose on
the alpha cell, or perhaps an effect of insulin, which is known to dampen
glucagon release. Another hormone well known to inhibit glucagon secretion is
somatostatin.
Diseases
associated with excessively high or low secretion of glucagon are rare. Cancers
of alpha cells (glucagonomas) are one situation known to cause excessive
glucagon secretion. These tumors typically lead to a wasting syndrome and,
interestingly, rash and other skin lesions.
Although
insulin deficiency is clearly the major defect in type 1 diabetes mellitus,
there is considerable evidence that aberrant secretion of glucagon contributes
to the metabolic derangements seen in this important disease. For example, many
diabetic patients with hyperglycemia also have elevated blood concentrations of
glucagon, but glucagon secretion is normally suppressed by elevated levels of
blood glucose.
What is
diabetes mellitus?
Diabetes
is a disease of the pancreas, an organ behind your stomach that produces the
hormone insulin. Insulin helps the body use food(glucose)
for energy. When a person has diabetes, the pancreas either cannot produce
enough insulin, or the body uses the insulin incorrectly, or both. Insulin
works together with glucose in the bloodstream to help it enter the body’s
cells to be burned for energy. If the insulin isn’t functioning properly,
glucose cannot enter the cells. This causes glucose levels in the blood to rise,
creating a condition of high blood sugar or hyperglycaemia which is the
hallmark of diabetes, and leaving the cells without fuel. When blood glucose
rises above a certain level, it spills over into the urine.
What are the
common types of diabetes?
There are two
common forms of diabetes: type 1 and type 2.
Generally,
type 2 diabetes is more common in people over age 40 who
are overweight. However, the increased prevalence of obesity has increased the
number of people under age 40 who are diagnosed with type 2 diabetes. Nine out
of 10 people with diabetes have type 2.
What causes
Diabetes Mellitus?
The following
factors may increase your chance of getting diabetes:
How is
diabetes diagnosed?
The preferred
method of diagnosing diabetes is the fasting blood sugar test (FBS). The FBS
measures your blood glucose level after you have fasted (not eaten anything)
for 10 to 12 hours.
Normal
fasting blood glucose is between 70 and 100 mg/dl for people who do not have
diabetes. The standard
diagnosis of diabetes is made when:
Other symptoms may include:
What are some
of the long-term complications of diabetes?
Retinopathy
(eye disease): All patients with diabetes should
see an ophthalmologist (eye specialist) yearly for a dilated eye examination.
Patients with known eye disease, symptoms of blurred vision in one eye or who
have blind spots may need to see their ophthalmologist more frequently.
Nephropathy
(kidney disease): Urine testing should be performed
yearly. Regular blood pressure checks also are important because control of
hypertension (high blood pressure) is essential in slowing kidney disease.
Generally, blood pressure should be maintained less than 130/80 in adults.
Persistent leg or feet swelling also may be a symptom of kidney disease and
should be reported to your doctor.
Neuropathy
(nerve disease): Numbness or tingling in your feet
should be reported to your doctor at your regular visits. You should check your
feet daily for redness, calluses, cracks or breakdown in skin tissue. If you
notice these symptoms before scheduled visits, notify your doctor immediately.
Other
long-term may complications include:
Because of
the link between obesity and type 2 diabetes, you can do a great deal to reduce
your chance of developing the disease by slimming down if you are overweight.
This is especially true if diabetes runs in your family.
In fact,
studies have shown that exercise and a healthy diet can prevent the development
of type 2 diabetes in people with impaired glucose tolerance — a condition that
often develops prior to full-blown type 2 diabetes.
Medications have also been shown to provide similar benefit. Both diabetes
drugs metformin and Precose have been shown to prevent the onset of type 2
diabetes in people with this pre-diabetes condition.
In someone who already has diabetes, exercise and a nutritionally balanced diet
can greatly limit the effects of both types 1 and 2 diabetes on your body. In
diabetics, stopping smoking is one of the best ways to help prevent the
damaging effects of diabetes. If you smoke, quit; smoking dramatically
increases the risk of heart disease, particularly for people with diabetes.
http://www.youtube.com/watch?v=FEsTIOIufiQ
Hormones of adrenal glands.
Adrenal glands consist of two parts: external -
cortex, internal - medulla.
http://www.youtube.com/watch?v=06jbq3bxKE0&feature=related
Epinephrine is
a hormone synthesized in the adrenal glands from tyrosine. Its release is
subject to neuronal control.This “emergency hormone” mainly acts on the blood vessels,
heart, andmetabolism. It constricts the blood vessels
and thereby increases blood pressure; it increases cardiac function; it
promotes the degradation of glycogen into glucose in the liver and muscles; and
it dilates the bronchia.
Each part secrets specific hormones.
Hormones of adrenal medulla –
catecholamines (epinephrine, norepinephrine, dopamine).
Chemical structure - these hormones are derivatives of
amino acid tyrosine.
Epinephrine, norepinephrine, dopamine exist in blood
in
Functions: causes very potent contraction of vessels
and increase the blood pressure, increase a pulse rate. Epinephrine relaxes the
smooth muscles of bronchi, intestine, promote the contraction of uterus smooth
muscle. Epinephrine play a great role in stress
reactions.
Catecholamine, any
of various naturally occurring amines that
function as neurotransmitters and hormones within the
body. Catecholamines are characterized by a catechol group
(a benzene ring with
two hydroxyl groups) to which is attached an amine (nitrogen-containing) group.
Among the catecholamines are dopamine, epinephrine (adrenaline), and norepinephrine
(noradrenaline).
All
catecholamines are synthesized from the amino acid l-tyrosine
according to the following sequence: tyrosine → dopa
(dihydroxyphenylalanine) → dopamine → norepinephrine
(noradrenaline) → epinephrine
(adrenaline). Catecholamines are synthesized in the brain, in the adrenal medulla, and
by some sympathetic nerve fibres. The particular catecholamine that is
synthesized by a nerve cell, or neuron, depends on
which enzymes are present
in that cell. For example, a neuron that has only the first two enzymes
(tyrosine hydroxylase and dopa decarboxylase) used in the sequence will stop at
the production of dopamine and is called a dopaminergic neuron (i.e., upon
stimulation, it releases dopamine into the synapse). In the
adrenal medulla the enzyme that catalyzes the transformation of norepinephrine
to epinephrine is formed only in the presence of high local concentrations of
glucocorticoids from the adjacent adrenal cortex; chromaffin cells in tissues
outside the adrenal medulla are incapable of synthesizing epinephrine.
l-Dopa is well
known for its role in the treatment of parkinsonism, but its
biological importance lies in the fact that it is a precursor of dopamine, a
neurotransmitter widely distributed in the central nervous system, including
the basal ganglia of the brain (groups of nuclei within the
cerebral hemispheres that collectively control muscle tone, inhibit movement,
and control tremour). A deficiency of dopamine in these ganglia leads to parkinsonism, and this deficiency is at least partially
alleviated by the administration of l-dopa.
Under
ordinary circumstances, more epinephrine than norepinephrine is released from
the adrenal medulla. In contrast, more norepinephrine is released from the sympathetic nervous system
elsewhere in the body. In physiological terms, a major action of the hormones
of the adrenal medulla and the sympathetic nervous system is to initiate a
rapid, generalized fight-or-flight response.
This response, which may be triggered by a fall in blood pressure or by pain, physical injury, abrupt
emotional upset, or hypoglycemia, is
characterized by an increased heart rate (tachycardia), anxiety, increased
perspiration, tremour, and increased blood glucose concentrations (due to glycogenolysis, or
breakdown of liver glycogen). These
actions of catecholamines occur in concert with other neural or hormonal
responses to stress, such as
increases in adrenocorticotropic hormone
(ACTH) and cortisol secretion.
Furthermore,
the tissue responses to different catecholamines depend on the fact that there
are two major types of adrenergic receptors
(adrenoceptors) on the surface of target organs and tissues. The receptors are
known as alpha-adrenergic and beta-adrenergic receptors, or alpha receptors and beta receptors,
respectively. In general, activation of alpha-adrenergic receptors results in
the constriction of blood vessels, contraction
of uterine muscles, relaxation of intestinal muscles, and dilation of the pupils. Activation of
beta-adrenergic receptors increases heart rate and stimulates cardiac contraction
(thereby increasing cardiac output), dilates the bronchi (thereby increasing
air flow into and out of the lungs), dilates
the blood vessels, and relaxes the uterus. Drugs that
block the activation of beta receptors (beta blockers), such as
propranolol, are often given to patients with tachycardia, high blood pressure,
or chest pain (angina pectoris). These
drugs are contraindicated in patients with asthma because they worsen bronchial
constriction.
Catecholamines
play a key role in nutrient metabolism and the generation of body heat
(thermogenesis). They stimulate not only oxygen consumption but also
consumption of fuels, such as glucose and free fatty acids, thereby
generating heat. They stimulate glycogenolysis and the breakdown of triglycerides, the stored
form of fat, to free fatty acids (lipolysis). They also have a role in the
regulation of secretion of multiple hormones. For example, dopamine inhibits prolactin secretion,
norepinephrine stimulates gonadotropin-releasing hormone secretion, and epinephrine inhibits insulin secretion by
the beta cells of the islets of Langerhans
of the pancreas.
The effect of epinephrine on carbohydrate metabolism:
- activates
the decomposition of glycogen in liver and muscles;
- activates
the glycolysis, Krebs cycle and tissue respiration;
- causes
the hyperglycemia.
The effect of
epinephrine on protein metabolism:
- activates
the protein decomposition.
The effect of epinephrine on lipid metabolism:
-
activates
the tissue lipase, mobilization of lipids and oxidation of fatty acids.
-
Norepinephrine (NE) and epinephrine
(E) are both sympathomimetic catecholamines that are synthesized in the adrenal
medulla and terminals of sympathetic neurons. Along the metabolic pathway, NE
is synthesized first -- and when an enzyme adds a methyl group, you have
epinephrine. NE is the main catecholamine in peripheral tissue and sympathetic
neurons. E is mostly
made in the adrenal medulla.
-
http://www.youtube.com/watch?v=4g25d7_Afmc
Tissue hormones
Hormonoids
(tissue hormones, histohormones) - organic trace substances produced by
different cells of different tissues (not by specific glands) that regulate
metabolism on the local level (some hormonoids are produced in the blood too
(serotonin, acetylcholine).
In
the organs, the hormones carry out physiological and biochemical regulatory
functions. In contrast to endocrine hormones, tissue hormones are only
active in the immediate vicinity of the cells that secrete them. The
distinctions between hormones and other signaling substances (mediators,
neurotransmitters, and growth factors) are fluid. Mediators is the term
used for signaling substances that do not derive from special hormone- forming
cells, but are formbymany cell types. Acetylcholine
is a neurotransmitter. What
that does is it releases chemicals into the brain and
plays a role in normal brain functions such as sleep. Also, it deals
with attention, learning, and memory skills. The mechanisms
that the transmitter controls was a mystery until now. Scientist now know that acetycholine deals with communication between
neurons. Located in the prefrontal cortex. This
is the formula for acetylcholine.
When
acetylcholine is released it binds to a specific reactor. Next it begins to
start a molecule cascade. Which then triggers physiological
alterations. Which deals with how prefrontal cortical
neurons are “wired” together. This
explains how acethlcholine is released into the brain. This process
may actually have an effect in the formation of new associative memories. Most
of this information can be found in the artical on acetylchonline.
The Professor of Cellular Neuroscience, Zafar Bashir was the one who
demonstrated how electron stimulation of the prefrontal cortex leads to the
release of acetylcholine. Then Dr. Douglas Caruana carried out another
experiment. He also found that acetylchonline when released into the prefrontal
cortex it helps you remember things. But when to much has
been released those memories start to be forgotten.
Just like the
article said in the Journal of Neuroscience. Acetylcholine is a
neurotransmitter which plays key roles in sleep and other normal functions.
This is basically what acetylcholine does before of course we found out that to
much of it is dangerous.
Gastrin is released
in response to certain stimuli. These include:
Gastrin release
is inhibited by:
The
presence of gastrin stimulates parietal cells
of the stomach to secrete hydrochloric acid
(HCl)/gastric acid. This is done both directly on the parietal cell and
indirectly via binding onto CCK2/gastrin
receptors on ECL cells
in the stomach, which then responds by releasing histamine, which in
turn acts in a paracrine manner on parietal cells stimulating them to secrete H+
ions. This is the major stimulus for acid secretion by parietal cells.
Along with
the above mentioned function, gastrin has been shown to have additional
functions as well:
Heparin, a highly-sulfated glycosaminoglycan, is widely used
as an injectable anticoagulant and has the highest negative charge density of
any known biological molecule; it consists of a variably-sulfated repeating
disaccharide unit: The most common disaccharide unit is composed of a
2-O-sulfated iduronic acid and 6-O-sulfated, N-sulfated glucosamine, IdoA-GlcN
Heparin is a
naturally-occurring anticoagulant produced by basophils and mast cells;
pharmaceutical grade heparin is derived from mucosal tissues of slaughtered
meat animals such as porcine intestine or bovine lung. Heparin allows the
body's natural clot lysis mechanisms to work normally to break down clots that have
already formed.
Heparin
binds to the enzyme inhibitor antithrombin (AT) causing
a conformational change that results in its activation
through an increase in the flexibility of its reactive site loop. The activated
AT then inactivates thrombin and other
proteases involved in blood clotting, most notably factor
Xa. The rate of inactivation of these proteases by AT
can increase by up to 1000-fold due to the binding of heparin.
The conformational change inAT on heparin-binding
mediates its inhibition of factor Xa. For thrombin inhibition however, thrombin
must also bind to the heparin polymer at a site proximal to the
pentasaccharide. The highly-negative charge density of heparin contributes to
its very strong electrostatic interaction with thrombin The
formation of a ternary complex between AT, thrombin,
and heparin results in the inactivation of thrombin. For this reason heparin's
activity against thrombin is size-dependent, the ternary complex requiring at
least 18 saccharide units for efficient formation. In contrast anti factor Xa
activity only requires the pentasaccharide binding site
This size difference has led to the development of low-molecular-weight heparins
and more recently to fondaparinux as
pharmaceutical anticoagulants. Low-molecular-weight heparins and fondaparinux
target anti-factor Xa activity rather than anti-thrombin (IIa) activity, with
the aim of facilitating a more subtle regulation of coagulation and an improved
therapeutic index
Secretin
hormone production is stimulated by acid chyme entering the duodenum. This
hormone stimulates the pancreas to release bicarbonate to
neutralize the acid.
Secretin is a hormone that both
controls the environment in the duodenum
by regulating secretions of the stomach and pancreas, and
regulates water homeostasis throughout
the body. It is produced in the S cells of the
duodenum, which are located in the crypts of
Lieberkühn. In humans, the secretin peptide is encoded
by the SCT gene. Secretin was also the first
hormone to be identified.
Secretin
regulates the pH within the duodenum by
inhibiting gastric acid secretion by
the parietal cells of the
stomach, and by stimulating bicarbonate
production by the centroacinar cells
and intercalated ducts of the pancreas.
In 2007,
secretin was discovered to play a role in osmoregulation by acting on
the hypothalamus, pituitary, and kidney.
Secretin
increases watery bicarbonate solution from pancreatic and bile duct epithelium.
Pancreatic centroacinar cells have secretin receptors in their plasma membrane.
As secretin binds to these receptors, it stimulates adenylate cyclase activity
and converts ATP to cyclic AMP.
Cyclic AMP acts as second messenger in intracellular signal transduction and
leads to increase in release of watery carbonate. It is known to promote the
normal growth and maintenance of the pancreas.
Secretin
increases water and bicarbonate secretion from duodenal Brunner's glands
to buffer the incoming protons
of the acidic chyme. It also enhances the effects of cholecystokinin to
induce the secretion of digestive enzymes and bile from pancreas
and gallbladder,
respectively.
It
counteracts blood glucose
concentration spikes by triggering increased insulin
release from pancreas, following oral glucose
intake.
Although
secretin releases gastrin from gastrinomas, it inhibits gastrin release from the
normal stomach. It reduces acid secretion from the stomach
by inhibiting gastrin
release from G cells.
This helps neutralize the pH of the digestive products entering the duodenum
from the stomach, as digestive enzymes
from the pancreas (e.g., pancreatic amylase and pancreatic lipase)
function optimally at slightly basic pH.
In addition,
secretin stimulates pepsin
secretion from chief cells, which can
help break down proteins in food digestion. It stimulates release of glucagon, pancreatic polypeptide
and somatostatin.
Histamine, an important mediator (local
signaling substance) and neurotransmitter, is mainly stored in tissue
mast cells and basophilic granulocytes in the blood. It is involved in
inflammatory and allergic reactions. “Histamine liberators” such as tissue
hormones, type E immunoglobulins (see p. 300), and drugs can release it.
Histamine acts via various types of receptor. Binding to H1 receptors promotes
contraction of smoothmuscle in the bronchia, and dilates the capillary vessels
and increases their permeability. Via H2 receptors, histamine slows down the
heart rate and promotes the formation of HCl in the gastric mucosa. In the
brain, histamine acts as a neurotransmitter.
They
have hormone-like effects in their immediate surroundings. Histamine and
prostaglandins are important examples of these substances. Histamine is
found in plant and animal tissue and is released from mast cells as part of an
allergic reaction in humans. Release of histamine stimulates gastric secretion
and causes dilation of capillaries, constriction of bronchial smooth muscle,
and decreases blood pressure.
Histamines are released from mast cells as an allergic response to
abnormal proteins found in the blood. The mast cells are found in connective
tissue that contains numerous basophilic granules and releases substances such
as heparin and histamine in response to injury or inflammation of body tissues.
How severe
can histamine reactions be?
It has
recently been discovered that histamines may play a much larger roll in human
disease than once thought. In the past, histamine production was blamed on some
very common allergic reactions such as hay fever, bee sting reactions, and
anaphylactic shock.
In
recent studies, histamine involvement in chronic inflammatory and degenerative
diseases such as Lupus, Arthritis, Gulf War Syndrome, Fibromyalgia, Leaky Gut
Syndrome, and some skin disorders like Psoriasis and obscure Rashes, has come
to light as causes of chronic inflammatory responses to abnormal proteins in
the blood of chronically ill patients!
Histamine is a biogenic
amine involved in local immune responses as well as regulating physiological function
in the gut and acting as a neurotransmitter. Histamine triggers the
inflammatory response. As part of an immune response to foreign pathogens,
histamine is produced by basophils and by mast cells found in nearby connective
tissues. Histamine increases the permeability of the capillaries to white blood
cells and other proteins, in order to allow them to engage foreign invaders in
the affected tissues. It is found in virtually all animal body cells.
Histamine
forms colorless hygroscopic crystals that melt at
Histamine has two basic centres, namely the aliphatic
amino group and whichever nitrogen atom of the imidazole ring does not already
have a proton. Under physiological conditions, the aliphatic amino group
(having a pKa around 9.4) will be protonated, whereas the second
nitrogen of the imidazole ring (pKa ≈ 5.8) will not be protonated.
Thus, histamine is normally protonated to a singly-charged cation. Histamine is
derived from the decarboxylation of the amino acid histidine, a reaction
catalyzed by the enzyme L-histidine decarboxylase. It is a hydrophilic
vasoactive amine.
Once
formed, histamine is either stored or rapidly inactivated. Histamine released
into the synapses is broken down by acetaldehyde dehydrogenase. It is the
deficiency of this enzyme that triggers an allergic
reaction as histamines pool in the synapses. Histamine is broken down by
histamine-N-methyltransferase and diamine oxidase. Some forms of foodborne
disease, so-called "food poisonings," are due to conversion of
histidine into histamine in spoiled food, such as fish.
Serotonin is a neurotransmitter
= Neurotransmitters are chemicals that are used to relay, amplify and modulate signals
between a neuron and another cell - cell to cell communicators
= Serotonin
is produced in the body from amino acids
= Serotonin
taken orally does not pass into the serotonergic pathways of the central
nervous system because it does not cross the blood-brain barrier. However,
tryptophan and its metabolite 5-hydroxytryptophan (5-HTP), from which serotonin
is synthesized, can and does cross the blood-brain
barrier. These agents are available as dietary supplements and may be effective
serotonergic agents
= Drugs
may hinder the natural use/loss of serotonin but Drugs do not incease the
supply of serotonin.
Serotonin or 5-hydroxytryptamine (5-HT) is a monoamine
neurotransmitter. Biochemically derived from tryptophan, serotonin
is primarily found in the gastrointestinal
(GI) tract, platelets, and in the central
nervous system (CNS) of animals including humans. It
is popularly thought to be a contributor to feelings of well-being and happiness.
Approximately
90% of the human body's total serotonin is
located in the enterochromaffin
cells in the alimentary canal (gut), where it
is used to regulate intestinal movements. The remainder is synthesized in serotonergic neurons of the CNS,
where it has various functions. These include the regulation of mood,
appetite, and sleep. Serotonin also has some cognitive functions, including
memory and learning. Modulation of serotonin at synapses is thought to be a
major action of several classes of pharmacological antidepressants.
Serotonin secreted from the enterochromaffin cells
eventually finds its way out of tissues into the blood. There, it is actively
taken up by blood platelets, which store
it. When the platelets bind to a clot, they disgorge serotonin, where it serves
as a vasoconstrictor and helps to
regulate hemostasis and blood
clotting. Serotonin also is a growth factor for some types of cells, which may
give it a role in wound healing.
Investigation of thyroid hormones in the
regulation of metabolism. Hormonal regulation of calsium and phosphorus homeostasis.
Hormones
of thyroid and parathyroid glands
Thyroid
synthesizes two kinds of hormones: iodine containing hormones and calcitonin.
Iodine containing hormones - thyroxine and triiodthyronine.
Thyroid hormone is produced by the thyroid gland, which consists of follicles in which thyroid hormone is synthesized through iodination of tyrosine residues in the glycoprotein thyroglobulin. Thyroid stimulating hormone (TSH), secreted by the anterior pituitary in response to feedback from circulating thyroid hormone, acts directly on the TSH receptor (TSH-R) expressed on the thyroid follicular cell basolateral membrane. TSH regulates iodide uptake mediated by the sodium/iodide symporter, followed by a series of steps necessary for normal thyroid hormone synthesis and secretion. Thyroid hormone is essential for normal development, growth, neural differentiation, and metabolic regulation in mammals.
The THs, T4 and the more potent T3, are
synthesized in the thyroid gland. Iodide is actively transported and
concentrated into the thyroid by
The secretion
of THs requires endocytosis of the stored iodinated Tg
from the apical surface of the thyroid follicular cell. The internalized Tg is incorporated in phagolysosomes and undergoes
proteolytic digestion, recapture of MIT and DIT, and release of T4 and T3 into the circulation via the basal
surface. The majority of released TH is in the form of T4, as total
serum T4 is 40-fold
higher than serum T3 (90
vs. 2 nM). Only 0.03% of the total serum T4 is free (unbound), with the remainder
bound to carrier proteins such as thyroxine binding globulin (TBG), albumin,
and thyroid binding prealbumin. Approximately 0.3% of the total serum T3 is free, with the remainder bound to
TBG and albumin. It is the free TH that enters target cells and generates a
biological response.
The major
pathway for the production of T3 is
via 5′-deiodination of the outer ring of T4 by deiodinases and accounts for the
majority of the circulating T3. Type I deioidinase is found in
peripheral tissues such as liver and kidney and is responsible for the
conversion of the majority of T4 to
T3 in circulation.
Type II deiodinase is found in brain, pituitary, and brown adipose tissue and
primarily converts T4 to
T3for intracellular use. These deiodinases recently have been cloned
and demonstrated to be selenoproteins. 5′-Deiodination by type I
deiodinase and type III deioidinase, which is found primarily in placenta,
brain, and skin, leads to the generation of rT3, the key step in the
inactivation of TH. rT3 and T3 can be further deiodinated in the
liver and are sulfo- and glucuronide-conjugated before excretion in the bile.
There also is an enterohepatic circulation of TH as intestinal flora
deconjugates some of these compounds and promotes the reuptake of TH.
Although THs
may exert their effects on a number of intracellular loci, their primary effect
is on the transcriptional regulation of target genes. Early studies showed that
the effects of THs at the genomic level are mediated by nuclear TRs, which are
intimately associated with chromatin and bind TH with high affinity and
specificity. Similar to steroid hormones that also bind to nuclear receptors,
TH enters the cell and proceeds to the nucleus. It then binds to TRs, which may
already be prebound to TREs located in promoter regions of target genes. The
formation of ligand-bound TR complexes that are also bound to TREs is the
critical first step in the positive or negative regulation of target genes and
the subsequent regulation of protein synthesis. Given their abilities to bind both ligand and DNA as well as their ability to regulate
transcription, TRs can be regarded as ligand-regulatable transcription factors.
Metabolism: Thyroid hormones stimulate diverse metabolic activities
most tissues, leading to an increase in basal metabolic rate. One consequence
of this activity is to increase body heat production, which seems to result, at
least in part, from increased oxygen consumption and rates of ATP hydrolysis.
By way of analogy, the action of thyroid hormones is akin to blowing on a
smouldering fire. A few examples of specific metabolic effects of thyroid
hormones include:
Lipid metabolism: Increased thyroid hormone levels stimulate fat
mobilization, leading to increased concentrations of fatty acids in plasma.
They also enhance oxidation of fatty acids in many tissues. Finally, plasma concentrations
of cholesterol and triglycerides are inversely correlated with thyroid hormone
levels - one diagnostic indiction of hypothyroidism is increased blood
cholesterol concentration.
Carbohydrate
metabolism: Thyroid hormones stimulate almost
all aspects of carbohydrate metabolism, including enhancement of
insulin-dependent entry of glucose into cells and increased gluconeogenesis and
glycogenolysis to generate free glucose.
Protein metabolism: in normal concentration stimulate the synthesis of proteins
and nucleic acids; in excessive concentration activate the catabolic processes.
Growth: Thyroid hormones are clearly necessary for normal
growth in children and young animals, as evidenced by the growth-retardation
observed in thyroid deficiency. Not surprisingly, the growth-promoting effect
of thyroid hormones is intimately intertwined with that of growth hormone, a clear indiction that complex physiologic processes like growth depend
upon multiple endocrine controls.
Development: Of critical importance in mammals is the fact
that normal levels of thyroid hormone are essential to the development of
the fetal and neonatal brain.
Other Effects: As mentioned above, there do not seem to be organs and
tissues that are not affected by thyroid hormones. A few additional,
well-documented effects of thyroid hormones include:
Cardiovascular
system: Thyroid hormones increases heart
rate, cardiac contractility and cardiac output. They also promote vasodilation,
which leads to enhanced blood flow to many organs.
Central nervous
system: Both decreased and increased
concentrations of thyroid hormones lead to alterations in mental state. Too
little thyroid hormone, and the individual tends to feel mentally sluggish,
while too much induces anxiety and nervousness.
Reproductive system:
Thyroid Disease
States
Disease is associated with both inadequate production and
overproduction of thyroid hormones. Both types of disease are relatively common
afflictions of man and animals.
Hypothyroidism is the result from any condition that results in
thyroid hormone deficiency. Two well-known examples include:
Iodine deficiency: Iodide is absolutely necessary for
production of thyroid hormones; without adequate iodine intake, thyroid
hormones cannot be synthesized. Historically, this problem was seen
particularly in areas with iodine-deficient soils, and frank iodine deficiency
has been virtually eliminated by iodine supplementation of salt.
Primary thyroid disease: Inflammatory diseases of the thyroid
that destroy parts of the gland are clearly an important cause of
hypothyroidism.
Common symptoms of
hypothyroidism arising after early childhood include lethargy, fatigue,
cold-intolerance, weakness, hair loss and reproductive failure. If these signs
are severe, the clinical condition is called myxedema.
In the case of iodide deficiency, the thyroid becomes inordinantly large and is
called a goiter.
About 95
percent of the active thyroid hormone is thyroxine, and most of the remaining 5
percent is triiodothyronine. Both of these require iodine for their synthesis.
Thyroid hormone secretion is regulated by a negative feedback mechanism that
involves the amount of circulating hormone, hypothalamus, and adenohypophysis.
If there is
an iodine deficiency, the thyroid cannot make sufficient hormone. This
stimulates the anterior pituitary to secrete thyroid-stimulating hormone, which
causes the thyroid gland to increase in size in a vain attempt to produce more
hormones. But it cannot produce more hormones because it does not have the
necessary raw material, iodine. This type of thyroid enlargement is called
simple goiter or iodine
deficiency goiter.
Calcitonin is
secreted by the parafollicular cells of the thyroid gland. This hormone opposes
the action of the parathyroid glands by reducing the calcium level in the
blood. If blood calcium becomes too high, calcitonin is secreted until calcium
ion levels decrease to normal.
The most severe and devestating form of hypothyroidism
is seen in young children with congenital thyroid deficiency. If that
condition is not corrected by supplemental therapy soon after birth, the child
will suffer from cretinism, a form of
irreversible growth and mental retardation.
Congenital
hypothyroidism can
be endemic, genetic, or sporadic. If untreated, it results in mild to severe
impairment of both physical and mental growth and development. Poor
length growth is apparent as early as the first year of life. Adult stature
without treatment ranges from 1 to
Sporadic
and genetic cretinism results from abnormal development or function of the
foetal thyroid gland. This type of cretinism has been almost completely
eliminated in developed countries by early diagnosis by newborn screening schemes
followed by lifelong treatment with thyroxine (T4).
Thyroxine must be dosed
as tablets only, even to newborns, as the liquid oral suspensions and compounded
forms cannot be depended on for reliable dosing. In the case of dosing infants,
the T4 tablets are generally crushed and mixed with breast milk, formula milk
or water. If the medication is mixed with formulas containing iron or soya
products, larger doses may be required, as these substances may alter the
absorption of thyroid hormone from the gut. Frequent
monitoring (every 2–3 weeks during the first months of life) is recommended to
ensure that infants with congenital hypothyroidism remain within the high end
of normal range, or euthyroid.
Cretinism arises from a diet deficient in iodine. It has affected many people worldwide and continues
to be a major public health problem in many countries. Iodine is
an essential trace element, necessary primarily for the synthesis of thyroid
hormones. Iodine deficiency is the most common preventable cause of brain
damage worldwide. Although iodine is found in many
foods, it is not universally present in all soils in adequate amounts. Most
iodine, in iodide form, is in the oceans where the iodide ions oxidize to
elemental iodine, which then enters the atmosphere and falls to earth as rain,
introducing iodine to soils. Earth deficient in iodine is most common inland
and in mountainous areas and areas of frequent flooding, but can also occur in
coastal regions owing to past glaciation, and leaching by snow, water and heavy
rainfall, which removes iodine from the soil.[8] Plants
and animals grown in iodine deficient soils are correspondingly deficient.
Populations living in those areas without outside food sources are most at risk
of iodine deficiency.
Diseases.
Calcitonin is
synthesized by the parafollicle cells of thyroid.
Functions: -
promotes the transition of calcium from blood in bones;
-
inhibits
the reabsorption of phosphorus in kidneys.
Thus,
calcitonin decreases the Ca and P contents in blood.
Parathyroid hormone. Chemical structure: protein.
1.
promotes
the transition of calcium from bones to blood;
2.
promotes
the absorption of Ca in the intestine;
3.
inhibits the reabsorption of phosphorus in
kidneys.
Thus,
parathyroid hormone increases the Ca amount in blood and decreases the P amount in blood.
Body Distribution of Calcium and Phosphate
There are
three major pools of calcium in the body:
Calcium in
blood and extracellular fluid:
Fluxes of
Calcium and Phosphate
·
Inhibition of bone resorption, which would minimize
fluxes of calcium from bone into blood.
http://www.youtube.com/watch?v=JwPVibQ6_3Y&feature=related
http://www.youtube.com/watch?v=n7vybcT9_F4
·
Stimulates production of the biologically-active form
of vitamin D within the kidney.
Adrenal
glands consist of two parts: external - cortex, internal - medulla.
Each part secrets specific hormones.
Hormones
synthesized in adrenal cortex are named corticosteroids.
Mechanism of
steroid hormones action (permeating into the cells):
http://www.youtube.com/watch?v=oOj04WsU9ko
http://www.youtube.com/watch?v=0ss8YIoKw0g
The synthesis of corticosteroids is
regulated by ACTH.
In the blood corticosteroids are
connected with proteins and transported to different organs.
Time half-life for corticosteroids is
about 1 hour.
Ways of
metabolism of corticosteroids:
2. Oxidation of
21-st carbon atom.
4. Corticosteroids can be excreted by kidneys in
native structure.
http://www.youtube.com/watch?v=0ss8YIoKw0g
The effect of glucocorticoids on
protein metabolism:
2. stimulate the activity
of aminotransferases;
3. activate the
synthesis of urea.
The effect of glucocorticoids on
carbohydrate metabolism:
1. activate the gluconeogenesis;
2. inhibit the activity of hexokinase;
3. activate the glycogen synthesis in liver.
Glucocorticoids
causes the hyperglycemia.
The effect of glucocorticoids on
lipid metabolism:
1. promote the absorption of lipids in intestine;
3.
activate the conversion of fatty acids in
carbohydrates.
http://www.youtube.com/watch?v=ku-QJyQ0j7M&feature=related
·
adults (8 A.M.): 6-28 mg/dL; adults (4 P.M.): 2-12 mg/dL
·
child one to six years (8 A.M.): 3-21 mg/dL; child one to
six years (4 P.M.): 3-10 mg/dL
·
adolescent: 5-55 mg/24 hours
ACTH Dependent
(80%)
Pituitary
Tumors (60%)
ACTH
Independent (20%)
Benign
Adrenal Tumors (adenoma) (25%)
Malignant Adrenal
Tumors (adrenal cell carcinoma) (10%)
Treatment of Cushings Syndrome
Functions of
mineralocorticoids.
Secretion of mineralocorticoids is
regulated by renin-angiotensine system
-
activates the reabsorption of Na+,
Cl- and water in kidney canaliculuses;
-
promote
the excretion of K+ by kidneys, skin and saliva.
Deficiency of corticosteroids causes Addison's disease.
http://www.youtube.com/watch?v=FK1pPqWMXjM
A rare disorder in which symptoms are
caused by a deficiency of hydrocortisone (cortisol) and aldosterone, two
corticosteroid hormones normally produced by a part of the adrenal glands
called the adrenal cortex. Symptoms include weakness, tiredness, vague
abdominal pain, weight loss, skin pigmentation and low blood pressure. Mineralocorticoids
Primary hyperaldosteronism has many causes, including adrenal hyperplasia and adrenal carcinoma.[2]
·
Bilateral
(micronodular) adrenal hyperplasia, 60%
·
Adrenal
(Conn's) adenoma, 40%
·
Glucocorticoid-remediable
hyperaldosteronism (dexamethasone-suppressible
hyperaldosteronism), <1%
·
rare forms, including disorders of the renin-angiotensin system, <1%
Male sex hormones are called androgens and female - estrogens.
Chemical
structure - steroids.
Estrogens, in females, are produced
primarily by the ovaries, and during
pregnancy, the placenta. Follicle-stimulating hormone(FSH)
stimulates the ovarian production of estrogens by the granulosa cells of the ovarian follicles and corpora lutea. Some
estrogens are also produced in smaller amounts by other tissues such as the liver, adrenal glands, and the breasts. These
secondary sources of estrogens are especially important in postmenopausal
women. Fat cells produce estrogen as
well.
·
Promote formation of female secondary sex characteristics
·
Stimulate endometrial growth
·
Increase vaginal lubrication
·
Maintenance of vessel and skin
·
Reduce bone resorption, increase bone formation
·
Increase hepatic production of binding proteins
·
Increase circulating level of factors 2, 7, 9, 10, plasminogen
·
Increase platelet adhesiveness
·
Decrease LDL, fat deposition
·
Salt (sodium) and water retention
·
Increase cholesterol in bile
·
Increase pheomelanin, reduce eumelanin
·
Support hormone-sensitive breast cancers (see section below)
·
Promotes lung function by
supporting alveoli (in rodents
but probably in humans).
·
Progesterone
decreases contractility of the uterine smooth muscle.
·
A drop in
progesterone levels is possibly one step that facilitates the onset of labor.
The fetus metabolizes placental progesterone in the
production of adrenal steroids.
http://www.youtube.com/watch?v=nLmg4wSHdxQ&feature=fvwrel
Besides testosterone, other androgens include:
Effect of sex
hormones on protein metabolism:
1.
stimulate the processes of protein, DNA, RNA
synthesis;
2.
cause the positive nitrogenous
equilibrium.
Effect
of sex hormones on carbohydrate metabolism:
2. activate the synthesis of glycogen in liver.
Effect of sex
hormones on lipid metabolism:
1. enhance the oxidation of lipids;
2. inhibit the synthesis of cholesterol.
Effect of sex
hormones on energy metabolism:
-
stimulate
the Krebs cycle, tissue respiration and ATP production.
There are a
variety of physiological effects including:
Basic
principles of metabolism: catabolism, anabolism. Common
pathways of proteins, carbohydrates and lipids transformation.
Investigation of Krebs cycle functioning.
Metabolism is the set of life-sustaining chemical transformations within the cells of living organisms. These enzyme-catalyzed reactions allow organisms to grow and
reproduce, maintain their structures, and respond to their environments. The
word metabolism can also refer to all chemical reactions that occur in living
organisms, including digestion and the transport of substances into and between
different cells, in which case the set of reactions within the cells is called intermediary
metabolism or intermediate metabolism.
The term metabolism is derived from the Greek –
"Metabolismos" for "change", or "overthrow". The history of the scientific study of
metabolism spans several centuries and has moved from examining whole animals
in early studies, to examining individual metabolic reactions in modern
biochemistry. The first controlled experiments in human metabolism were published by Santorio Santorioin 1614 in his book Ars de statica medicina. He described how he weighed himself
before and after eating, sleep, working, sex, fasting, drinking, and
excreting. He found that most of the food he took in was lost through what he
called "insensible perspiration".
In these early studies, the mechanisms of these metabolic
processes had not been identified and a vital force was thought to animate living tissue. In the 19th century, when studying the fermentation of sugar to alcohol by yeast, Louis Pasteur concluded that fermentation was
catalyzed by substances within the yeast cells he called "ferments".
He wrote that "alcoholic fermentation is an act correlated with the life
and organization of the yeast cells, not with the death or putrefaction of the
cells."] This
discovery, along with the publication by Friedrich Wöhler in 1828 of the chemical synthesis of urea, notable for being the first organic
compound prepared from wholly inorganic precursors, proved that the organic
compounds and chemical reactions found in cells were no different in principle
than any other part of chemistry.
It was the discovery of enzymes at
the beginning of the 20th century by Eduard Buchner that separated the study of the
chemical reactions of metabolism from the biological study of cells, and marked
the beginnings of biochemistry. The mass of biochemical knowledge grew
rapidly throughout the early 20th century. One of the most prolific of these
modern biochemists wasHans Krebs who made huge contributions to the
study of metabolism. He
discovered the urea cycle and later, working with Hans Kornberg, the citric acid cycle and
the glyoxylate cycle. Modern
biochemical research has been greatly aided by the development of new
techniques such as chromatography, X-ray diffraction, NMR spectroscopy, radioisotopic
labelling, electron microscopy andmolecular dynamics simulations. These techniques have
allowed the discovery and detailed analysis of the many molecules and metabolic
pathways in cells.
Metabolism is a term that is used to describe all
chemical reactions involved in maintaining the living state of the cells and
the organism. Metabolism can be conveniently divided into two categories:
· Catabolism - the breakdown of
molecules to obtain energy
· Anabolism -
the synthesis of all compounds needed by the cells
Anabolism is the set of
constructive metabolic processes where the energy released by catabolism is
used to synthesize complex molecules. In general, the complex molecules that
make up cellular structures are constructed step-by-step from small and simple
precursors. Anabolism involves three basic stages. Firstly, the production of
precursors such as amino acids, monosaccharides,isoprenoids and nucleotides, secondly, their activation
into reactive forms using energy from ATP, and thirdly, the assembly of these
precursors into complex molecules such as proteins, polysaccharides, lipids and nucleic acids.
Metabolism refers to the highly integrated network of
chemical reactions by which living cells grow and sustain themselves. This
network is composed of two major types of pathways: anabolism and catabolism.
Anabolism uses energy stored in the form of adenosine triphosphate (ATP) to
build larger molecules from smaller molecules. Catabolic reactions degrade
larger molecules in order to produce ATP and raw materials for anabolic
reactions.
Together, these two general metabolic networks have three
major functions:
(1) to extract energy from nutrients or solar
energy;
(2) to synthesize the building blocks that make up the
large molecules of life: proteins, fats, carbohydrates, nucleic acids, and
combinations of these substances;
(3) to synthesize and degrade molecules required for
special functions in the cell.
These reactions are controlled by enzymes, protein
catalysts that increase the speed of chemical reactions in the cell without
themselves being changed. Each enzyme catalyzes a specific chemical reaction by
acting on a specific substrate, or raw material. Each reaction is just one in a
sequence of catalyticsteps known as metabolic pathways. These sequences may be
composed of up to20 enzymes, each one creating a product that becomes the
substrate--or raw material--for the subsequent enzyme. Often, an additional
molecule called a coenzyme is required for the enzyme to function. For example,
some coenzymes accept an electron that is released from the substrate during
the enzymatic reaction. Most of the water-soluble vitamins of the B complex
serve as coenzymes;riboflavin (Vitamin B2) for example, is a
precursor of the coenzyme flavine adenine dinucleotide, while pantothenate is a
component of coenzyme A, an important intermediate metabolite.
The series of products created by the sequential
enzymatic steps of anabolismor catabolism are called metabolic intermediates,
or metabolites. Each steprepresents a small change in the molecule, usually the
removal, transfer, oraddition of a specific atom, molecule, or group of atoms
that serves as a functional group, such as the amino groups (-NH2) of proteins.
Most such metabolic pathways are linear, that is, they
begin with a specificsubstrate and end with a specific product. However, some
pathways, such as the Krebs cycle, are cyclic. Often, metabolic pathways also
have branches thatfeed into or out of them. The specific sequences of
intermediates in the pathways of cell metabolism are called intermediary
metabolism.
Among the many hundreds of chemical reactions there are
only a few that are central to the activity of the cell, and these pathways are
identical in mostforms of life.
All
reactions of metabolism, however, are part of the overall goal of the organism to
maintain its internal orderliness, whether that organism is a singlecelled
protozoan or a human. Organisms maintain this orderliness by removingenergy
from nutrients or sunlight and returning to their environment an equal amount
of energy in a less useful form, mostly heat. This heat becomes dissipated
throughout the rest of the organism's environment.
According to the first law of thermodynamics, in any physical or chemical
change, the total amount of energy in the universe remains constant, that is,
energy cannot be created or destroyed. Thus, when the energy stored in
nutrientmolecules is released and captured in the form of ATP, some energy is
lost as heat. But the total amount of energy is unchanged.
The second law of thermodynamics states that physical and chemical changes
proceed in such a direction that useful energy undergoes irreversible
degradation into a randomized form--entropy. The dissipation of energy during
metabolism represents an increase in the randomness, or disorder, of the
organism's environment. Because this disorder is irreversible, it provides the
driving force and direction to all metabolic enzymatic reactions.
Even in the simplest cells, such as bacteria, there are at least a thousand
such reactions. Regardless of the number, all cellular reactions can be
classified as one of two types of metabolism: anabolism and catabolism. These
reactions, while opposite in nature, are linked through the common bond of
energy.Anabolism, or biosynthesis, is the synthetic phase of metabolism during
which small building block molecules, or precursors, are built into large
molecular components of cells, such as carbohydrates and proteins.
Catabolic reactions are used to capture and save
energy from nutrients, as well as to degrade larger molecules into smaller,
molecular raw materials for reuse by the cell. The energy is stored in the form
of energy-rich ATP, whichpowers the reactions of anabolism. The useful energy
of ATP is stored in theform of a high-energy bond between the second and third
phosphate groups of ATP. The cell makes ATP by adding a phosphate group to the
molecule adenosinediphosphate (ADP). Therefore, ATP is the major chemical link
between the energy-yielding reactions of catabolism, and the energy-requiring
reactions of anabolism.
In some cases, energy is also conserved as
energy-rich hydrogen atoms in thecoenzyme nicotinamide adenine dinucleotide
phosphate in the reduced form of NADPH. The NADPH can then be used as a source
of high-energy hydrogen atoms during certain biosynthetic reactions of
anabolism.
In addition to the obvious difference in the
direction of their metabolic goals, anabolism and catabolism differ in other
significant ways. For example, the various degradative pathways of catabolism
are convergent. That is, many hundreds of different proteins, polysaccharides,
and lipids are broken down into relatively few catabolic end products. The
hundreds of anabolic pathways,however, are divergent. That is, the cell uses
relatively few biosynthetic precursor molecules to synthesize a vast number of
different proteins, polysaccharides, and lipids.
The opposing pathways of anabolism and catabolism
may also use different reaction intermediates or different enzymatic reactions
in some of the steps. Forexample, there are 11 enzymatic steps in the breakdown
of glucose into pyruvic acid in the liver. But the liver uses only nine of
those same steps in thesynthesis of glucose, replacing the other two steps with
a different set ofenzyme-catalyzed reactions. This occurs because the pathway
to degradation ofglucose releases energy, while the anabolic process of glucose
synthesis requires energy. The two different reactions of anabolism are
required to overcome the energy barrier that would otherwise prevent the
synthesis of glucose.
Another reason for having slightly different
pathways is that the corresponding anabolic and catabolic routes must be
independently regulated. Otherwise,if the two phases of metabolism shared the
exact pathway (only in reverse) aslowdown in the anabolic pathway would slow
catabolism, and vice versa.
Some reactions can be either catabolic or anabolic,
depending on the circumstances. Such reactions are called amphibolic reactions.
Many of the reactions interconverting the “simple molecules” fall in this
category.
Catabolic and anabolic pathways are interrelated in
three ways:
Matter (catabolic pathways furnish the precursor
compounds for anabolism. Energy (catabolic pathways furnish the energy to
“drive” anabolism). Electrons (catabolic pathways furnish the reducing power
for anabolism).
Linear pathways convert one compound through a
series of intermediates to another compound. An example would be glycolysis,
where glucose is converted to pyruvate.
Branched pathways may either be divergent (an
intermediate can enter several linear pathways to different end products) or
convergent (several precursors can give rise to a common intermediate).
Biosynthesis of purines and of some amino acids are examples
of divergent pathways. There is usually some regulation at the branch point.
The conversion of various carbohydrates into the glycolytic pathway would be an
example of convergent pathways.
In a cyclic pathway, intermediates are regenerated,
and so some intermediates act in a catalytic fashion. In this illustration, the
cyclic pathway carries out the net conversion of X to Z. The Tricarboxylic Acid
Cycle is an example of a cyclic pathway.
A pool of compounds in equilibrium with each other
provides the intermediates for converting compounds to a variety of products,
depending on what is fed “into” the pool and what is “withdrawn” from the pool.
The phosphogluconate pathway is an example of such a pool of intermediates. The
pathway can convert glucose to CO2, hexoses to pentoses, pentoses to hexoses,
pentoses to trioses, etc. depending on what the cell requires in a particular
situation. NADPH as a source of reducing power for anabolic reactions is also a
main product of the phosphogluconate pathway.
Organisms differ in how many of the molecules in
their cells they can construct for themselves. Autotrophs such as plants can construct the
complex organic molecules in cells such as polysaccharides and proteins from
simple molecules like carbon dioxide and water. Heterotrophs, on the other hand, require a
source of more complex substances, such as monosaccharides and amino acids, to
produce these complex molecules. Organisms can be further classified by
ultimate source of their energy: photoautotrophs and photoheterotrophs obtain
energy from light, whereas chemoautotrophs and chemoheterotrophs obtain energy
from inorganic oxidation reactions.
Metabolism is closely linked to nutrition and the
availability of nutrients. Bioenergetics is a term which describes the
biochemical or metabolic pathways by which the cell ultimately obtains energy.
Energy formation is one of the vital components of metabolism.
The speed of
metabolism, the metabolic rate,
influences how much food an organism will require, and also affects how it is
able to obtain that food.
A striking feature of metabolism is the similarity of the
basic metabolic pathways and components between even vastly different species. For example, the set of carboxylic acids that are best known as the
intermediates in the citric acid cycle are present in all known organisms,
being found in species as diverse as the unicellular bacterium Escherichia coli and huge multicellular organisms.
Nutrition is the key to metabolism. The pathways of
metabolism rely upon nutrients that they breakdown in order to produce energy.
This energy in turn is required by the body to synthesize new proteins, nucleic
acids (DNA, RNA) etc.
Nutrients in relation to metabolism encompass bodily
requirement for various substances, individual functions in body, amount
needed, level below which poor health results etc.
Essential nutrients supply energy (calories) and supply
the necessary chemicals which the body itself cannot synthesize. Food provides
a variety of substances that are essential for the building, upkeep, and repair
of body tissues, and for the efficient functioning of the body.
The diet needs
essential nutrients like carbon, hydrogen, oxygen, nitrogen, phosphorus,
sulfur, and around 20 other inorganic elements. The major elements are supplied
incarbohydrates, lipids, and protein. In addition, vitamins, minerals and
water are necessary.
The fate of dietary components after
digestion and absorption constitutes
metabolism—the metabolic pathways taken by individual
molecules, their interrelationships, and the
mechanisms that regulate the flow of metabolites
through the pathways. Metabolic pathways fall
into three categories: (1) Anabolic pathways are those
involved in the synthesis of compounds. Protein synthesis
is such a pathway, as is the synthesis of fuel reserves
of triacylglycerol and glycogen. Anabolic pathways are
endergonic. (2) Catabolic pathways are involved in
the breakdown of larger molecules, commonly involving
oxidative reactions; they are exergonic, producing reducing equivalents and,
mainly via the respiratory chain, ATP.
Amphibolic pathways occur at
the “crossroads” of metabolism, acting as links between the
anabolic and catabolic pathways, eg, the citric acid
cycle.
A knowledge of normal
metabolism is essential for an
understanding of abnormalities underlying disease. Normal
metabolism includes adaptation to periods of starvation,
exercise, pregnancy, and lactation. Abnormal metabolism
may result from nutritional deficiency, enzyme
deficiency, abnormal secretion of hormones, or the actions of drugs and toxins.
An important example of a metabolic disease is diabetes
mellitus.
PATHWAYS THAT
PROCESS THE MAJOR PRODUCTS OF
DIGESTION
The nature of the diet sets the basic
pattern of metabolism. There is a
need to process the products of digestion of
dietary carbohydrate, lipid, and protein. These are
mainly glucose, fatty acids and glycerol, and amino acids,
respectively. In ruminants (and to a lesser extent in
other herbivores), dietary cellulose is fermented by symbiotic
microorganisms to short-chain fatty acids (acetic,
propionic, butyric), and metabolism in these animals
is adapted to use these fatty acids as major substrates.
All the products of digestion are
metabolized to a common product, acetyl-CoA, which
is then oxidized by the citric acid cycle .
Carbohydrate
Metabolism Is Centered on the
Provision & Fate of Glucose
Glucose is
metabolized to pyruvate by the pathway of glycolysis,
which can occur anaerobically (in the absence of
oxygen), when the end product is lactate. Aerobic tissues metabolize pyruvate
to acetyl-CoA, which can enter the
citric acid cycle for complete oxidation to
CO2 and H2O, linked to the formation of ATP.
Glucose
and its metabolites also take part in other processes.
Examples: (1) Conversion to the storage polymer
glycogen in skeletal muscle and liver. (2) The pentose phosphate
pathway, an alternative to part of the pathway
of glycolysis, is a source of reducing equivalents (NADPH)
for biosynthesis and the source of ribose for nucleotide and nucleic
acid synthesis. (3) Triose phosphate gives rise to
the glycerol moiety of
triacylglycerols. (4) Pyruvate and intermediates of the
citric acid cycle provide the carbon skeletons for the
synthesis of amino acids; and acetyl-CoA, the precursor of
fatty acids and cholesterol (and hence of all steroids
synthesized in the body). Gluconeogenesis is the
process of forming glucose from noncarbohydrate precursors, eg, lactate, amino
acids, and glycerol.
Foods supply carbohydrates in three forms: starch, sugar,
and cellulose (fiber). Starches and sugars form major and essential sources of
energy for humans. Fibers contribute to bulk in diet.
Body tissues depend on glucose for all activities.
Carbohydrates and sugars yield glucose by digestion or metabolism.Most people consume
around half of their diet as carbohydrates.
Lipid
Metabolism Is Concerned Mainly With
Fatty Acids & Cholesterol
The source of long-chain fatty acids is either dietary lipid
or de novo synthesis from acetyl-CoA derived from carbohydrate.
Fatty acids may be oxidized to acetyl- CoA
(β-oxidation) or esterified with glycerol, forming triacylglycerol
(fat) as the body’s main fuel reserve. Acetyl-CoA
formed by β-oxidation
may undergo several fates:
(1) As with acetyl-CoA arising from glycolysis, it is oxidized
to CO2 + H2O via the citric
acid cycle.
(2) It is the precursor for synthesis of cholesterol
and other steroids.
(3) In the liver, it forms ketone bodies (acetone,
acetoacetate, and 3 hydroxybutyrate)
that are important fuels in prolonged starvation.
Fats are concentrated sources of energy. They produce
twice as much energy as either carbohydrates or protein on a weight basis.
Carbohydrate catabolism is the breakdown of
carbohydrates into smaller units. Carbohydrates are usually taken into cells
once they have been digested intomonosaccharides. Once inside, the major route of
breakdown is glycolysis, where sugars such as glucose and fructose are
converted into pyruvate and
some ATP is generated.
Pyruvate is an intermediate
in several metabolic pathways, but the majority is converted to acetyl-CoA and fed into the citric acid cycle. Although some more ATP
is generated in the citric acid cycle, the most important product is NADH,
which is made from NAD+ as
the acetyl-CoA is oxidized. This oxidation releases carbon dioxide as a waste product. In anaerobic
conditions, glycolysis produces lactate, through the enzyme lactate
dehydrogenase re-oxidizing
NADH to NAD+ for re-use in glycolysis. An alternative route for glucose
breakdown is the pentose
phosphate pathway, which reduces the coenzyme NADPH and produces pentose sugars
such asribose, the sugar component of nucleic acids.
Fats are catabolised by hydrolysis to free fatty acids and glycerol. The
glycerol enters glycolysis and the fatty acids are broken down bybeta oxidation to release acetyl-CoA, which then is
fed into the citric acid cycle. Fatty acids release more energy upon oxidation
than carbohydrates because carbohydrates contain more oxygen in their
structures.
Amino acids are either used to synthesize proteins
and other biomolecules, or oxidized to urea and carbon dioxide as a source of
energy. The oxidation pathway
starts with the removal of the amino group by a transaminase. The amino group is fed into
the urea cycle, leaving a deaminated carbon
skeleton in the form of a keto acid. Several of these keto acids are
intermediates in the citric acid cycle, for example the deamination of glutamate forms
α-ketoglutarate. The glucogenic amino
acids can also be
converted into glucose, through gluconeogenesis .
Much of Amino
Acid Metabolism Involves
Transamination
The amino acids are required for protein synthesis. Some
must be supplied in the diet (the essential amino acids)
since they cannot be synthesized in the body. The
remainder are nonessential amino acids that are supplied
in the diet but can be formed from metabolic intermediates
by transamination, using the amino nitrogen from other amino acids.
After deamination, amino
nitrogen is excreted as urea, and the carbon skeletons
that remain after transamination (1) are oxidized to
CO2 via the citric acid cycle, (2) form glucose (gluconeogenesis),
or (3) form ketone bodies.
Several amino acids are also the precursors of other compounds,
eg, purines, pyrimidines, hormones such as
epinephrine and thyroxine, and neurotransmitters.
Proteins
are the main tissue builders in the body. They are part of every cell in the
body. Proteins help in cell structure, functions, haemoglobin formation to
carry oxygen, enzymes to carry out vital reactions and a myriad of other
functions in the body. Proteins are also vital in supplying nitrogen for DNA
and RNA genetic material and energy production.
Catabolism can be broken down into 3 main stages.
Stage 1 – Stage of Digestion
The large organic molecules like proteins, lipids and
polysaccharides are digested into their smaller components outside cells. This
stage acts on starch, cellulose or proteins that cannot be directly absorbed by
the cells and need to be broken into their smaller units before they can be
used in cell metabolism.
Digestive enzymes include glycoside hydrolases that
digest polysaccharides into monosaccharides or simple sugars.
The primary enzyme involved in protein digestion is pepsin
which catalyzes the nonspecific hydrolysis of peptide bonds at an optimal pH of
2. In the lumen of the small intestine, the pancreas secretes
zymogens of trypsin, chymotrypsin, elastase etc. These proteolytic
enzymes break the proteins down into free amino acids as well as dipeptides and
tripeptides. The free amino acids as well as the di and tripeptides are
absorbed by the intestinal mucosa cells which subsequently are released into
the blood stream where they are absorbed by other tissues.
The amino acids and sugars are then pumped into cells by
specific active transport proteins.
Stage 2 – Release of energy
Once broken down these molecules are taken up by cells
and converted to yet smaller molecules, usually acetyl coenzyme A (acetyl-CoA),
which releases some energy.
Stage 3 - The acetyl group on the CoA is
oxidised to water and carbon dioxide in the citric acid cycle and electron
transport chain, releasing the energy that is stored by reducing the coenzyme
nicotinamide adenine dinucleotide (NAD+) into NADH.
When complex carbohydrates are broken they form simple sugars or
monosaccharides. This is taken up by the cells. Once inside these sugars
undergo glycolysis, where sugars such as glucose and fructose are converted
into pyruvate and some ATP is generated. Pyruvate is an intermediate in several
metabolic pathways, but the majority is converted to acetyl-CoA and fed into
the citric acid cycle or the Kreb’s cycle.
Within the citric acid cycle more ATP is generated by the
monosaccharides. The most important product is NADH, which is made from
NAD+ as the acetyl-CoA is oxidized. This oxidation releases carbon dioxide
as a waste product.
When there is no oxygen, glycolysis produces lactate,
through the enzyme lactate dehydrogenase, re-oxidizing NADH to NAD+ for re-use
in glycolysis.
Glucose can also be broken down by pentose phosphate
pathway, which reduces the coenzyme NADPH and produces pentose sugars such as
ribose, the sugar component of nucleic acids.
Proteins are broken down into amino acids. Amino acids
are either used to synthesize proteins and other biomolecules, or oxidized to
urea and carbon dioxide as a source of energy.
In the process of oxidation, first the amino group is
removed by a transaminase. The amino group is fed into the urea cycle, leaving
a deaminated carbon skeleton in the form of a keto acid.
These keto acids enter the citric acid cycle.
Glutamate, for example, forms α-ketoglutarate. Some of the amines may also
be converted into glucose, through gluconeogenesis.
Some proteins are incredibly stable, others are very
short lived. The short lived proteins usually play important metabolic
roles. The short life times of these proteins allow the cell to rapidly
adjust to changes in the metabolic state of the cell.
Lipid breakdown
Fats are catabolised by hydrolysis to free fatty acids
and glycerol. The glycerol enters glycolysis and the fatty acids are broken
down by beta oxidation to release acetyl-CoA. This acetyl co-A reaches the
citric acid cycle next. Fatty acids release more energy upon oxidation than
carbohydrates because carbohydrates contain more oxygen in their structures.
The chemical reactions of metabolism are organized into
metabolic pathways. These allow the basic chemicals from nutrition to be
transformed through a series of steps into another chemical, by a sequence of
enzymes.
Enzymes are crucial to metabolism because they allow
organisms to drive desirable reactions that require energy. These reactions
also are coupled with those that release energy. As enzymes act as catalysts
they allow these reactions to proceed quickly and efficiently. Enzymes also
allow the regulation of metabolic pathways in response to changes in the cell's
environment or signals from other cells.
Each metabolic pathway consists of a series of
biochemical reactions that are connected by their intermediates: the products
of one reaction are the substrates for subsequent reactions, and so on.
Metabolic pathways are often considered to flow in one direction. Although all
chemical reactions are technically reversible, conditions in the cell are often
such that it is thermodynamically more favorable for flux to flow in one direction of a
reaction. For example, one pathway may be responsible for the synthesis of a
particular amino acid, but the breakdown of that amino acid may occur via a
separate and distinct pathway. One example of an exception to this
"rule" is the metabolism of glucose. Glycolysis results in the breakdown of glucose,
but several reactions in the glycolysis pathway are reversible and participate
in the re-synthesis of glucose (gluconeogenesis).
· Glycolysis was the first metabolic pathway
discovered:
1. As glucose enters
a cell, it is immediately phosphorylated by ATP to glucose 6-phosphate in the irreversible first step.
2. In times of excess lipid or protein energy
sources, certain reactions in the glycolysis pathway may run in reverse in order to
produce glucose 6-phosphate which is then used for storage as glycogen or starch.
· Metabolic pathways are often regulated by feedback inhibition.
· Some metabolic pathways flow in a 'cycle' wherein each
component of the cycle is a substrate for the subsequent reaction in the cycle,
such as in the Krebs Cycle (see below).
· Anabolic and catabolic pathways
in eukaryotes often occur independently of each
other, separated either physically by compartmentalization within organelles or separated biochemically by the
requirement of different enzymes and co-factors.
Several distinct but linked metabolic pathways are
used by cells to transfer the energy released by breakdown of fuel molecules into ATPand
other small molecules used for energy (e.g. GTP, NADPH, FADH).
These pathways occur within all living organisms in some
form:
1. Glycolysis
2. Aerobic respiration and/or Anaerobic
respiration
3. Citric acid cycle / Krebs cycle (not in most obligate
anaerobic organisms)
4. Oxidative
phosphorylation (not in
obligate anaerobic organisms)
Catabolism is
characterized by convergence of three major routs toward a final
common pathway.
Different proteins, fats and
carbohydrates enter the same pathway – tricarboxylic acid cycle.
Anabolism can also be divided
into stages, however the anabolic pathways are
characterized by divergence.
Monosaccharide synthesis begin with CO2, oxaloacetate, pyruvate or
lactate. Amino acids are synthesized from acetyl CoA, pyruvate or keto acids of Krebs cycle. .
Fatty acids are constructed from acetyl CoA.
On the next stage
monosaccharides, amino acids and fatty acids are used for the synthesis of polysaccharides, proteins and fats.
Compartmentation of metabolic
processes permits:
- separate pools
of metabolites within a cell
- simultaneous operation
of opposing metabolic paths
- high local
concentrations of metabolites
Example: fatty acid synthesis
enzymes (cytosol), fatty acid breakdown enzymes
(mitochondria).
METABOLIC PATHWAYS MAY BE STUDIED
AT DIFFERENT LEVELS
OF ORGANIZATION
In addition to studies in the whole organism, the location and
integration of metabolic pathways is revealed by
studies at several levels of organization. At the tissue and organ level, the
nature of the substrates entering and
metabolites leaving tissues and organs is defined. At the
subcellular level, each cell organelle (eg, the mitochondrion) or
compartment (eg, the cytosol) has specific roles
that form part of a subcellular pattern of metabolic
pathways.
At the Tissue
and Organ Level, the Blood Circulation
Integrates Metabolism
Amino acids resulting from the digestion of
dietary protein and glucose resulting from
the digestion of carbohydrate are absorbed
and directed to the liver via the hepatic portal vein. The liver has the
role of regulating the blood concentration of most
water-soluble metabolites In the case
of glucose, this is achieved
by taking up glucose in excess of immediate requirements
and converting it to glycogen.
Between meals, the liver acts to
maintain the blood glucose concentration from glycogen (glycogenolysis) and,
together with the kidney, by converting
noncarbohydrate metabolites such as lactate,
glycerol, and amino acids to glucose (gluconeogenesis). Maintenance
of an adequate concentration of blood
glucose is vital for those tissues in which it is
the major fuel (the brain) or the only fuel (the erythrocytes).
The liver also synthesizes the major plasma proteins
(eg, albumin) and deaminates amino acids that
are in excess of requirements, forming urea, which is transported to the kidney
and excreted. Skeletal muscle utilizes
glucose as a fuel, forming both lactate
and CO2. It stores glycogen as a fuel for its use
in muscular contraction and synthesizes muscle protein
from plasma amino acids. Muscle accounts for approximately
50% of body mass and consequently represents a
considerable store of protein that can be drawn
upon to supply amino acids for gluconeogenesis in
starvation.
Lipids in the diet are mainly triacylglycerol and
are hydrolyzed to monoacylglycerols and fatty
acids in the gut, then reesterified in the intestinal mucosa.
Here they are packaged with protein and secreted into
the lymphatic system and thence into the
blood stream as chylomicrons,
the largest of the plasma lipoproteins.
Chylomicrons also contain other lipidsoluble nutrients,
eg, vitamins. Unlike glucose and amino acids,
chylomicron triacylglycerol is not taken up directly
by the liver. It is first metabolized by tissues that have
lipoprotein lipase, which hydrolyzes the triacylglycerol, releasing
fatty acids that are incorporated into tissue lipids
or oxidized as fuel. The other major source of
long-chain fatty acid is synthesis (lipogenesis) from carbohydrate,
mainly in adipose tissue and the liver. Adipose
tissue triacylglycerol is the main fuel reserve of
the body. On hydrolysis (lipolysis) free fatty acids are released
into the circulation. These are taken up by most tissues (but not brain or
erythrocytes) and esterified to acylglycerols
or oxidized as a fuel. In the liver, triacylglycerol arising
from lipogenesis, free fatty acids, and chylomicron remnants is secreted into
the circulation as very low density lipoprotein (VLDL).
This triacylglycerol undergoes a fate similar to
that of chylomicrons. Partial oxidation of fatty acids
in the liver leads to ketone body production Ketone bodies are
transported to extrahepatictissues, where they act as a fuel source in starvation.
Glycolysis enzymes are
located in the cytosol of cells. Pyruvate enters the mitochondrion to be metabolized further.
Pyruvate dehydrogenase complex is a bridge between
glycolysis and aerobic metabolism – citric acid cycle.
Flow diagram
depicting the overall activity of the pyruvate
dehydrogenase complex. During the oxidation of
pyruvate to CO2 by pyruvate dehydrogenase the electrons flow from
pyruvate to the lipoamide moiety of dihydrolipoyl transacetylase then to the
FAD cofactor of dihydrolipoyl dehydrogenase and finally to reduction of NAD+
to NADH. The acetyl group is linked to coenzyme
A (CoASH) in a high energy thioester bond. The acetyl-CoA then enters the
TCA cycle for complete oxidation to CO2 and H2O.
Pyruvate freely diffuses through the outer membrane of
mitochon-dria through the channels formed by transmembrane proteins porins.
Pyruvate Dehydrogenase catalyzes
oxidative decarboxylation of pyruvate, to form acetyl-CoA. The overall reaction is shown below.
Pyruvate
dehydrogenase complex is giant,
with molecular mass ranging from 4 to 10 million daltons.
Pyruvate Dehydrogenase is a large complex containing many copies of each of
three enzymes, E1, E2, and E3.
The inner core of the mammalian Pyruvate
Dehydrogenase complex is an icosahedral structure consisting of 60 copies of E2.
At the periphery of
the complex are:
· 30 copies of E1 (itself a tetramer with
subunits a2b2) and
· 12 copies of E3 (a homodimer), plus 12 copies of an E3 binding protein that links E3 to E2.
Prosthetic groups are listed below
Enzyme |
Abbreviated |
Prosthetic Group |
Pyruvate Dehydrogenase |
E1 |
Thiamine pyrophosphate (TPP) |
Dihydrolipoyl Transacetylase |
E2 |
Lipoamide |
Dihydrolipoyl Dehydrogenase |
E3 |
FAD |
Thiamine pyrophosphate (TPP)
is a derivative of thiamine (vitamin B1). Nutritional
deficiency of thiamine leads to the disease beriberi.
Beriberi affects especially the brain, because TPP is required for carbohydrate
metabolism, and the brain depends on glucose metabolism for energy.
A proton readily dissociates from the C that is between N and S in the thiazole ring of TPP. The
resulting carbanion (ylid) can attack the
electron-deficient keto carbon of pyruvate.
Lipoamide includes
a dithiol that undergoes oxidation and
reduction.
The carboxyl group at the end of lipoic acid's
hydrocarbon chain forms an amide
bond to the side-chain amino
group of a lysine residue of E2.
A long
flexible arm, including hydrocarbon chains of lipoate and the lysine
R-group, links the dithiol of each lipoamide to one of two lipoate-binding
domains of each E2.
Lipoate-binding domains are themselves part of a flexible strand of E2 that extends out from the core of the complex.
The long flexible attachment allows lipoamide functional groups to swing back and forth between E2 active sites in the core of the
complex and active sites of E1 & E3 in the outer shell of the complex.
The E3
binding protein (that binds
E3 to E2) also has attached lipoamide that can exchange reducing equivalents
with lipoamide on E2.
FAD (Flavin Adenine Dinucleotide) is a derivative of the B-vitamin
riboflavin (dimethylisoalloxazine-ribitol). The flavin ring system undergoes oxidation/reduction as shown below. Whereas NAD+ is a coenzyme that reversibly binds to
enzymes, FAD is a prosthetic group,
that is permanently part
of the complex.
FAD accepts and donates 2 electrons with 2
protons (2 H):
FAD + 2 e- + 2 H+ �� FADH2
Organic arsenicals are potent
inhibitors of lipoamide-containing enzymes such as Pyruvate Dehydrogenase.
These highly toxic compounds react with "vicinal" dithiols such as the
functional group of lipoamide as shown below.
In
the overall reaction, the acetic acid generated is transferred to coenzyme A.
The final electron acceptor is NAD+.
The keto carbon of pyruvate reacts with the
carbanion of TPP on E1 to yield an addition compound. The electron-pulling
positively charged nitrogen of the thiazole ring promotes loss of CO2. What
remains is hydroxyethyl-TPP.
The hydroxyethyl carbanion on TPP of E1 reacts
with the disulfide of lipoamide on E2. What was the keto carbon of pyruvate is
oxidized to a carboxylic acid, as the disulfide of lipoamide is reduced to a
dithiol. The acetate formed by oxidation of the hydroxyethyl moiety is linked
to one of the thiols of the reduced lipoamide as a thioester (~).
The acetate is transferred from the thiol of
lipoamide to the thiol of coenzyme A, yielding acetyl CoA.
The reduced lipoamide swings over to the E3
active site. Dihydrolipoamide is reoxidized to the disulfide, as 2 e- + 2 H+
are transferred to a disulfide on E3 (disulfide interchange).
The dithiol on E3 is reoxidized as 2 e- + 2 H+
are transferred to FAD. The resulting FADH2 is reoxidized by electron transfer
to NAD+, to yield NADH + H+.
Acetyl CoA,
a product of the Pyruvate Dehydrogenase reaction, is a central compound in metabolism.
The "high energy" thioester linkage makes it an excellent donor of
the acetate moiety.
For
example, acetyl CoA functions as:
· input to the Krebs Cycle,
where the acetate moiety is further degraded to CO2.
· donor of acetate
for synthesis of fatty acids, ketone bodies,
and cholesterol.
The
first enzyme of the complex is PDH itself which oxidatively decarboxylates
pyruvate. During the course of the reaction the acetyl group derived from
decarboxylation of pyruvate is bound to TPP. The next reaction of the complex
is the transfer of the 2--carbon acetyl group from acetyl-TPP to lipoic acid,
the covalently bound coenzyme of lipoyl transacetylase. The transfer of the
acetyl group from acyl-lipoamide to CoA results in the formation of 2
sulfhydryl (SH) groups in lipoate requiring reoxidation to the disulfide (S-S)
form to regenerate lipoate as a competent acyl acceptor. The enzyme
dihydrolipoyl dehydrogenase, with FAD+ as a cofactor, catalyzes that
oxidation reaction. The final activity of the PDH complex is the transfer of
reducing equivalents from the FADH2 of dihydrolipoyl dehydrogenase
to NAD+. The fate of the NADH is oxidation via mitochondrial
electron transport, to produce 3 equivalents of ATP:
The
net result of the reactions of the PDH complex are:
Pyruvate + CoA + NAD+
------> CO2 + acetyl-CoA + NADH + H+
Regulation of the PDH Complex The
reactions of the PDH complex serves to interconnect
the metabolic pathways of glycolysis, gluconeogenesis and fatty acid synthesis
to the TCA cycle. As a consequence, the activity of the PDH complex is highly
regulated by a variety of allosteric effectors and by covalent modification.
The importance of the PDH complex to the maintenance of homeostasis is evident
from the fact that although diseases associated with deficiencies of the PDH
complex have been observed, affected individuals often do not survive to
maturity. Since the energy metabolism of highly aerobic tissues such as the
brain is dependent on normal conversion of pyruvate to acetyl-CoA, aerobic
tissues are most sensitive to deficiencies in components of the PDH complex.
Most genetic diseases associated with PDH complex deficiency are due to
mutations in PDH. The main pathologic result of such mutations is moderate to
severe cerebral lactic acidosis
and encephalopathies.
The
main regulatory features of the PDH complex are diagrammed below.
|
Factors regulating
the activity of pyruvate dehydrogenase, (PDH). PDH activity is regulated by
its' state of phosphorylation, being most active in the dephosphorylated
state. Phosphorylation of PDH is catalyzed by a specific PDH kinase. The
activity of the kinase is enhanced when cellular energy charge is high which
is reflected by an increase in the level of ATP, NADH and acetyl-CoA.
Conversely, an increase in pyruvate strongly inhibits PDH kinase. Additional
negative effectors of PDH kinase are ADP, NAD+ and CoASH, the
levels of which increase when energy levels fall. The regulation of PDH
phosphatase is not completely understood but it is known that Mg2+
and Ca2+ activate the enzyme. In adipose tissue insulin increases PDH activity and
in cardiac muscle PDH activity is increased by catecholamines. |
Two products of the complex,
NADH and acetyl-CoA, are negative allosteric effectors on PDH-a, the
non-phosphorylated, active form of PDH. These effectors reduce the affinity of the
enzyme for pyruvate, thus limiting the flow of carbon through the PDH complex.
In addition, NADH and acetyl-CoA are powerful positive effectors on PDH kinase,
the enzyme that inactivates PDH by converting it to the phosphorylated PDH-b
form. Since NADH and acetyl-CoA accumulate when the cell energy charge is high,
it is not surprising that high ATP levels also up-regulate PDH kinase activity,
reinforcing down-regulation of PDH activity in energy-rich cells. Note,
however, that pyruvate is a potent negative effector on PDH kinase, with the
result that when pyruvate levels rise, PDH-a will be favored even with high
levels of NADH and acetyl-CoA.
Concentrations of pyruvate
which maintain PDH in the active form (PDH-a) are sufficiently high so that, in
energy-rich cells, the allosterically down-regulated, high Km form
of PDH is nonetheless capable of converting pyruvate to acetyl-CoA. With large
amounts of pyruvate in cells having high energy charge and high NADH, pyruvate
carbon will be directed to the 2 main storage forms of carbon (glycogen via
gluconeogenesis and fat production via fatty acid synthesis) where acetyl-CoA
is the principal carbon donor.
Although the regulation of
PDH-b phosphatase is not well understood, it is quite likely regulated to maximize
pyruvate oxidation under energy-poor conditions and to minimize PDH activity
under energy-rich conditions.
Regulation of Pyruvate
Dehydrogenase complex.
Allosteric Regulation
Pyruvate dehydrogenase is a major regulatory point
for entry of materials into the citric acid cycle.. The enzyme is regulated allosterically
and by covalent modification.
E2 - inhibited by acetyl-CoA, activated by CoA-SH
E3 - inhibited by NADH, activated by NAD+.
ATP is an allosteric inhibitor of the complex,
and AMP is an activator. The activity of this key reaction is coordinated with
the energy charge, the [NAD+]/[NADH] ratio,
and the ratio of acetylated to free coenzyme A.
Covalent Regulation
Part of the pyruvate
dehydrogenase complex,
pyruvate dehydrogenase kinase, phosphorylates three specific E1 serine
residues, resulting in loss of activity of pyruvate dehydrogenase. NADH and
acetyl-CoA both activate the kinase. The serines are dephosphorylated by a
specific enzyme called pyruvate dehydrogenase phosphatase that hydrolyzes the
phosphates from the E1 subunit of the pyruvate
dehydgrogenase complex. This has the effect of activating the complex. The
phosphatase is activated by Ca2+and Mg2+. Because ATP and
ADP differ in their affinities for Mg2+, the concentration of free
Mg2+ reflects the
ATP/ADP ratio within the mitochondrion. Thus, pyruvate dehydrogenase responds
to ATP levels by being turned off when ATP is abundant and further energy
production is unneeded.
In mammalian tissues at rest, much less than half of the total pyruvate
dehydrogenase is in the active, nonphosphorylated form. The complex can be
turned on when low ATP levels signal a need to generate more ATP. The kinase
protein is an integral part of the pyruvate dehydrogenase complex, whereas the
phosphatase is but loosely bound.
At the Subcellular Level, Glycolysis
Occurs in the Cytosol & the Citric Acid
Cycle in the Mitochondria
Compartmentation
of pathways in separate subcellular compartments
or organelles permits integration and regulation of
metabolism. Not all pathways are of equal importance in all cells. Depicts the subcellular compartmentation
of metabolic pathways in a hepatic parenchymal
cell.
The central role of the mitochondrion
is immediately apparent, since it acts as the focus
of carbohydrate, lipid, and amino acid metabolism. It
contains the enzymes of the citric acid cycle, â-oxidation
of fatty acids, and ketogenesis, as well as the
respiratory chain and ATP synthase. Glycolysis,
the pentose phosphate pathway, and fatty acid
synthesis are all found in the cytosol. In gluconeogenesis, substrates
such as lactate and pyruvate, which are formed in
the cytosol, enter the mitochondrion to
yield oxaloacetate
before formation of glucose. The membranes
of the endoplasmic reticulum contain the
enzyme system for acylglycerol synthesis, and the
ribosomes are responsible for protein synthesis.
• The products of digestion provide
the tissues with the building blocks for the
biosynthesis of complex molecules and
also with the fuel to power the living processes.
• Nearly all products of digestion of
carbohydrate, fat, and protein are metabolized to a
common metabolite, acetyl-CoA, before final oxidation to
CO2 in the citric acid cycle.
• Acetyl-CoA
is also used as the precursor for biosynthesis of
long-chain fatty acids; steroids, including cholesterol;
and ketone bodies.
• Glucose provides carbon skeletons
for the glycerol moiety of fat and of several
nonessential amino acids.
• Water-soluble products of digestion
are transported directly to the liver via the hepatic
portal vein. The liver regulates the blood
concentrations of glucose and amino acids.
• Pathways are compartmentalized
within the cell. Glycolysis, glycogenesis, glycogenolysis,
the pentose phosphate pathway, and lipogenesis
occur in the cytosol.
The mitochondrion contains the
enzymes of the citric acid cycle, β-oxidation of
fatty acids, and of oxidative phosphorylation. The
endoplasmic reticulum also contains
the enzymes for many other processes,
including protein synthesis, glycerolipid
formation,
and drug metabolism.
• Metabolic pathways are regulated by
rapid mechanisms affecting the activity of existing
enzymes, eg, allosteric and covalent modification
(often in response
to
hormone action); and slow mechanisms affecting the
synthesis of enzymes.
Krebs Cycle
The Krebs cycle, also known as the tricarboxylic acid cycle (TCA), was first recognized in 1937 by
the man for whom it is named, German biochemist Hans Adolph Krebs.
Krebs was educated at the universities of
Göttingen, Freiburg,
Krebs is best known for his discovery of the Krebs
cycle (or tricarboxylic acid
cycle) in 1937. This is a continuation of the work of
Carl and Gerty Cori,
who had shown howcarbohydrates,
such as glycogen, are broken
down in the body to lactic acid;
Krebs completed the process by working out how the lactic acid is metabolized to carbon dioxideand
water. When he began this work little was known apart from the fact that the
process involved the consumption of oxygen, which could be increased, according
to AlbertSzent-Györgyi,
by the four-carbon compounds succinic acid, fumaric acid, malic acid,
and oxaloacetic acid.
Krebs himself showed in 1937 that the six-carbon citric acid is also
involved in the cycle.
By studying the process in pigeon breast muscle Krebs
was able to piece together the clues already collected into a coherent scheme. The
three-carbon lactic acid is first broken down to a two-carbon molecule
unfamiliar to Krebs; it was in fact later identified by Fritz Lipmann as coenzyme A. This
then combines with the four-carbon oxaloacetic acid to form the six-carbon citric acid. The
citric acid then undergoes a cycle of reactions to be converted to oxaloacetic
acid once more. During this cycle two molecules of carbondioxide are given up
and hydrogen atoms are released; the hydrogen is then oxidized in the electron
transport chain with the production of energy. Much of the detail of this aspect
of the cycle was later filled in by Lipmann, with whom Krebs shared the 1953
Nobel Prize for physiology or medicine.
Krebs fully appreciated the significance of the cycle, pointing out the
important fact that it is the common terminal pathway for the
chemical breakdown of all foodstuffs.
In 1932, with K. Henselheit, Krebs was responsible for the introduction
of another cycle. This was the urea cycle,
whereby amino acids (the constituents of proteins) eliminate their nitrogen in
the form of urea, which is excreted in urine.
This left the remainder of the amino acid to give up its potential energy and
participate in a variety of metabolic pathways.
Hans A. Krebs, the son of Georg Krebs, an
otolaryngologist, was born in Hildesheim,
In 1935 Krebs went to the
The Ornithine Cycle
To keep organs and tissues alive for biochemical
tests, they had been perfused with
physiological salines as a substitute for blood. The results were often unsatisfactory.
Early in his career Krebs devised the tissue-slice technique. The organ,
rapidly removed after the death of the test animal, was cut into thin slices
and kept in fresh saline forbiochemical testing.
He used this technique in his study of the synthesis of urea by the liver.
It was known that urea is produced in a liver
undergoing autolysis,
and in 1904 it was shown that the autolysis produces the amino acid arginine, which is
acted on catalytically by the enzyme arginase to produce urea.
In 1932 Krebs found that, when an amino acid is added to liver, ammonia is liberated
and is converted approximately quantitatively into urea. All the amino acids
tested gave this result except two. When ornithine was added, the urea
production was 10 times the expected amount, and arginine also gave an excess
yield of urea. He therefore suggested that ornithine reacted with
added ammonia and carbon dioxide to form
arginine. Under the action of arginase, the arginine was broken down to urea
and ornithine. If ammonia was omitted, there was no appreciable formation of
urea. Further, ornithine was not observed to disappear while, with added
ammonia, the synthesis of urea was in progress. Krebs therefore concluded that
the ornithine acted as a catalyst. Many other
substances were tested, but the only one that acted like ornithine was citrulline,
and he suggested that citrulline formed a stage midway between ornithine and
arginine. His ornithine cycle is still regarded as a sound explanation of the
synthesis of urea in the body.
The Citric
Acid Cycle
Krebs then turned to the intermediary oxidation of carbohydrates.
In 1935 Albert von Szent-Györgyi elucidated the sequence of oxidations of
the C4-dicarboxylic acids as follows:
succinic acid→fumaric
acid→malic acid→maoxaloacetic acid
He also showed that these reactions were at least in part
catalytic. This was later proved, but the manner of action remained unknown. In 1936 C. Martius and F. Knoop showed
that in biological material citrate yields
alphaketoglutarate on oxidation.
They further suggested that the intermediate products were cis -aconitic
acid, isocitric acid,
andoxalosuccinic acid.
It was already known that alpha-ketoglutarate forms succinate.
In 1937, when Krebs started his work, the following sequence of reactions was
therefore known:
citric acid→cis -aconitic acid→iso-citric acid→oxalosuccinic acid→alpha-ketoglutamic acid→succinic acid→fumaric acid→malic acid→oxaloacetic acid
Krebs and W. A. Johnson found that citrate was not only
rapidly broken down in muscle but was also readily formed provided that
oxaloacetate was added. The assumption was that some of the oxaloacetate was
broken down to pyruvate or acetate and that the
formation of citrate was due to a combination of the remaining oxaloacetate
with pyruvate or acetate. But pyruvate or acetate could be derived from
carbohydrate. In 1937 Krebs conceived the whole process as a cycle in which an undefined derivative of
pyruvate, resulting from the breakdown of carbohydrate, condensed with
oxaloacetate to form citric acid.
The citric acid then passed through the changes noted above untiloxaloacetic acid was
regenerated, and the cycle was repeated. The full cycle is therefore as
follows:
citric acid→cis
-aconitic acid→iso-citric acid→oxalosuccinic acid→alpha-ketoglutamic
acid→succinic acid→fumaric acid→malic acid→oxaloacetic
acid+pyruvic acid→citric acid
Since Krebs originally described this cycle, he and
others did further work on it. In 1950 Fritz Lipmann showed that the derivative
of pyruvic acid that combines
with oxaloacetate to form citrate is acetyl-coenzyme A and that this coenzyme is also active
at two other points in the cycle. It was shown that acetyl-coenzyme A, in
addition to its formation from carbohydrate, is also formed from fatty acids
and many amino acids. The Krebs cycle is therefore a most important concept of
biochemistry. Krebs shared with Lipmann the Nobel Prize in Physiology or
Medicine in 1953.
Among Krebs's other important contributions to
biochemistry were his studies of the synthesis of glutamine in brain
tissue under the influence of the enzyme glutaminase(1935),
the passage of ions across cell membranes (1950), and the effect of primitive intrinsic regulating
mechanisms in controlling the metabolism of metazoan
cells (1957).
Later Life
In 1967 Krebs, having reached
Krebs received many honors in addition to his Nobel
Prize. In 1947 he was elected a Fellow of the Royal Society, and he was awarded
its Royal (1954) and Copley (1961) Medals. He delivered its Croonian Lecture in
1963. He was a member of many foreign scientific societies, and he held
honorary doctorates from 14 universities. He received the Gold Medal of the
Royal Society of Medicine in 1965, and he was knighted in 1958.
Read more: http://www.answers.com/topic/hans-adolf-krebs#ixzz2QQAAV519
http://www.youtube.com/watch?v=A1DjTM1qnPM&feature=related
The Krebs cycle refers
to a complex series of chemical reactions that produce carbon dioxide and Adenosine triphosphate
(ATP), a compound rich in energy. The cycleoccurs by essentially linking two carbon coenzyme with
carbon compounds; the created compound then goes through a series of changes
that produce energy. This cycle occurs
in all cells that utilize oxygen as part of
their respiration process; this includes those cells of creatures from the
higher animal kingdom such as humans. Carbon dioxide is important for various
reasons, the main one being that it stimulates breathing, while ATP provides
cells with the energy required for the synthesis of proteins from amino acids
and the replication of deoxyribonucleic acid (DNA);
both are vital for energy supply and for life to continue. In short, the Krebs cycle constitutes the discovery of the major
source of energy in all living organisms.
Within the Krebs cycle,
energy in the form of ATP is usually derived from the breakdown of glucose,
although fats and proteins can also be utilized as energy sources. Since
glucose can pass through cell membranes, it transports energy from one part of
the body to another. The Krebs cycle affects
all types of life and is, as such, the metabolic pathway within the cells. This
pathway chemically converts carbohydrates, fats, and proteins into carbon
dioxide, and converts water into serviceable energy.
The Krebs cycle is
the second stage of aerobic respiration, the first being glycolysis and last
being the electron transport
chain; the cycle is
a series of stages that every living cell must undergo in order to produce
energy. The enzymes that cause
each step of the process to occur are all located in the cell's "power
plant"; in animals, this power plant is the mitochondria; in plants, it is
the chloroplasts; and in microorganisms, it can be found in the cell membrane.
The Krebs cycle is also known as the citric acid cycle, because
citric acid is the very first product generated by this sequence of chemical
conversions, and it is also regenerated at the end of the cycle.
The pyruvate molecules produced during glycolysis
contains a lot
of energy in the bonds between their molecules. In order to use that energy,
the cell must convert it into the form of ATP. To do so, pyruvate molecules are
processed through the Kreb Cycle, also known as the citric acid cycle.
http://www.youtube.com/watch?v=7gR4s8ool1Y
(Kerbs Cycle as a drawing)
1. Prior to entering the Krebs Cycle,
pyruvate must be converted into acetyl CoA. This is achieved by removing a CO2
molecule from pyruvate and then removing an electron to reduce an NAD+ into
NADH. An enzyme called coenzyme A is combined with the remaini ow:
2. Citrate is formed when the acetyl
group from acetyl CoA combines with oxaloacetate from the previous Krebs cycle.
3. Citrate is converted into its
isomer isocitrate.
4. Isocitrate is oxidized to form the
5-carbon α-ketoglutarate. This step releases one molecule of CO2 and
reduces NAD+ to NADH2+.
5. The α-ketoglutarate is
oxidized to succinyl CoA, yielding CO2 and NADH2+.
The a-Ketoglutarate Dehydrogenase Complex
is
Similar to pyruvate dehydrogenase
complex
Same coenzymes, identical mechanisms
E1 - a-ketoglutarate dehydrogenase
(with TPP)
E2 – dihydrolipoyl
succinyltransferase (with flexible lipoamide prosthetic group)
E3 - dihydrolipoyl dehydrogenase
(with FAD)
6. Succinyl CoA releases coenzyme A
and phosphorylates ADP into ATP.
In the succinyl CoA synthetase
reaction, the thioester bond between HS-CoA and the succinyl group is
hydrolyzed.
Since it is a rich in energy
bond, the energy released is enough for synthesizing GTP from GDP + (P).
This GTP is equivalent, from
the energetic point of view, to ATP. In fact, GTP can transfer the (P) group to
ADP to form ATP:
GTP + ADP ————–à GDP +
ATP
Since ATP can be produced from
this reaction, without participation of the respiratory chain, this process is
called Substrate Level Phosphorylation (SLP) in contrast to the Oxidative
Phosphorylation (ATP synthesis using the energy released in the Electron
Transport Chain).
A few other reactions in
metabolism are also coupled with ATP synthesis without participation of the
respiratory chain. They are considered also SLP reactions.
7. Succinate is oxidized to fumarate,
converting FAD to FADH2.
The Succinate Dehydrogenase Complex
of several polypeptides, an FAD prosthetic group and iron-sulfur clusters,
embedded in the inner mitochondrial membrane. Electrons are transferred from
succinate to FAD and then to ubiquinone (Q) in electron transport chain.
Dehydrogenation is stereospecific; only the trans isomer
is formed
8. Fumarate is hydrolized to form
malate.
9. Malate is oxidized to
oxaloacetate, reducing NAD+ to NADH2+.
We are now back at the
beginning of the Krebs Cycle. Because glycolysis produces two pyruvate
molecules from one glucose, each glucose is processes
through the kreb cycle twice. For each molecule of glucose,
six NADH2+, two FADH2, and two ATP.
The sum of all reactions in the citric
acid cycle is:
Acetyl-CoA + 3 NAD+ + FAD + GDP + Pi + 2 H2O → CoA-SH + 3
NADH + 3 H+ + FADH2 + GTP + 2 CO2
(the above
reaction is equilibrated if Pi represents
the H2PO4- ion,
GDP the GDP2- ion and
GTP the GTP3- ion).
Two carbons are oxidized to
CO2, and the energy from these reactions is stored in GTP,
NADH and FADH2. NADH and FADH2 are coenzymes (molecules
that enable or enhance enzymes) that store energy and are utilized in oxidative
phosphorylation.
· Electrons are also
transferred to the electron acceptor FAD, forming FADH2.
· At the end of
all cycles, the products are two GTP, six NADH, two FADH2, four CO2.
http://www.youtube.com/watch?v=hw5nWB0xN0Y&feature=related
INVESTIGATION OF BIOLOGICAL
OXIDATION, OXIDATIVE
PHOSPHORYLATION AND ATP SYNTHESIS. INHIBITORS
AND UNCOUPLERS OF OXIDATIVE PHOSPHORYLATION.
Bioenergetic
Bioenergetics is the part of biochemistry
concerned with the energy involved in making and breaking of chemical bonds in
the molecules
found in biological organisms.
Growth, development and metabolism
are some of the central phenomena in the study of biological organisms. The
role of energy is fundamental to such biological processes. The ability to harness
energy from a variety of metabolic pathways is a property of all living
organisms. Life
is dependent on energy transformations; living organisms
survive because of exchange of energy within and without.
In a living organism, chemical
bonds are broken and made as part of the exchange and transformation
of energy. Energy is available for work (such as mechanical work) or for other
processes (such as chemical synthesis and anabolic
processes in growth), when weak bonds are broken and stronger bonds are made.
The production of stronger bonds allows release of usable energy.
Living organisms obtain energy from organic
and inorganic materials. For example, lithotrophs
can oxidize minerals such as nitrates or forms of sulfur, such as
elemental sulfur, sulfites,
and hydrogen sulfide to produce ATP. In photosynthesis,
autotrophs
can produce ATP using light energy. Heterotrophs
must consume organic compounds. These are mostly carbohydrates,
fats, and proteins.
The amount of energy actually obtained by the organism is lower than the amount
present in the food; there are losses in digestion, metabolism, and thermogenesis.
The materials are generally combined with oxygen to
release energy, although some can also be oxidized anaerobically by various
organisms. The bonds holding the molecules of nutrients
together and the bonds holding molecules of free oxygen together are all
relatively weak compared with the chemical bonds holding carbon dioxide and
water together. The utilization of these materials is a form of slow combustion.
That is why the energy content of food can be estimated with a bomb calorimeter.
The materials are oxidized slowly enough that the organisms do not actually
produce fire. The oxidation releases energy because stronger bonds have been
formed. This net energy may evolve as heat, or some of which may be used by the
organism for other purposes, such as breaking other bonds to do chemistry.
Living organisms produce ATP from energy sources via oxidative phosphorylation. The terminal
phosphate bonds of ATP are relatively weak compared with the stronger bonds
formed when ATP is broken down to adenosine monophosphate and phosphate,
dissolved in water. Here it is the energy of hydration that results in energy
release. This hydrolysis of ATP is used as a battery to store energy in cells,
for intermediate metabolism. Utilization of chemical energy from such molecular
bond rearrangement powers biological processes in every biological organism.
Exergonic
and endergonic processes. Mechanism of energy releasing and storage in the organism.
Exergonic refers to chemical reactions that
proceed spontaneously from reactants to products with the release of energy.
Endergonic reactions require energy input to proceed. Although the terms are
often used rather loosely, they are precisely defined thermodynamic concepts
based on changes in an entity called Gibbs free energy (G) accompanying
reactions. Reactions in which -G decreases are exergonic, and those in which -G
increases are endergonic. Exergonic reactions often involve the breakdown of
organic compounds found in food, whereas endergonic reactions frequently entail
synthesis of complicated molecules.
Biological metabolism
contains many examples of both types, and living organisms have developed
elaborate techniques for coupling the two.
Exergonic reactions
release free energy while endergonic reactions consume free energy
Although a negative -G
indicates that energy must be added to the system before a reaction will occur,
it tells us nothing about the rate at which it will progress. As is often the
case, it may go very slowly if substantial activation energy is required to
start the reaction. Living organisms have found a way around this problem by
forming protein catalysts, called enzymes, that effectively reduce the amount
of activation energy needed, and allow the reaction to proceed at a
satisfactory rate. Enzymes do not affect the free energy of the reaction, and
will not enable reactions to proceed that are not energetically feasible.
By coupling exergonic and endergonic reactions, organisms are
able to use the available energy in food they consume to construct complex
proteins, lipids, nucleic acids and carbohydrates needed for their growth and
development. A well-known example involves coupling the formation of
energy-rich adenosine triphosphate (ATP) from adenosine diphosphate (ADP) and
phosphate (an endergonic reaction), with the transfer of hydrogen, removed from
organic food materials, to oxygen (an exergonic reaction). The process is
called oxidative phosphorylation. Energy stored in ATP may be used subsequently
when the exergonic conversion of ATP back to ADP and phosphate is coupled with
the endergonic synthesis of a needed cellular component.
What is macroergic bond? Examples of high
energy compounds.
A bond in chemical compounds,
which acts as an energy accumulator. Macroergic bond
is present in some phosphorus-containing
compounds in living organisms, e. g.* adenosinetriphosphate.
Macroergic bonds
are formed as a result of complex biochemical processes and break up when
energy is released. The processes are reversible and can be repeated.
High-energy
phosphate can mean one of two things:
·
The
phosphate-phosphate bonds formed when compounds such as adenosine diphosphate
and adenosine triphosphate are created.
·
The compounds
that contain these bonds, which include the nucleoside
diphosphates and nucleoside triphosphates, and the high-energy storage
compounds of the muscle, the phosphagens. When people speak of a high-energy phosphate
pool, they speak of the total concentration of these compounds with these
high-energy bonds.
·
High-energy phosphate bonds are pyrophosphate
bonds, acid anhydride
linkages formed by taking phosphoric acid derivatives and dehydrating
them. As a consequence, the hydrolysis of these bonds is exergonic under physiological conditions,
releasing energy.
Energy released by high energy
phosphate reactions |
|
Reaction |
ΔG [kJ/mol] |
ATP + H2O → ADP +
Pi |
-30.5 |
ADP + H2O → AMP + Pi |
-30.5 |
ATP + H2O → AMP +
PPi |
-40.6 |
PPi + H2O →
2 Pi |
-31.8 |
AMP + H2O → A + Pi |
-12.6 |
Except for PPi → 2 Pi, these
reactions are, in general, not allowed to go uncontrolled in the human cell but
are instead coupled to other processes needing energy to drive them to
completion. Thus, high-energy phosphate reactions can:
·
provide energy to cellular processes,
allowing them to run;
·
couple processes to a particular
nucleoside, allowing for regulatory control of the process;
·
drive the
reaction to the right, by taking a reversible process and making it
irreversible.
The one exception is of value because it allows a single
hydrolysis, ATP + 2H2O → AMP + PPi, to effectively
supply the energy of hydrolysis of two high-energy bonds, with the hydrolysis
of PPi being allowed to go to completion in a separate reaction. The
AMP is regenerated to ATP in two steps, with the equilibrium reaction ATP + AMP
↔ 2ADP, followed by regeneration of ATP by the usual means, oxidative phosphorylation or other
energy-producing pathways such as glycolysis.
Often, high-energy phosphate bonds are
denoted by the character '~'. In this "squiggle" notation, ATP
becomes A-P~P~P. The squiggle notation was invented by Fritz Albert Lipmann, who first
proposed ATP as the main energy transfer molecule of the cell, in 1941. It
emphasizes the special nature of these bonds.
Stryer states:
ATP is often called a high energy
compound and its phosphoanhydride bonds are referred to as high-energy bonds.
There is nothing special about the bonds themselves. They are high-energy
bonds in the sense that free energy is released when they are hydrolyzed,
for the reasons given above.
Lipmann’s term “high-energy bond” and his symbol ~P
(squiggle P) for a compound having a high phosphate group transfer potential
are vivid, concise, and useful notations. In fact Lipmann’s squiggle did much
to stimulate interest in bioenergetics.
The term 'high energy' with respect to
these bonds can be misleading because the negative free energy change is not due
directly to the breaking of the bonds themselves. The breaking of these bonds,
as with the breaking of most bonds, is an endergonic step (i.e., it absorbs
energy, not releases it). The negative free energy change comes instead from
the fact that the bonds formed after hydrolysis-or the phosphorylation of a
residue by ATP-are lower in energy than the bonds present before hydrolysis
(this includes all of the bonds involved in the reaction, not just the
phosphate bonds themselves). This effect is due to a number of factors
including increased resonance stabilization and solvation
of the products relative to the reactants.
Besides the adenosine nucleotide phosphates,
uracil, cytosine and guanine phosphates occur, too:
UMP, UDP, UTP, CMP, CDP, CTP,
GMP, GDP, GTP.
The triphosphate nucleosides of these compounds and those of ATP are
components of RNA. They are integrated into the polymer by splitting off
pyrophosphate ( = PP). The corresponding desoxyribose
derivatives (dATP, dGTP, dCTP....) are necessary for DNA synthesis, where dTTP
is used instead of dUTP. The terminal phosphate residues of all nucleoside di-
and triphosphates are equally rich in energy. The energy set free by their
hydrolysis is used for biosyntheses. They share the work equally: UTP is needed
for the synthesis of polysaccharides, CTP for that of lipids and GTP for the
synthesis of proteins and other molecules. These specificities are the results
of the different selectivities of the enzymes, that
control each of these metabolic pathways.
ATP formation
The Function of ATP
The ATP is used for many cell
functions including transport work moving substances across cell
membranes. It is also used for mechanical work, supplying the energy
needed for muscle contraction. It supplies energy not only to heart muscle (for
blood circulation) and skeletal muscle (such as for gross body movement), but
also to the chromosomes and flagella to enable them to carry out their many
functions. A major role of ATP is in chemical work, supplying the needed
energy to synthesize the multi-thousands of types of macromolecules that the
cell needs to exist.
ATP is also used as an on-off switch both to control
chemical reactions and to send messages. The shape of the protein chains that
produce the building blocks and other structures used in life is mostly
determined by weak chemical bonds that are easily broken and remade. These
chains can shorten, lengthen, and change shape in response to the input or
withdrawal of energy. The changes in the chains alter the shape of the protein
and can also alter its function or cause it to become either active or inactive.
The ATP molecule can bond to
one part of a protein molecule, causing another part of the same molecule to
slide or move slightly which causes it to change its conformation, inactivating
the molecule. Subsequent removal of ATP causes the protein to return to its
original shape, and thus it is again functional. The cycle can be repeated
until the molecule is recycled, effectively serving as an on and off switch.
Both adding a phosphorus (phosphorylation) and removing a phosphorus from a
protein (dephosphorylation) can serve as either an on or an off switch.
How is ATP Produced?
ATP is manufactured as a result of
several cell processes including fermentation, respiration and photosynthesis.
Most commonly the cells use ADP as a precursor molecule and then add a phosphorus to it. In eukaryotes this can occur either in
the soluble portion of the cytoplasm (cytosol) or in special energy-producing
structures called mitochondria. Charging ADP to form ATP in the mitochondria is
called chemiosmotic phosphorylation. This process occurs in specially
constructed chambers located in the mitochondrion’s inner membranes.
ATP-synthase
converts ADP into ATP, a process called charging.
Adenosine triphosphate (ATP) is an
organic molecule which stores energy used to carry out life processes. ATP
is made of an adenine nucleoside, ribose sugar, and three phosphate groups. The
high energy bonds between phosphate groups are broken when hydrolyzed, thus
releasing energy in the system. Either one or two phosphate groups can break
off, releasing Gibb's free energy, which
can then be used to drive other reactions.
The molecular structure of ATP which is formed from
a adenine nucleoside, ribose sugar, and three
phosphate groups
ATP
can be formed from bonding either adenosine monophosphate (AMP) and two
inorganic phosphate groups (PPi) together or by bonding adenosine
diphosphate (ADP) and one inorganic phosphate group (Pi) together.
Energy is required to bond the adenosine to the phosphate groups, making it an
endergonic reaction. The energy used to bond the two molecules together is then
stored within covalent bonds between phosphate groups in ATP. ATP
can be formed through two different endergonic processes, either through substrate-level
phosphorylation or chemiosmosis.
ATP is needed
- as a source of energy for biochemical syntheses
- for
transport processes (active transport) and
- for
mechanical work like movements (ciliar movements, plasma currents etc.)
How
the Hydrolysis of ATP Performs Work
•The
bonds between the phosphate groups of ATP’s tail can be broken by hydrolysis
•Energy
is released from ATP when the terminal phosphate bond is broken
•This
release of energy comes from the chemical change to a state of lower free
energy, not from the phosphate bonds themselves
•The
three types of cellular work (mechanical, transport, and chemical) are powered
by the hydrolysis of ATP
•In
the cell, the energy from the exergonic reaction of ATP hydrolysis can be used
to drive an endergonic reaction
•Overall,
the coupled reactions are exergonic
ATP + H2O →
ADP + Pi
Releases -30.5
kJ/mol= ΔG˚
(when one phosphate group breaks off)
ATP + H2O →
AMP + PPi
Releases -45.6 kJ/mol= ΔG˚ (when two phosphate
groups break off)
Hydrolysis of ATP
The Regeneration of ATP
•ATP
is a renewable resource that is regenerated by addition of a phosphate group to
adenosine diphosphate (ADP)
•The
energy to phosphorylate ADP comes from catabolic reactions in the cell
•The
ATP cycle is a revolving door through which energy passes during its transfer
from catabolic to anabolic pathways
http://www.youtube.com/watch?v=_PgjsfY71AM&feature=related
http://www.youtube.com/watch?v=YndC0gS3t6M&feature=related
Substrate-level and oxidative phosphorylation
Substrate-level phosphorylation
is a type of metabolic reaction that results in the formation of adenosine
triphosphate (ATP) or guanosine
triphosphate (GTP) by the direct transfer and
donation of a phosphoryl (PO3)
group to adenosine
diphosphate (ADP) or guanosine
diphosphate (GDP) from a phosphorylated reactive intermediate.
Note that the phosphate group does not have to come directly from the
substrate. By convention, the phosphoryl group that is transferred is referred
to as a phosphate group.
An alternative way to create ATP is
through oxidative
phosphorylation, which takes place during the
process of cellular
respiration, in addition to the substrate-level
phosphorylation that occurs during glycolysis and the Krebs cycle. During
oxidative phosphorylation, NADH is oxidized to NAD+, yielding 2.5
ATPs, and FADH2 yields 1.5 ATPs when it is oxidized. Oxidative phosphorylation
uses an electrochemical or chemiosmotic gradient of protons (H+)
across the inner mitochondrial membrane to generate ATP from ADP, which is a
key difference from substrate-level phosphorylation.
Unlike oxidative phosphorylation, oxidation
and phosphorylation
are not coupled in the process of substrate-level phosphorylation, although
both types of phosphorylation result in the formation of ATP and reactive
intermediates are most often gained in course of oxidation
processes in catabolism.
However, usually most of the ATP is generated by oxidative phosphorylation in
aerobic or anaerobic respiration. Substrate-level phosphorylation serves as
fast source of ATP independent of external electron acceptors and respiration.
This is the case for example in human erythrocytes, which have no mitochondria, and in
the muscle during oxygen depression.
oxidative phosphorylation
The main part of substrate-level
phosphorylation occurs in the cytoplasm of cells as part of glycolysis
and in mitochondria as part of the Krebs Cycle under both aerobic
and anaerobic
conditions. In the pay-off phase of glycolysis, two ATP are
produced by substrate-level phosphorylation: two and only two 1,3-bisphosphoglycerate
are converted to 3-phosphoglycerate by transferring a
phosphate group to ADP by a kinase; two phosphoenolpyruvate are converted to pyruvate
by the transfer of their phosphate groups to ADP by another kinase. The first
reaction occurs after the generation of 1,3-bisphosphoglycerate
from 3-phosphoglyceraldehyde and an organic phosphate via a dehydrogenase.
ATP is generated in a following separate step (key
difference from oxidative phosphorylation) by transfer of the high-energy
phosphate on 1,3-bisphosphoglycerate to ADP via the
enzyme phosphoglycerate kinase,
generating 3-phosphoglycerate. As ATP is formed of a former inorganic phosphate
group, this step leads to the energy yield of glycolysis. The second
substrate-level phosphorylation occurs later by means of the reaction of
phosphenolpyruvate (PEP) to pyruvate via the pyruvate kinase.
This reaction regenerates the ATP that has been used in the preparatory phase
of glycolysis to activate glucose to glucose-6-phosphate and
fructose-6-phosphate to fructose-1,6-bisphosphate,
respectively.
ATP
can be generated by substrate-level phosphorylation in the mitochondrial
matrix, a pathway that
is independent from the protonmotive force, pmf. In the mitochondrial matrix
there are two reactions capable of substrate-level phosphorylation: the
mitochondrial phosphoenolpyruvate carboxykinase (PEPCK), and the succinate-CoA
ligase (SUCL or succinate thiokinase or succinyl-CoA synthetase). Mitochondrial
PEPCK is thought to participate in the transfer of the phosphorylation
potential from the matrix to cytosol and vice versa. The enzyme is a
heterodimer, being composed of an invariant alpha subunit encoded by SUCLG1,
and a substrate-specific beta subunit, encoded by either SUCLA2 or SUCLG2. This
dimer combination results in either an ADP-forming succinate-CoA ligase
(A-SUCL, EC 6.2.1.5) or a GDP-forming succinate-CoA ligase (G-SUCL, EC
6.2.1.4). The ADP-forming succinate-CoA ligase is potentially the only matrix
enzyme generating ATP in the absence of a pmf, capable of maintaining matrix
ATP levels under energy-limited conditions, such as transient hypoxia.
Another
form of substrate-level phosphorylation is also seen in working skeletal
muscles and the brain. Phosphocreatine
is stored as a readily available high-energy phosphate supply, and the enzyme creatine
phosphokinase transfers a phosphate from
phosphocreatine to ADP to produce ATP. Then the ATP releases giving chemical
energy.
Apart
from this substrate-level phosphorylation can also be observed in fermentation,
for example, heterolactic
fermentation, butyric acid fermentation, and
propanoic acid fermentation.
The modern views on the biological
oxidation
Biological oxidation is that oxidation which occurs in
biological systems to produce energy.
Oxidation can occur by:
1-Addition of oxygen (less common)
2-Removal of hydrogen (common)
3-Removal of electrons (most common)
Electrons are
not stable in the free state, so their removal form a substance
(oxidation) must be accompanied by their acceptance by another substance
(reduction) hence the reaction is called oxidation-reduction reaction or redox
reaction and the involved enzymes are called oxido-reductases
Redoxpotential
It is the affinity of a substance to accept electrons i.e. it is
the potential for a substance to become reduced. Hydrogen has the lowest
redoxpotential (-0.42 volt), while oxygen has the highest redoxpotential (+0.82
volt). The redoxpotentials of all other substances lie between that of hydrogen
and oxygen.
Electrons
are transferred from substances with low redoxpotential to substances with
higher redoxpotential.This transfer of electrons is an energy yielding process
and the amount of energy liberated depends on the redoxpotential difference between
the electron donor and acceptor.
Oxido-reductases
These enzymes catalyze oxidation-reduction
reactions.
They are classified into five groups:
1-oxidases. 2-aerobic dehydrogenises.
3-anaerobic dehydrogenises.
4-hydroperoxidasesand
5-oxygenases.
1. Oxidases
An oxidase
is any enzyme
that catalyzes
an oxidation-reduction
reaction involving molecular oxygen (O2) as the electron acceptor. In these
reactions, oxygen is reduced to water (H2O) or hydrogen
peroxide (H2O2). The oxidases are a subclass
of the oxidoreductases.
OXIDATION-REDUCTION OR REDOX
REACTIONS
2.
Aerobic Dehydrogenases(FlavoproteinLinked Oxidases).
The coenzyme of aerobic dehydrogenasesmay be:
•FMN (Flavinadenine mononucleotide) as in L-amino acid oxidase.
•FAD (Flavinadenine dinucleotide) as in D-amino acid oxidase,
xanthineoxidase, aldehydedehydrogenaseand glucose oxidase.
3. Anaerobic
Dehydrogenases
Anaerobic dehydrogenasesare further classified according to their
coenzymes into:
•NAD+linked
anaerobic dehydrogenasese.g.
a)Cytoplasmicglycerol-3-phosphate dehydrogenase
b)Isocitratedehydrogenase.
c)Malatedehydrogenase.
d)β-HydroxyacylCoAdehydrogenase.
e)β-Hydroxybutyrate dehydrogenase.
•NADP linked anaerobic dehydrogenasese.g.
a)Glucose-6-phosphate dehydrogenase.
b)Malicenzyme.c)Cytoplasmicisocitratedehydrogenase
•FAD linked anaerobic dehydrogenasese.g.
a)Succinatedehydrogenase.
b)Mitochondrial glycerol-3-phosphate
dehydrogenase.
c)Acy1 CoAdehydrogenase.
4. Hydroperoxidases
These enzymes use hydrogen peroxide (H2O2) as
substrate changing it into water to get rid of its harmful effects.
They are further classified into peroxidasesand
catalases.
•Peroxidases: These enzymes need a reduced substrate as
hydrogen donorperoxidase
H2O2 + XH2 (reduced substrate) ----------→ X(oxidized
substrate)+ 2H2O Example:
-Glutathione peroxidasegets rid of H2O2from red cells to protect
them from haemolysis
Glutathione Peroxidase
H2O2 + 2 G-S H -----------→2H2O + G-S-S-G
•Catalases:These enzymes act on 2 molecules of hydrogen
peroxide; one molecule is hydrogen donor & the other molecule is
hydrogenaccepetor.
2H2O2 + catalase---------→ 2H2O + O2
Hydrogen peroxide is continuously produced by the action of aerobic
dehydrogenasesand some oxidases. It is also produced by action of superoxide
dismutase on superoxide (O•2). It is removed by the action of peroxidasesand
catalasesto protect cells against its harmful effects.
5. Oxygenases
These enzymes catalyze direct incorporation
(addition) of oxygen into substrate.
They are further classified into dioxygenasesand
monooxygenases.
A. Dioxygenases(true oxgenases)
These enzymes catalyze direct incorporation of two atoms of oxygen
molecule into substrate e.g. tryptophan pyrrolase, homnogentisicacid
dioxygenase, carotenaseand β-hydroxyanthranilicacid dioxygenase.
Dioxygenase
A
+ O2 →
AO2
B. Mono-oxygenases(pseudo-oxygenases; hydroxylases; mixed:
function oxygenases)
AH
+ O2+ XH2 ------→A-OH + H2O + X
Fuctionsof cytochromeP450
Functions of microsomalcytochromeP450
1-It is important for detoxicationof xenobioticsby hydroxylation.
e.g. insecticides,carcinogens,mutagensand drugs.
2-It is also important for metabolism of some drugs by hydroxylation
e.g. morphine, aminopyrine, benzpyrineand aniline.
drug-H + O2+ XH2 drug-OH + H2O + X→ drug-OH + H2O + X
Function of mitochondrial cytochromeP450
1-It has a role in biosynthesis of steroid hormones from
cholesterol in adrenal cortex, testis, ovary and placenta by hydroxylation
2-It has a role in biosynthesis of bile acids from cholesterol in
the liver by hydroxylation at C26 by 26 hydroxylase.
3-It is important for activation of vitamin D
CytochromeP450
It is a group of hydroxylaseswhich are collectively referred to as
cytochromeP450.
They are so called because their reduced forms exhibit an intense
absorption band at wavelength 450 nm when complexedto carbon monoxide.
They are conjugated protein containing haeme(haemoproteins).
According to their intracellular localization they may be
classified into:
•MicrosomalcytochromeP450.
It is present mainly in the microsomes of liver cells. It
represents about 14% of the microsomalfraction of liver cells.
•Mitochondrial cytochromeP450.
It is present in mitochondria of many tissues but it is
particularly abundant in liver and steroidogenictissues as adrenal cortex,
testis, ovary, placenta and kidney.
Tissue
Respiration.
Tissue respiration is the release of
energy, usually from glucose, in the tissues of all animals, green plants,
fungi and bacteria. All these living things require energy for other processes
such as growth, movement, sensitivity, and reproduction.
The most efficient form of
respiration is aerobic respiration: this requires oxygen. When oxygen is not
available, some organisms can respire anaerobically i.e. without air or oxygen.
Yeast can respire in both ways. Yeast gets more energy from aerobic
respiration, but when it runs out of oxygen it does not die. It can continue to
respire anaerobically, but it does not get so much energy from the sugar. Yeast
produces ethanol (alcohol) when it respires anaerobically and ultimately the
ethanol will kill the yeast.
We can respire in both ways too. Normally we use
oxygen, but when we are running in a race, we may not get enough oxygen into
our blood, so our muscles start to respire anaerobically. Unlike yeast we
produce lactic acid. Of course if we produced alcohol in our muscles it would
make us drunk! Fine thing if you are running away from a predator and you end
up drunk! Making lactic acid is not much better. Lactic acid causes cramp.
Glucose
+ Oxygen = Carbon Dioxide + Water + Energy
This word
equation means: “sugar and oxygen are turned into carbon dioxide and water
releasing energy”. You must memorise the word equation (and the balanced
chemical equation if you want a grade A, B or C). Get help memorising the
equations
Glucose
= Carbon Dioxide + Ethanol + Energy
This word
equation means: “glucose is turned into carbon dioxide and ethanol releasing energy”. You must
memorise this word equation.
Composition of respiratory chain
NADH
dehydrogenase (EC 1.6.5.3) (also
referred to as "NADH:quinone reductase" or "Complex I") is
an enzyme located in the inner mitochondrial membrane
that catalyzes the transfer of electrons
from NADH to coenzyme Q (CoQ). It is
the "entry enzyme" of oxidative
phosphorylation in the mitochondria.
NADH
Dehydrogenase is the first enzyme (Complex I) of the mitochondrial electron
transport chain. There are three energy-transducing
enzymes in the electron transport chain - NADH dehydrogenase (Complex I), Coenzyme Q –
cytochrome c reductase (Complex III), and cytochrome c
oxidase (Complex IV). NADH dehydrogenase is the largest
and most complicated enzyme of the electron transport chain..
The reaction of NADH dehydrogenase is:
In
this process, the complex translocates four protons across the
inner membrane per molecule of oxidized NADH,
helping to build the electrochemical
potential used to produce ATP.
The
reaction can be reversed - referred to as aerobic succinate-supported NAD+
reduction - in the presence of a high membrane potential, but the exact
catalytic mechanism remains unknown.
Complex
I may have a role in triggering apoptosis. [5] In fact,
there has been shown to be a correlation between mitochondrial activities and programmed
cell death (PCD) during somatic embryo
development.
All redox
reactions take place in the extramembranous portion of NADH dehydrogenase. NADH
initially binds to NADH dehydrogenase, and transfers two
electrons to the flavin
mononucleotide (FMN) prosthetic group of complex I,
creating FMNH2. The electron acceptor - the isoalloxazine ring - of
FMN is identical to that of FAD. The
electrons are then transferred through the second prosthetic group of NADH
dehydrogenase via a series of iron-sulfur (Fe-S) clusters, and finally to coenzyme Q
(ubiquinone). This electron flow causes four hydrogen ions to be pumped out of
the mitochondrial matrix. Ubiquinone
(CoQ) accepts two electrons to be reduced to ubiquionol (CoQH2). [2]
FADH
dehydrogenase
In biochemistry, flavin
adenine dinucleotide (FAD) is a redox cofactor
involved in several important reactions in metabolism. FAD can
exist in two different redox states, which it converts between by accepting or
donating electrons. The molecule consists of a riboflavin moiety
(vitamin B2) bound to the phosphate group of an ADP
molecule. The flavin group is bound to ribitol,
a sugar alcohol, by a carbon-nitrogen bond, not a glycosidic bond. Thus,
riboflavin is not technically a nucleotide; the name flavin adenine
dinucleotide is a misnomer.[1]
FAD can be reduced to FADH2, whereby it
accepts two hydrogen atoms (a net gain of two electrons):
FAD (fully
oxidized form, or quinone
form) accepts two electrons and two protons to become FADH2
(hydroquinone form). FADH2 can then be oxidized to the semireduced
form (semiquinone) FADH by donating one electron and one proton. The
semiquinone is then oxidized once more by losing an electron and a proton and
is returned to the initial quinone form (FAD).
FAD is an aromatic ring system,
whereas FADH2 is not. This means that FADH2 is
significantly higher in energy, without the stabilization that aromatic
structure provides. FADH2 is an energy-carrying molecule, because,
if it is oxidized, it will regain aromaticity and release all the energy
represented by this stabilization.
The
primary biochemical role of FADH2 in eukaryotes is to carry
high-energy electrons used for oxidative
phosphorylation. Its hydrogens remain in the
mitochondrial matrix, whilst FAD is tightly bound to a dehydrogenase enzyme
i.e. the second protein complex in the oxidative phosphorylation chain. FAD is
a prosthetic group in the
enzyme complex succinate
dehydrogenase (complex II) that oxidizes succinate to fumarate in the
eighth step of the citric acid cycle.
The high-energy electrons from this oxidation are stored momentarily by
reducing FAD to FADH2. FADH2 then reverts to FAD, sending
its two high-energy electrons through the electron
transport chain; the energy in FADH2 is
enough to produce 1.5 equivalents of ATP[2] by oxidative
phosphorylation. Another metabolic source of FADH2 is beta oxidation, where FAD
serves as a coenzyme to acyl CoA
dehydrogenase. A flavoprotein is a protein
that contains a flavin moiety, this may be in the form of FAD or FMN (Flavin
mononucleotide) .
There are many flavoproteins besides components of the succinate dehydrogenase
complex, including α-ketoglutarate dehydrogenase
and a component of the pyruvate dehydrogenase
complex.
Ubiquinones
These molecules are also known as coenzyme Q or mitoquinones. They are
involved in electron transport in mitochondrial preparations playing an
essential role in the oxidation of succinate or NADH via the cytochrome system.
They serves not only as a coenzyme but also, in their
reduced forms, as antioxidants. They are synthesized de novo in all animal
tissues and cannot thus be regarded as vitamins. Ubiquinones are present in all
aerobic organisms, plants, animals (the name ubiquinone was proposed with
reference to their ubiquitous occurrence) and bacteria, but are absent from
Gram-positive eubacteria and the archaebacteria. They were discovered by the Morton's group
in animal fat but their quinonoid structure was revealed by Crane two years
later in extracts from beef heart mitochondria.
The
compound had a 2,3-dimethoxy-5-methylbenzoquinone
nucleus and a side chain of 10 isoprenoid units and
was referred to as coenzyme Q 10 . Later, homologues with 6, 7, 8 and 9 units
were isolated from other organisms, bacteria or higher organisms. The main form
in man has 10 units but in rat has 9 units. Another system of nomenclature is
used: ubiquinone(x) in which x designates the total number of carbon atoms in
the side chain, it can be a multiple of 5.
Ubiquinones accept one electron and are transformed into semiquinone
radicals (UQH°) or two electrons to give ubiquinol (UQH2)
Coenzyme Q is
reducible by sodium dithionite or borohydride to its hydroquinone form, and can
in turn be reoxidized to the quinone by Ag2O or more slowly by oxygen. The
absorption spectra of the two forms are shown below. The quinone form has a
strong absorption band at 275 nm which disappears in the reduced form.
Cytochromes are, in
general, membrane-bound (i.e. inner mitochondrial membrane) hemeproteins
containing heme
groups and are primarily responsible for the generation of ATP
via electron transport.
They are found either as monomeric proteins
(e.g., cytochrome c)
or as subunits
of bigger enzymatic complexes that catalyze redox
reactions.
Cytochromes
were initially described in 1884 by MacMunn as respiratory pigments (myohematin
or histohematin).[1] In
the 1920s, Keilin
rediscovered these respiratory pigments and named them the cytochromes, or
“cellular pigments”, and classified these heme proteins, on the basis of the
position of their lowest energy absorption band in the reduced state, as
cytochromes a (605 nm), b (~565 nm), and c
(550 nm). The UV-visible spectroscopic signatures of hemes are still used
to identify heme type from the reduced bis-pyridine-ligated state, i.e., the
pyridine hemochrome method. Within each class, cytochrome a, b,
or c, early cytochromes are numbered consecutively, e.g. cyt c,
cyt c1, and cyt c2, with more recent
examples designated by their reduced state R-band maximum, e.g. cyt c559.[2]
The
heme
group is a highly-conjugated ring system (which allows its electrons
to be very mobile) surrounding a metal ion, which readily interconverts between
the oxidation states. For many cytochromes, the metal ion present is that of iron,
which interconverts between Fe2+ (reduced) and Fe3+
(oxidised) states (electron-transfer processes) or between Fe2+
(reduced) and Fe3+ (formal, oxidized) states (oxidative processes).
Cytochromes are, thus, capable of performing oxidation and reduction.
Because the cytochromes (as well as other complexes) are held within membranes
in an organized way, the redox
reactions are carried out in the proper sequence for maximum efficiency.
In the
process of oxidative phosphorylation,
which is the principal energy-generating process undertaken by organisms, other
membrane-bound and -soluble complexes and cofactors are involved
in the chain of redox reactions, with the additional net effect that protons (H+)
are transported across the mitochondrial inner membrane. The resulting transmembrane proton gradient
([protonmotive force]) is used to generate ATP, which is
the universal chemical energy currency of life. ATP is consumed to drive cellular
processes that require energy (such as synthesis of macromolecules, active
transport of molecules across the membrane, and assembly of flagella).
Several kinds of cytochrome exist and can be
distinguished by spectroscopy, exact
structure of the heme group, inhibitor sensitivity, and reduction potential.
Three types of cytochrome are distinguished by their
prosthetic groups:
Type |
Prosthetic group |
The
definition of cytochrome c is not
defined in terms of the heme group. There is no "cytochrome e," but
there is a cytochrome f, which is
often considered a type of cytochrome c.
In mitochondria and chloroplasts, these
cytochromes are often combined in electron
transport and related metabolic pathways:
Cytochromes |
Combination |
a and a3 |
Cytochrome
c oxidase ("Complex IV") with
electrons delivered to complex by soluble cytochrome c
(hence the name) |
b and c1 |
Coenzyme Q - cytochrome c reductase
("Complex III") |
b6 and f |
A completely
distinct family of cytochromes is known as the cytochrome
P450 oxidases, so named for the characteristic Soret peak formed by
absorbance of light at wavelengths near 450 nm when the heme iron is
reduced (with sodium dithionite) and
complexed to carbon monoxide. These
enzymes are primarily involved in steroidogenesis
and detoxification.
Respiratory chain
The system of
mitochondrial enzymes and redox carrier molecules which ferry reducing
equivalents from substrates to oxygen are collectively known as the electron
transport system, or the respiratory chain. This system captures the free energy available
from substrate oxidation so that it may later be applied to the synthesis of
ATP. Many respiratory chain components were first identified in crude
homogenates through their spectral properties, which frequently change when a
carrier is oxidised or reduced. Fractionation of mitochondria in the presence
of mild detergents or chaotropic salts dissected the respiratory chain into four
large multi-subunit complexes containing the principal respiratory carriers,
named Complex 1 to Complex 4.
These
substantial protein "icebergs" float in the sheet of inner membrane lipids,
often presenting one face to the mitochondrial matrix
and another to the inter - membrane space.
Many of their components have now been isolated in a relatively pure form.
Other membrane bound enzymes such as the energy linked transhydrogenase (ELTH)
are also present which fulfil ancillary roles.
The main
components participate in the approximate order of their redox potentials, and the
bulky complexes are linked by low molecular weight mobile carriers which ferry
the reducing equivalents from one complex to the next. Except for succinate
dehydrogenase (complex 2) all these complexes pump protons from the matrix
space into the cytosol as they transfer reducing equivalents (either hydrogen
atoms or electrons) from one carrier to the next. The diagram above shows the
flow of reducing equivalents in purple, and movement of the positively charged
protons in red. Proton pumping is an arduous task which creates substantial pH
and electrical gradients across the mitochondrial inner membrane. These protons
eventually re-enter the matrix space via the F1 ATPase, driving the
synthesis of ATP as they return.
The number of
protons and the number of positive charges crossing the inner membrane need not
necessarily agree for each individual transmembrane protein, although the
accounts must balance for the whole ensemble. This discrepancy is illustrated
on the diagram above, and is explained in greater detail below.
Electrons
flow through the electron transport chain to molecular oxygen; during this
flow, protons are moved across the inner membrane from the matrix to the
intermembrane space. This model for ATP synthesis is called the chemiosmotic
mechanism, or Mitchell hypothesis. Peter Mitchell, a British
biochemist, essentially by himself and in the face of contrary opinion,
proposed that the mechanism for ATP synthesis involved the coupling between
chemical energy (ATP) and osmotic potential (a higher concentration of protons
in the intermembrane space than in the matrix). The inner membrane of the
mitochondrion is tightly packed with cytochromes and proteins capable of
undergoing redox changes. There are four major protein-membrane complexes.
Complex I and Complex II direct electrons to coenzyme Q. Complex I, also
called NADH-coenzyme Q reductase, accepts electrons from NADH. The NADH
releases a proton and two electrons. The electrons flow through a flavoprotein
containing FMN and an iron-sulfur protein. First, the flavin coenzyme (flavin
mononucleotide) and then the iron-sulfur center undergo cycles of reduction and
then oxidation, transferring their electrons to a quinone molecule, coenzyme
Q (see Figure ).
Complex
I is capable of transferring protons from the matrix
to the intermembrane space while undergoing these redox cycles. One possible
source of the protons is the release of a proton from NADH as it is oxidized to
NAD, although this is not the only explanation. Apparently, conformational
changes in the proteins of Complex I also are involved
in the mechanism of proton translocation during electron transport.
|
Complex
II, also known as succinate-coenzyme Q reductase, accepts electrons from succinate
formed during the TCA cycle. Electrons flow from succinate to FAD (the
flavin-adenine dinucleotide) coenzyme, through an iron-sulfur protein and a
cytochrome b550 protein (the number refers to the wavelength where the
protein absorbs), and to coenzyme Q. No protons are translocated by Complex II.
Because translocated protons are the source of the energy for ATP synthesis,
this means that the oxidation of a molecule of FADH2 inherently
leads to less ATP synthesized than does the oxidation of a molecule of NADH.
This experimental observation also fits with the difference in the standard
reduction potentials of the two molecules. The reduction potential of FAD is
-0.22 V, as opposed to -0.32 V for NAD.
Coenzyme Q is
capable of accepting either one or two electrons to form
either a semiquinone or hydroquinone form. Coenzyme Q is not
bound to a protein; instead it is a mobile electron carrier and can float
within the inner membrane, where it can transfer electrons from Complex I and
Complex II to Complex III.
Complex
III is also known as coenzyme Q-cytochrome c reductase. It
accepts electrons from reduced coenzyme Q, moves them within the complex
through two cytochromes b, an iron-sulfur protein, and cytochrome c1.
Electron flow through Complex II transfers proton(s) through the membrane into
the intermembrane space. Again, this supplies energy for ATP synthesis. Complex
III transfers its electrons to the heme group of a small, mobile electron
transport protein, cytochrome c.
Cytochrome
c transfers its electrons to the final electron transport component, Complex
IV, or cytochrome oxidase. Cytochrome oxidase
transfers electrons through a copper-containing protein, cytochrome a, and
cytochrome a3, and finally to molecular oxygen. The overall pathway for electron transport is therefore:
|
||
|
|
The
number n is a fudge factor to account for the fact that the exact
stoichiometry of proton transfer isn't really known. The important point is
that more proton transfer occurs from NADH oxidation than from FADH2
oxidation.
A theory
postulated by the biochemist Peter Mitchell in 1961 to describe ATP synthesis
by way of a proton electrochemical coupling is called chemiosmotic
hypothesis.
Accordingly, hydrogen ions (protons) are
pumped from the mitochondrial matrix to the intermembrane space via the
hydrogen carrier proteins while the electrons are transferred along the
electron transport chain in the mitochondrial inner membrane. As the hydrogen
ions accumulate in the intermembrane space, an energy-rich proton gradient is
established. As the proton gradient becomes sufficiently intense the hydrogen
ions tend to diffuse back to the matrix (where hydrogen ions are less) via the
ATP synthase (a transport protein). As the hydrogen ions diffuse (through the
ATP synthase) energy is released which is then used to drive the conversion of
ADP to ATP (by phosphorylation).
Chemiosmotic Hypothesis in a simple form
In the 1960s,
ATP
was known to be the energy currency of life, but the mechanism by which ATP was
created in the mitochondria was assumed
to be by substrate-level
phosphorylation. Mitchell's chemiosmotic hypothesis
was the basis for understanding the actual process of oxidative
phosphorylation. At the time, the biochemical
mechanism of ATP synthesis by oxidative phosphorylation was unknown.
Mitchell
realised that the movement of ions across an electrochemical
membrane potential could provide the energy needed to
produce ATP. His hypothesis was derived from information that was well known in
the 1960s. He knew that living cells had a membrane potential;
interior negative to the environment. The movement of charged ions across a
membrane is thus affected by the electrical forces (the attraction of positive
to negative charges). Their movement is also affected by thermodynamic
forces, the tendency of substances to diffuse from regions
of higher concentration. He went on to show that ATP synthesis was coupled to
this electrochemical
gradient.
His
hypothesis was confirmed by the discovery of ATP synthase, a
membrane-bound protein that uses the potential energy of the electrochemical
gradient to make ATP.
The passage
back occurs via a specific proton channel. This passage is coupled to
ATP-synthesis, using the potential energy of the proton gradient for the
formation of the third phosphate bond of ATP.
We can now
calculate the end result of glucose degradation: the oxidation is coupled to a
decrease of the free energy; 686 kcal/mol (= 2881
kJ/mol) are obtained by the complete oxidation of glucose. How much of this energy can
the cell use?
1. Six
mol ATP per mol glucose are generated (substrate chain
phosphorylation). This is because all steps after the breaking down of
fructose-1,6-phosphate have to be counted twice (once
for each of the two resulting C3 molecules), so it is 3 x 2 ATPs. Of
these six ATPs, two are needed to start glycolysis. That leaves four.
2.
During the course of glycolysis up to
acetyl-CoA, 2 x 2 NADH + H+ are generated. An additional 3 x 2 NADH
+ H+ and 1 x 2 FADH2 are produced in the citric acid
cycle. One NADH + H+ gives three, one FADH2 two ATPs when
fed into the respiratory chain. This sums up to 34
ATPs plus the 4 ATPs of glycolysis. A total of 38 mol ATP
are thus gained by the cell's degradation of one mol glucose. Since each
energy-rich bond of ATP contains 7.3 kcal/mol (= -30.6 kJ/mol), the 38 ATP
equal 277 kcal/mol (ca 1163 kJ/mol). This is 40.6% of the theoretically
possible gain. The other 59.4 percent are set free as heat. This is a very high
percentage compared to the gain of technical machines like steam or petrol
engines that is around or below 20 percent.
The ATP synthase enzymes have been
remarkably conserved through evolution. The bacterial enzymes are essentially
the same in structure and function as those from mitochondria of animals, plants
and fungi, and the chloroplasts of plants. The early ancestory of the enzyme is
seen in the fact that the Archaea have an enzyme which is clearly closely
related, but has significant differences from the Eubacterial branch. The H+-ATP-ase
found in vacuoles of the eukaryote cell cytoplasm is similar to the archaeal
enzyme, and is thought to reflect the origin from an archaeal ancestor.
In most systems, the ATP synthase sits
in the membrane (the "coupling" membrane), and catalyses the
synthesis of ATP from ADP and phosphate driven by a flux of protons across the
membrane down the proton gradient generated by electron transfer. The flux goes
from the protochemically positive (P) side (high proton electrochemical
potential) to the protochemically negative (N) side. The reaction catalyzed by
ATP synthase is fully reversible, so ATP hydrolysis generates a proton gradient
by a reversal of this flux. In some bacteria, the main function is to operate
in the ATP hydrolysis direction, using ATP generated by fermentative metabolism
to provide a proton gradient to drive substrate accumulation, and maintain
ionic balance.
ADP + Pi + nH+P
<=> ATP + nH+N
Because the structures seen in EM, the
subunit composition, and the sequences of the subunits appeared to be so
similar, it had been assumed that the mechanisms, and hence the
stoichiometries, would be the same. In this context, the evidence suggesting that
the stoichiometry of H+/ATP (n above) varied depending on system was
surprising. Values based on measure ATP/2e- ratios, and H+/2e-
ratios had suggested that n was 3 for mitochondria, and 4 for chloroplasts, but
these values were based on the assumption of integer stoichiometries. Although
all the F1F0-type ATP-synthases likely had a common
origin, both the assumption that the stoichiometries are the same, and that n
is integer, are called into question by emerging structural data (see below).
The structure of the soluble (F1) portion of the
ATP synthase from beef heart mitochondria has been solved by X-ray
crystallography. The pictures below are from Abrahams, J.P., Leslie, A.G.,
Lutter, R. and Walker, J.E.
(1994) Structure at 2.8 Å resolution of F1-ATPase from bovine
heart mitochondria
The ATP synthase operates through a
mechanism in which the three active sites undergo a change in binding affinity
for the reactants of the ATP-ase reaction, ATP, ADP and phosphate, as
originally predicted by Paul Boyer. The change in affinity accompanies
a change in the position of the g-subunit
relative to the a, b-ring, which
involves a rotation of the one relative to the other. In the direction of ATP
synthesis, the rotation is driven by a flux of H+ down the proton
gradient, through a coupling between the g-subunit, and
the c-subunit of FO. This rotation has now been demonstrated
experimentally.
Respiratory control
The dependence of oxidative phosphorylation on ADP reveals an important
general feature of this process: Respiration is tightly coupled to the
synthesis of ATP. Not only is ATP synthesis
absolutely dependent on continued electron flow from substrates to oxygen, but
electron flow in normal mitochondria occurs only when ATP is being synthesized
as well. This regulatory phenomenon, called respiratory control, makes
biological sense, because it ensures that substrates will not be oxidized
wastefully. Instead, their utilization is controlled by the physiological need
for ATP.
In most aerobic
cells the level of ATP exceeds that of ADP by 4- to 10-fold. Respiration
depends on ADP as a substrate for phosphorylation. When ATP is consumed at high
rates, accumulation of ADP stimulates respiration, with concomitant activation
of ATP resynthesis. Conversely, in a relaxed and well-nourished cell, ATP
accumulates at the expense of ADP, and the depletion of ADP limits the rate of
both electron transport and its own phosphorylation to ATP. Thus, the
energy-generating capacity of the cell is closely attuned to its energy
demands.
Experimentally, respiratory control is demonstrated by following
oxygen utilization in isolated mitochondria. In the absence
of added substrate or ADP, oxygen uptake, caused by oxidation of endogenous
substrates, is slow. Addition of an oxidizable substrate, such as glutamate or
malate, has but a small effect on the respiration rate. If ADP is then added,
however, oxygen uptake proceeds at an enhanced rate until all of the added ADP
has been converted to ATP, and then oxygen uptake returns to the basal rate.
This stimulation of respiration is stoichiometric; that is, addition of
twice as much ADP causes twice the amount of oxygen uptake at the enhanced
rate. If excess ADP is present instead of oxidizable substrate, the addition of
substrate in limiting amounts will stimulate oxygen uptake until the substrate
is exhausted.
Two mechanisms of the control of
respiration and ATP synthesis in mitochondria according to the utilization of
energy (ATP). The first mechanism of respiratory control is based on the proton
motive force Δp across the
inner mitochondrial membrane (grey). Activation of the ATP-synthase (blue) by
ADP, taken up via the ATP/ADP carrier (margenta), decreases Δp which in consequence stimulates the
three proton pumps of the respiratory chain (complexes I, III and IV). For
simplicity, only complex IV (cytochrome c
oxidase) and its substrate (cytochrome c)
are shown in green and red, respectively. The second mechanism of respiratory
control is based on the intramitochondrial ATP/ADP ratio. High ATP/ADP ratios
inhibit cytochrome c oxidase
activity allosterically. Uptake of ADP decreases the intramitochondrial ATP/ADP
ratio accompanied by exchange of bound ATP by ADP at the matrix domain of
subunit IV of cytochrome c
oxidase, with subsequent stimulation of respiration
Maintenance of respiratory control
depends on the structural integrity of the mitochondrion. Disruption of the
organelle causes electron transport to become uncoupled from ATP synthesis.
Under these conditions, oxygen uptake proceeds at high rates even in the
absence of added ADP. ATP synthesis is inhibited, even though electrons are
being passed along the respiratory chain and used to reduce O2 to water.
Uncoupling of
respiration from phosphorylation can also be achieved chemically. Chemical
uncouplers such as DNP
or FCCP
act by dissipating the proton gradient. Addition of an uncoupler to
mitochondria stimulates oxygen utilization even in the absence of added ADP. No
phosphorylation occurs under these conditions because there is no ADP to be
phosphorylated.
The phenomenon of respiratory
control is the subject of today's studio
exercise. An oxygen electrode may be used to record [O2]
in a closed vessel (diagram p. 804). Electron transfer, e.g., from NADH to O2,
is monitored by recording the rate of disappearance of O2. At
right is an idealized representation of an oxygen electrode recording while
mitochondria respire in the presence of Pi, along with an electron
donor (e.g., succinate, or a substrate of a reaction that will generate NADH).
The dependence of respiration rate on availability of ADP, the substrate for
the ATP Synthase, is called respiratory control. The respiratory
control ratio is the ratio of slopes after and before ADP addition (b/a).
The P/O ratio is the moles of ADP added, divided by the moles of
O consumed (based on c) while phosphorylating the added ADP.
Chemiosmotic explanation of
respiratory control:
Electron transfer is obligatorily
coupled to H+ ejection from the matrix. Whether this coupled reaction
is spontaneous depends on the pH and electrical gradients.
Reaction |
Free energy change |
e-
transfer (e.g., NADH to O2) |
a negative value* |
H+ ejection from
the matrix |
a positive value that varies
with the H+ gradient** |
e- transfer
coupled to H+ ejection |
algebraic sum of the above |
*DGo' =
- nFDEo' = -218 kJ/mol, for transfer of 2 e- from NADH to O2.
** For ejection of one H+
from the matrix:
DG = RT ln ([H+]cytosol/[H+]matrix)
+ F DY = 2.3 RT (pHmatrix - pHcytosol) + F DY
In the absence of ADP, H+
cannot flow back to the matrix through Fo. The pH and electrical
gradients (DpH & DY) are maximal.
As respiration with outward H+ pumping proceeds, the free energy
change for H+ ejection (positive DG) increases and
approaches the magnitude of that for electron transfer (negative DG). When the coupled reaction becomes non-spontaneous, respiration stops.
This is referred to as a static head. In fact there is usually a
low rate of respiration in the absence of ADP, attributed to H+
leaks. Protons pumped out are carried by the uncoupler back
into the mitochondrial matrix, preventing development of a pH or electrical
gradient.
Inhibitors of tissue respiration
Chemiosmosis can be disrupted by a variety of
chemicals. In oxidative phosphorylation, some of these inhibitors are quite
infamous:
The
electron transport chain was determined by studying the effects of particular
inhibitors.
Rotenone
is a common insecticide that strongly inhibits the electron transport of
complex I.
Rotenone
is a natural product obtained from the roots of several species of plants.
Tribes in certain parts of the world beat the roots of trees along riverbanks
to release rotenone into the water which paralyzes fish and makes them easy
prey.
Amytal
is a barbiturate that inhibits the electron transport of complex I. Demerol is
painkiller that also inhibits complex
Antibiotic. Induces apoptosis, which is not prevented by the presence of Bcl-2.
Inhibits mitochondrial electron transport specifically
between cytochromes b and c1. All three of these complex I inhibitors
block the oxidation of the Fe-S clusters of complex I.
2-Thenoyltrifluoroacetone
and carboxin specifically block electron transport in Complex II
Antimycin
A is an antibiotic that inhibits electron transfer in complex III by blocking
the transfer of electrons between Cyt bH and coenzyme
Q bound at the QN site. Antibiotic. Induces
apoptosis, which is not prevented by the presence of Bcl-2. Inhibits mitochondrial electron transport specifically between
cytochromes b and c1.
Cyanide,
azide and carbon monoxide all inhibit electron transport in Complex IV. The all
inhibit electron transfer by binding tightly with the iron coordinated in Cyt a
This complex oxidizes cytochrome c and also reduces O2 to H2O.
Remember that cytochromes have heme cofactors -- this is important in our
discussion of cyanide and azide. Cytochrome c is a soluble
protein and also is a mobile carrier. Other inhibitors of cytochrome c
oxidase will not be discussed here, but are important biologically, such as
sulfide, formate, and nitric oxide.
Azide
and cyanide bind to the iron when the iron is in the ferric state. Carbon
Monoxide binds to the iron when it is in the ferrous state. Cyanide and azide
are potent inhibitors at this site which accounts for there acute toxicity.
Carbon monoxide is toxic due to its affinity for the heme iron of hemoglobin.
Animals carry many molecules of hemoglobin, therefore
it takes a large quantity of carbon monoxide to die from carbon monoxide
poisoning.
CO
competes with oxygen for binding to the reduced form of cytochrome c
oxidase. Once bound to the cytochrome oxidase, oxygen cannot attach, and
electron transport is stopped. CO is a colorless, tasteless,
non-irritating toxic gas. When inhaled, the toxic gas enters the
bloodstream, depriving the heart and brain of the oxygen necessary to function
correctly. Sensing the body's need for more oxygen, the victim's heart rate
increases to pump more blood to the body's organs. If a person continues to
inhale CO, he or she faces the risk of breathing difficulty, cardiac trauma,
brain damage, coma and even death.
CN-,
like azide, binds to the iron atom of oxidized cytochrome, preventing binding
of oxygen. Again, since cytochrome oxidase is inhibited, oxygen
metabolism is prevented and thus so is energy generation. Cyanide has
long been known as a poison, sometimes used in warfare. As mentioned
above, lethal doses cause death in 15 minutes.
They
all cause similar toxic repercussions (with the exception of Carbon Monoxide,
which can bind to Hemoglobin causing your body to be unable to bind oxygen
properly and causing you to suffocate);
However, these three also cause chemical suffocation. They bind a protein in
our electron transport chain (cytochrome c oxidase). The electron transport
chain is our body's best way to create energy in the form of ATP. Without it,
we literally have next to no energy. The electron transport chain is the reason
we have pain in our muscles when we work out--if our body doesn't have oxygen
in those tissues and can't make energy, we make lactic acid, which causes our
muscles to hurt/burn. Anyway, cyanide, azide and carbon monoxide bind this
protein (cytochrome c oxidase) in the electron transport chain. Doing this
causes all of the electrons to stop transferring and no energy to be made. This
causes cell death. The first place to experience cell death from cyanide, azide
and carbon monoxide is your central nervous system--which is made up of your
brain and spinal cord.
So the first thing to die when someone dies of cyanide, azide or carbon
monoxide poisoning?
The
coupling between electron transport and oxidative phosphorylation depends on
the impermeability of the inner mitochondrial membrane to H+translocation. The
only way for protons to go from the intermembrane space to the matrix is
through ATP synthase. Uncouplers uncouple electron transport from oxidative
phosphorylation. They collapse the chemiosmotic gradient by dissipating protons
across the inner mitochondrial membrane. All of the uncouplers shown to the left,
collapse the pH gradient by binding a proton on the acidic side of the
membrane, diffusing through the inner mitochondrial membrane and releasing the
proton on the membranes alkaline side.
Uncouplers
of oxidative phosphorylation stimulate the rate of electron flow but not
ATP synthesis.
(a)At
relatively low levels of an uncoupling agent, P/O ratios drop somewhat, but the
cell can compensate for this by increasing the rate of electron flow; ATP
levels can be kept relatively normal. At high levels of uncoupler, P/O ratios
approach zero and the cell cannot maintain ATP levels.
(b)
As amounts of an uncoupler increase, the P/O ratio decreases and the body
struggles to make sufficient ATP by oxidizing more fuel. The heat produced by
this increased rate of oxidation raises the body temperature. The P/O ratio is
affected as noted in (a).
(c)
Increased activity of the respiratory chain in the presence of an uncoupler
requires the degradation of additional energy stores (glycogen and fat). By
oxidizing more fuel in an attempt to produce the same amount of ATP, the
organism loses weight. If the P/O ratio nears zero, the lack of ATP will be
lethal
2,4-Dinitrophenol, dicumarol and carbonyl
cyanide-p-trifluorocarbonyl-cyanide methoxyphenyl hydrazone (FCCP) all have
hydrophobic character making them soluble in the bilipid membrane. All of these
decouplers also have dissociable protons allowing them to carry protons from
the intermembrane space to the matrix which collapses the pH gradient. The
potential energy of the proton gradient is lost as heat DNP is a
chemical uncoupler of electron transport and oxidative phosphorylation.
DNP
permeabilizes the inner mitochondrial membrane to protons, destroying the
proton gradient and, in doing so, uncouples the electron transport system from
the oxidative phosphorylation. In this situation, electrons continue to pass
through the electron transport system and reduce oxygen to water, but ATP is not
synthesized in the process. The compound, trifluorocarbonylcyanide
phenylhydrazone (FCCP), is also an
uncoupler.
The phenolic
group of DNP is usually dissociated at intracellular pH. However, a DNP
molecule that approaches the inner mitochondrial membrane from the outside
becomes protonated (because the pH is lower there). Protonation increases the
hydrophobicity of DNP, allowing it to diffuse into the membrane and, by
mass action, to pass through. Once inside, the higher pH of the
matric deprotonates the phenolic hydroxyl again. Thus, DNP has
the effect of transporting H+ back
into the matrix, bypassing the F0 proton channel and thereby preventing ATP
synthesis.
The
link between electron transport and ATP synthesis is below. (a) In the presence
of excess phosphate and substrate and intact mitochondria, oxygen is consumed
only when ADP is added. When all of the added ADP has been converted into ATP,
electron transport stops and oxygen consumption ceases. (b) The addition of 2,4-dintrophenol uncouples electron transfer from ATP
synthesis. The oxygen is completely consumed in the absence of ADP. Endogenous
Uncouplers Enable Organisms to Generate Heat
.
The
uncoupling of oxidative phosphorylation from electron transport generates heat.
Hibernating animals and newborne animals (including human beings) contain brown
adipose tissue. The adipose tissue is brown due to the high mitochondria
content of the tissue. An endogenous protein called thermogenin uncouples ATP
synthesis from electron transport by opening up a passive proton channel
(UCP-1) through the inner mitochondrial membrane. The collapse of the pH
gradient generates heat. An uncoupling protein
(also called thermogenin) is produced in brown adipose tissue of newborn
mammals and hibernating mammals .
This protein
of the inner mitochondrial membrane functions as a H+
carrier. The uncoupling protein blocks development of a H+ electrochemical gradient, thereby
stimulating respiration. The free energy change associated with respiration is
dissipated as heat. This "non-shivering thermogenesis" is costly in
terms of respiratory energy unavailable for ATP synthesis, but it provides
valuable warming of the organism.
Valinomycin
combines with K ions to form a complex that passes through the inner
Valinomycin is a potent antibiotic which acts as a
potassium (K+) ionophore. Induces K+
conductivity in cell membranes. Also active in vitro against
Mycobacterium Tuberculosis, and as an apoptosis inducer.
Valinomycin is obtained from the cells of several Streptomyces
strains, among which "S. tsusimaensis" and S. fulvissimus.
It is a member of the group of natural neutral ionophores because it does not
have a residual charge. It consists of enantiomers D- and L-valine (Val),
D-hydroxyvaleric acid and L-lactic acid. Structures are alternately bound via
amide and ester bridges. Valinomycin is highly selective for potassium ions
over sodium ions within the cell membrane. It functions as a potassium-specific
transporter and facilitates the movement of potassium ions through lipid
membranes "down" an electrochemical potential gradient. The stability
constant K for the potassium-valinomycin complex is 106 and for the
sodium-valinomycin complex only 10. This difference is important for
maintaining the selectivity of valinomycin for the transport of potassium ions
(and not sodium ions) in biological systems.
Oligomycin is a natural antibiotic isolated from Streptomyces
diastatochromogenes which inhibits mitochondrial H+-ATP
synthase. It is primarily found to act as an inhibitor of mitochondrial
respiration and swelling. This antibiotic is widely used as an
inhibitor of oxidative phosphorylation.1-2
Oligomycin inhibits the H+- ATP-synthase by binding to the
Oligomycin sensitivity-conferring protein (OSCP) at the F(o)
subunits 6 and 9 which are found in the stalk of the F1F0-ATPase
complex. This binding blocks the proton conductance and inhibits the synthesis
of mitochondrial ATP.3-4 Because of its activity,
it can also be used to reduce the number of parameters (such as ER Ca2+
release, exocytotoxicity and apopotosis) which are affected by mitochondrial
depolarization.
Oligomycin, at high concentrations may also inhibit the plasma membrane
Na+-K+-ATPase. Interaction of Oligomycin with the Na+
occlusion site on the extracellular side of Na/K-ATPase, delays Na+
release to the extracellular side without inducing a conformational change.
Although Oligomycin stimulated Na+ binding to Na+/K+-ATPase,
it inhibited Na+/Na+ exchange, and did not affect
either Na+-dependent AD/-ATP exchange or K+-dependent
phosphatase activity.6-16
Proton
Motive Force Drives Transport The primary purpose of the proton gradient is
togenerate ATP by oxidative phosphorylation. The potential energy of the
gradient can also be used for active transport. The inner mitochondrial
membrane is impermeable to charged molecules.
ATP-ADP
Translocase There are two specific systems to
transport ADP and Pi into the mitochondrial matrix. A specific transport
protein ATP-ADP translocase enables ATP and ADP to transverse the inner
mitochondrial membrane. The transport of ADP in and ATP out are coupled. ADP
only enters the matrix if ATP exits or vice versa. ATP-ADP translocase has a
single nucleotide binding site which binds ADP and ATP with equal affinity. Due
to the negative electrostatic charge of the matrix, ATP-is bound on the N phase
of the membrane because it has greater negative charge than ADP.
Pentachlorophenol
(PCP) acts in a similar way to DNP. It was widely used
as a biocide, especially in pallet board manufacture as a fungicide, but is now
banned by the Biocidal Products Directive, because of its extreme toxicity and
environmental persistence.
DNP
PCP
Ways of energy usage in the organism.
o
1. From the smallest, single-celled
organism to the biggest and most complex mammals--including people--all living
things require energy for life. It's easy enough to understand that we and
other animals eat. Things get a little more puzzling when we think about fungi,
which absorb their food as organic molecules, from the surrounding environment.
Where do those molecules come from? Furthermore, where does the food come from
that we humans convert to energy? At the most basic level, all energy traces
back to plants. Plants are the basis of all the world's food systems, and their
unique ability to make organic materials from sunlight--called
photosynthesis--is what sustains nearly every other life form on the planet.
o
The powerhouse of energy production
in all plants is called a chloroplast. More than a million of these handy
devices occur in every quarter-inch of a leaf. They contain the pigment called
chlorophyll that makes most leaves green--and drives photosynthesis. The
reaction isn't all that complicated, as far as chemical reactions go. The
chloroplasts take in carbon dioxide, sunlight and water. They release oxygen
and a bit less water than they took in. The conversion of carbon dioxide to
oxygen is one life-sustaining function that plants perform for Earth and all of
its life. But plants do something equally as important when they keep a third
product behind: glucose, the sugar that sustains the plants---and anything, in
turn, that eats the plants.
o
In cellular respiration, glucose is
broken down by the removal of its hydrogen atoms. That process releases energy
in the form of electrons, negatively charged particles that fuel all of a
cell's other work in later reactions. So, plants make the glucose and
everything down the line---from plant-eaters to the carnivores that eat
them---break the glucose down again, and use its energy. That's the simple story.
Of course, life is rarely so simple, and there are exceptions to every rule.
Every so often, a new discovery comes along about living things that use a
non-living substance other than sunlight to make energy--like ammonia, or even
sulfur. These less-common organisms can harness electrons from chemical sources
instead of the sun. More amazing life forms have the potential to be discovered
at any time, anywhere on our planet---or beyond.
Cytochrome P450
The
active site of cytochrome P450 contains a heme
iron center. The iron is tethered to the P450 protein via a thiolate
ligand derived from a cysteine
residue. This cysteine and several flanking residues are highly conserved in
known CYPs and have the formal PROSITE
signature consensus pattern [FW] - [SGNH] - x - [GD] - {F} - [RKHPT] - {P} - C
- [LIVMFAP] - [GAD].[7]
Because of the vast variety of reactions catalyzed by CYPs, the activities and
properties of the many CYPs differ in many aspects. In general, the P450 catalytic cycle
proceeds as follows:
1.
The substrate binds to the active
site of the enzyme, in close proximity to the heme group, on the side opposite
to the peptide chain. The bound substrate induces a change in the conformation
of the active site, often displacing a water molecule from the distal axial
coordination position of the heme iron,[8] and sometimes changing the state of the heme iron from low-spin to
high-spin.[9]
This gives rise to a change in the spectral properties of the enzyme, with an
increase in absorbance at 390 nm and a decrease at 420 nm. This can
be measured by difference spectrometry and is referred to as the
"type I" difference spectrum (see inset graph in figure). Some
substrates cause an opposite change in spectral properties, a "reverse
type I" spectrum, by processes that are as yet unclear. Inhibitors
and certain substrates that bind directly to the heme iron give rise to the
type II difference spectrum, with a maximum at 430 nm and a minimum
at 390 nm (see inset graph in figure). If no reducing equivalents are
available, this complex may remain stable, allowing the degree of binding to be
determined from absorbance measurements in vitro
2.
The change in the electronic state of
the active site favors the transfer of an electron from NAD(P)H via cytochrome P450 reductase
or another associated reductase[11]
This takes place by way of the electron transfer chain, as described above,
reducing the ferric heme iron to the ferrous state.
3.
Molecular oxygen binds covalently to
the distal axial coordination position of the heme iron. The cysteine ligand is
a better electron donor than histidine, which is normally found in
heme-containing proteins. As a consequence, the oxygen is activated to a
greater extent than in other heme proteins. However, this sometimes allows the
iron-oxygen bond to dissociate, causing the so-called "uncoupling
reaction", which releases a reactive superoxide radical and interrupts the
catalytic cycle.
4.
A second electron is transferred via
the electron-transport system, from either cytochrome
P450 reductase,
ferredoxins, or cytochrome b5, reducing the dioxygen adduct to a negatively charged
peroxo group. This is a short-lived intermediate state.
5.
The peroxo group formed in step 4 is
rapidly protonated twice by local transfer from water or from surrounding
amino-acid side-chains, releasing one water molecule, and forming a highly
reactive species commonly referred to as P450. This highly reactive
intermediate was not "seen in action" until 2010,[12]
although it had been studied theoretically for many years. P450 Compound 1 is
most likely a iron(IV)oxo (or ferryl)
species with an additional oxidizing equivalent delocalized over the porphyrin
and thiolate ligands. Evidence for the alternative perferryl iron(V)-oxo [8]
is lacking.
6.
Depending on the substrate and enzyme
involved, P450 enzymes can catalyze any of a wide variety of reactions. A
hypothetical hydroxylation is shown in this illustration. After the product has
been released from the active site, the enzyme returns to its original state,
with a water molecule returning to occupy the distal coordination position of
the iron nucleus.
S: An
alternative route for mono-oxygenation is via the "peroxide shunt":
Interaction with single-oxygen donors such as peroxides and hypochlorites can
lead directly to the formation of the iron-oxo intermediate, allowing the
catalytic cycle to be completed without going through steps 2, 3, 4, and
C: If carbon
monoxide (CO) binds to reduced P450, the catalytic cycle is interrupted. This
reaction yields the classic CO difference spectrum with a maximum at
450 nm.