DIGESTION OF PROTEINS.
GENERAL PATHWAYS OF AMINO ACIDS TRANSFORMATION.
Detoxification of ammonia and biosynthesis
of urea. Specific pathways of amino acid metabolism. Mechanisms of hormonal
regulation and pathologies of protein metabolism.
Proteins are essential nutrients for the human
body. They are one of the building blocks of body tissue, and can also serve as
a fuel source. As fuel, proteins contain 4 kcal
per gram, just like carbohydrates and unlike lipids, which contain 9
kcal per gram.
Proteins are polymer chains made of amino acids
linked together by peptide bonds. In nutrition, proteins
are broken down in the stomach during digestion
by enzymes
known as proteases
into smaller polypeptides to provide amino acids for the
body, including the essential amino acids that cannot be biosynthesized
by the body itself.
Amino acids can be divided into three
categories: essential amino acids, non-essential amino acids and conditional
amino acids. Essential amino acids cannot be made by the body, and must be
supplied by food. Non-essential amino acids are made by the body from essential
amino acids or in the normal breakdown of proteins. Conditional amino acids are
usually not essential, except in times of illness, stress or for someone
challenged with a lifelong medical condition.
Essential amino acids are leucine,
isoleucine,
valine,
lysine,
threonine,
tryptophan,
methionine,
phenylalanine
and histidine.
Non-essential amino acids include alanine, asparagine, aspartic acid
and glutamic acid.
Conditional amino acids include arginine, cysteine, glutamine, glycine, proline, serine, and tyrosine.
Amino acids are found in animal sources such as meats,
milk, fish and eggs, as well as in plant sources such as whole grains, pulses,
legumes, soy, fruits, nuts and seeds. Vegetarians and vegans can get enough
essential amino acids by eating a variety of plant proteins.
Protein is a nutrient needed by the human body for growth
and maintenance. Aside from water, proteins are the most abundant kind of
molecules in the body. Protein can be found in all cells of the body and is the
major structural component of all cells in the body, especially muscle. This
also includes body organs, hair and skin. Proteins also are utilized in
membranes, such as glycoproteins. When broken down into amino
acids, they are used as precursors to nucleic acid,
co-enzymes, hormones, immune response, cellular repair and molecules essential
for life. Finally, protein is needed to form blood cells.
Proteins are very important molecules in our cells. They
are involved in virtually all cell functions. Each protein within the body has
a specific function. Some proteins are involved in structural support, while
others are involved in bodily movement, or in defense against germs. Proteins
vary in structure as well as function. They are constructed from a set of 20
amino acids and have distinct three-dimensional shapes. Below is a list of
several types of proteins and their functions.
Protein
Functions
Antibodies - are
specialized proteins involved in defending the body from antigens (foreign
invaders). They can travel through the blood stream and are utilized by the
immune system to identify and defend against bacteria, viruses, and other
foreign intruders. One way antibodies counteract antigens is by immobilizing
them so that they can be destroyed by white blood cells.
Contractile
Proteins - are responsible for movement. Examples include actin
and myosin. These proteins are involved in muscle contraction and movement.
Enzymes
- are proteins that facilitate biochemical reactions. They are often
referred to as catalysts because they speed up chemical reactions. Examples
include the enzymes lactase and pepsin. Lactase breaks down the sugar lactose
found in milk. Pepsin is a digestive enzyme that works in the stomach to break
down proteins in food.
Hormonal
Proteins - are messenger proteins which help to coordinate certain
bodily activities. Examples include insulin, oxytocin, and somatotropin.
Insulin regulates glucose metabolism by controlling the blood-sugar
concentration. Oxytocin stimulates contractions in females during childbirth.
Somatotropin is a growth hormone that stimulates protein production in muscle
cells.
Structural
Proteins - are fibrous and stringy and provide support. Examples
include keratin, collagen, and elastin. Keratins strengthen protective coverings
such as hair, quills, feathers, horns, and beaks. Collagens and elastin provide
support for connective tissues such as tendons and ligaments.
Storage
Proteins - store amino acids. Examples include ovalbumin and
casein. Ovalbumin is found in egg whites and casein is a milk-based protein.
Transport
Proteins - are carrier proteins which move molecules from one
place to another around the body. Examples include hemoglobin and cytochromes.
Hemoglobin transports oxygen through the blood. Cytochromes operate in the
electron transport chain as electron carrier proteins.
Proteins
are one of the key nutrients for success in terms of sports. Amino acids, the
building blocks of proteins, are used for building tissue, including muscle, as
well as repairing damaged tissues. Proteins usually only provide a small source
of fuel for the exercising muscles, being used as fuel typically only when
carbohydrates and lipid resources are low.
Animal sources of protein.
A wide range of foods are a source of protein. The best
combination of protein sources depends on the region of the world, access,
cost, amino acid types and nutrition balance, as well as acquired tastes. Some
foods are high in certain amino acids, but their digestibility and the anti-nutritional factors present in these
foods make them of limited value in human nutrition. Therefore, one must
consider digestibility and secondary nutrition profile such as calories,
cholesterol, vitamins and essential mineral density of the protein source. On a
worldwide basis, plant protein foods contribute over 60 percent of the per
capita supply of protein, on average. In
Meat, eggs and fish are sources of complete protein. Milk
and milk-derived foods are also good sources of protein.
Whole grains and cereals are another source of proteins.
However, these tend to be limiting in the amino acid lysine or threonine, which
are available in other vegetarian sources and meats. Examples of food staples
and cereal sources of protein, each with a concentration greater than 7
percent, are (in no particular order) buckwheat, oats, rye, millet, maize
(corn), rice, wheat, spaghetti, bulgar, sorghum, amaranth, and quinoa.
Vegetarian sources of proteins include legumes, nuts,
seeds and fruits. Legumes,
some of which are called pulses in certain parts of the world, have
higher concentrations of amino acids and are more complete sources of protein
than whole grains and cereals. Examples of vegetarian foods with protein
concentrations greater than 7 percent include soybeans, lentils, kidney beans,
white beans, mung beans, chickpeas, cowpeas, lima beans, pigeon peas, lupines,
wing beans, almonds, Brazil nuts, cashews, pecans, walnuts, cotton seeds,
pumpkin seeds, sesame seeds, and sunflower seeds.
Dietary
requirements
The amount of protein required in a person's diet is
determined in large part by overall energy intake, the body's need for nitrogen
and essential amino acids, body weight and composition, rate of growth in the
individual, physical activity level, individual's energy and carbohydrate
intake, as well as the presence of illness or injury. Physical activity and
exertion as well as enhanced muscular mass increase the need for protein. Requirements
are also greater during childhood for growth and development, during pregnancy
or when breast-feeding in order to nourish a baby, or when the body needs to
recover from malnutrition or trauma or after an operation.
If enough energy is not taken in through diet, as in the
process of starvation,
the body will use protein from the muscle mass to meet its energy needs,
leading to muscle wasting over time. If the individual does not consume
adequate protein in nutrition, then muscle will also waste as more vital
cellular processes (e.g. respiration enzymes, blood cells) recycle muscle
protein for their own requirements.
According to US & Canadian Dietary Reference Intake guidelines, women
aged 19–70 need to consume
Several studies have concluded that active people and
athletes may require elevated protein intake (compared to 0.8 g/kg) due to
increase in muscle mass and sweat losses, as well as need for body repair and
energy source. Suggested amounts vary between 1.6 g/kg and 1.8 g/kg, while a
proposed maximum daily protein intake would be approximately 25% of
energy requirements i.e. approximately 2 to 2.5 g/kg. However, many questions
still remain to be resolved.
The result of limited synthesis and normal rates
of protein degradation is that the balance of nitrogen intake and nitrogen
excretion is rapidly and significantly altered. Normal, healthy adults are
generally in nitrogen balance, with intake and excretion being very well
matched. Young growing children, adults recovering from major illness, and
pregnant women are often in positive nitrogen balance. Their intake of nitrogen
exceeds their loss as net protein synthesis proceeds. When more nitrogen is
excreted than is incorporated into the body, an individual is in negative
nitrogen balance. Insufficient quantities of even one essential amino acid is
adequate to turn an otherwise normal individual into one with a negative
nitrogen balance. The biological value of dietary proteins is related to the
extent to which they provide all the necessary amino acids. Proteins of animal
origin generally have a high biological value; plant proteins have a wide range
of values from almost none to quite high. In general, plant proteins are
deficient in lysine, methionine, and tryptophan and are much less concentrated
and less digestible than animal proteins. The absence of lysine in low-grade
cereal proteins, used as a dietary mainstay in many underdeveloped countries,
leads to an inability to synthesize protein (because of missing essential amino
acids) and ultimately to a syndrome known as kwashiorkor, common among children
in these countries.
A child in Africa suffering from kwashiorkor
– one of the three protein energy malnutrition ailments
afflicting over 10 million children in developing countries.
Protein deficiency and malnutrition can
lead to variety of ailments including mental retardation and kwashiorkor. Symptoms of
kwashiorkor include apathy, diarrhea, inactivity, failure to grow, flaky skin,
fatty liver, and edema of the belly and legs. This edema is explained by the
action of lipoxygenase on arachidonic acid to form leukotrienes and the normal
functioning of proteins in fluid balance and lipoprotein transport.
Although protein energy malnutrition is more common in
low-income countries, children from higher-income countries are also affected,
including children from large urban areas in low socioeconomic neighborhoods.
This may also occur in children with chronic diseases, and children who are
institutionalized or hospitalized for a different diagnosis. Risk factors
include a primary diagnosis of mental retardation, cystic fibrosis, malignancy,
cardiovascular disease, end stage renal disease, oncologic disease, genetic
disease, neurological disease, multiple diagnoses, or prolonged
hospitalization. In these conditions, the challenging nutritional management
may get overlooked and underestimated, resulting in an impairment of the
chances for recovery and the worsening of the situation.
Deficiency of protein leads to
following:
1. Shortage of protein leads to
retardation of growth and in extreme cases failure of growth. This is
manifested as marasmus and kwashiorkor among infants and children.
2. Protein deficiency affects the
intestinal mucosa and the gland that secret digestive enzymes. This results in
the failure to digest and absorb the food, consequently leading to diarrhea and
loss of fluid and electrolyte.
3. The normal structure and function of
liver is disturbed leading fat accumulation and fatty livers. Liver fails to
synthesis plasma albumin thus leading to Oedema.
4. Muscle wasting and anemia due to the
shortage of hemoglobin are common feature due to the deficiency of protein.
6. The amino acids presents in the protein
help in tissue synthesis during growth period e.g. infancy childhood and
adolescence. The body goes into negative N2 -balance due to the shortage of
protein in the diet. This results in muscle wastage
7. Proteins from important constituents
of hormones. How-ever the deficiency of proteins leads to no marked and
characteristic changes in the functioning of endocrine glands.
8. Proteins furnish 10-12per cent of
calories required daily. However the major part of proteins is essentially for
body-building purposes only.
Digestion of proteins starts in the
stomach and accomplishes in the small intestine. Several hormones take part in
protein digestion. They include trypsin, chymotrypsin, pepsin, etc.
Digestion of Protein in Stomach
Protein digestion does not start with chewing of food in
the mouth. It begins in the stomach. The stomach is especially designed for the
purpose of digestion of foods. Its walls are composed of strong muscles. These
muscles mix and churn the ingested food. They do it with the help of rhythmic
contractions, occurring at the average rate of 3 per min.
http://www.youtube.com/watch?v=AEsQxzeAry8
The lining of the stomach contains
glands. Their function is to secrete gastric juice. It is a colorless and
strong acidic liquid at a pH of 1-3. The main components of gastric juice are
digestive enzymes, hydrochloric acid and mucus.
Hydrochloric acid produced in the
stomach is a very strong acid. It is produced by the type of epithelial cells
called parietal cells present in the lining of the stomach. HCl is so strong
that it can easily digest the stomach itself. But such a destructive process is
prevented from occurring by another secretion of the stomach called mucus. It
protects the delicate cell lining of the stomach as well as moistens the food
present there. However, the cells in the stomach lining keep getting destroyed
by hydrochloric acid. It gets replaced by newer cells. According to studies,
the lining of the stomach gets completely replaced every third day. Protein
digestion in the stomach occurs mainly by the action of hydrochloric acid (HCl)
and enzyme called pepsin. The enzyme pepsin forms in the stomach when its
precursor pepsinogen reacts with HCl. Pepsin and HCl breaks the protein bonds.
The foods containing proteins are separated from each other. The proteins get
separated out, which is necessary for the action of enzymes. The enzymes needed
for digesting proteins are proteinases and proteases. These enzymes break down
the molecules of proteins into its constituents, amino acids by a
depolymerisation process called hydrolysis. It is described as a chemical
reaction wherein a water molecule breaks down into hydrogen cations and
hydroxide anions. The rate of action of these protein digestive enzymes is
influenced by a number of factors. Some of them are concentration and amount of
the enzyme, amount of protein food needed to be digested, temperature of the
food, acidity of the food, acidity of the stomach and presence of antacids or
other inhibitors of digestion. The task of enzymes is to breakdown of protein
molecules into simpler structures called peptones and proteose. They leave the
stomach and enter the small intestine with the help of peristalsis movement of
the body. It is called chyme. The entire process of protein digestion in the
stomach takes about 4 hours.
Digestion of Protein in Small Intestine
The chyme first enters duodenum, which
is a part of small intestine. It is a C-shaped structure about
Trypsin is a serine
protease found in the digestive
system of many vertebrates, where it hydrolyses
proteins.
Trypsin is produced in the pancreas as the inactive proenzyme
trypsinogen.
Trypsin cleaves peptide
chains mainly at the carboxyl side of the amino acids
lysine
or arginine,
except when either is followed by proline. It is used for numerous biotechnological
processes. The process is commonly referred to as trypsin proteolysis
or trypsinisation,
and proteins that have been digested/treated with trypsin are said to have been
trypsinized.
In the duodenum,
trypsin catalyzes
the hydrolysis
of peptide bonds,
breaking down proteins into smaller peptides. The peptide products are then
further hydrolyzed into amino acids via other
proteases, rendering them available for absorption into the blood
stream. Tryptic digestion is a necessary step in protein absorption as proteins
are generally too large to be absorbed through the lining of the small
intestine.
Trypsin is produced in the pancreas,
in the form of the inactive zymogen trypsinogen. When the pancreas is stimulated by
cholecystokinin,
it is then secreted into the first part of the small intestine (the duodenum)
via the pancreatic duct. Once in the small intestine,
the enzyme enteropeptidase activates it into trypsin by proteolytic cleavage. Auto catalysis does
not happen with trypsin since trypsinogen is a poor substrate for trypsin. This
activation mechanism is common for most serine proteases, and serves to prevent
autodegradation of the
pancreas.
Activation of trypsin from proteolytic cleavage of
trypsinogen in the pancreas can lead to a series of events that cause
pancreatic self-digestion, resulting in pancreatitis.
One consequence of the autosomal recessive disease cystic
fibrosis is a deficiency in transport of trypsin and other digestive
enzymes from the pancreas. This leads to the disorder termed meconium
ileus. This disorder involves intestinal obstruction (ileus) due to overly thick
meconium,
which is normally broken down by trypsins and other proteases, then passed in
faeces.
Trypsin is available in high quantity in
pancreases, and can be purified rather easily. Hence it has been used widely in
various biotechnological processes.
In a tissue
culture lab, trypsins are used to re-suspend cells adherent to the
cell culture dish wall during the process of harvesting cells. Some cell types
have a tendency to "stick" - or adhere - to the sides and bottom of a
dish when cultivated in vitro. Trypsin is used to cleave proteins bonding the
cultured cells to the dish, so that the cells can be suspended in fresh
solution and transferred to fresh dishes.
Trypsin can also be used to dissociate
dissected cells (for example, prior to cell fixing and sorting).
Trypsins can be used to break down
casein in breast milk. If trypsin is added to a solution of milk powder, the
breakdown of casein will cause the milk to become translucent.
The rate of reaction can be measured by using the amount of time it takes for
the milk to turn translucent.
Trypsin is commonly used in biological
research during proteomics experiments to digest proteins into peptides for
mass spectrometry analysis, e.g. in-gel
digestion. Trypsin is particularly suited for this, since it has a
very well defined specificity, as it hydrolyzes only the peptide bonds in which
the carbonyl group is contributed either by an Arg or
Trypsin can also be used to dissolve
blood clots in its microbial form and treat inflammation in its pancreatic
form.
THE KINDS OF
GASTRIC JUICE ACIDITY.
Gastric acid is a digestive
fluid, formed in the stomach. It has a pH of 1.5 to 3.5 and is composed of hydrochloric
acid (HCl) (around 0.5%, or 5000 parts per
million) as high as 0.1 N, and large quantities of potassium chloride (KCl) and sodium
chloride (NaCl). The acid plays a key role in digestion of proteins,
by activating digestive enzymes, and making ingested proteins
unravel so that digestive enzymes break down the long chains of amino acids.
Gastric acid is produced by cells lining the stomach,
which are coupled to systems to increase acid production when needed. Other
cells in the stomach produce bicarbonate, a base, to buffer
the fluid, ensuring that it does not become too acidic. These cells also
produce mucus,
which forms a viscous physical barrier to prevent
gastric acid from damaging the stomach. Cells in the beginning of the
small intestine, or duodenum, further produce large amounts of bicarbonate to
completely neutralize any gastric acid that passes further down into the
digestive tract.
Gastric acid is produced by parietal
cells (also called oxyntic cells) in the stomach. Its secretion is a
complex and relatively energetically expensive process. Parietal cells contain
an extensive secretory network (called canaliculi) from which the gastric acid is
secreted into the lumen of the stomach. These cells are part of epithelial
fundic glands
in the gastric mucosa. The pH of gastric acid is 1.35
to 3.5 [2]
in the human stomach lumen, the acidity being maintained by the proton pump
H+/K+ ATPase. The
parietal cell releases bicarbonate into the blood stream in the
process, which causes a temporary rise of pH in the blood, known as alkaline tide.
The resulting highly acidic environment in the stomach
lumen causes proteins
from food to lose their characteristic folded structure (or denature). This exposes the protein's peptide bonds.
The chief cells
of the stomach secrete enzymes for protein breakdown (inactive pepsinogen
and rennin).
Hydrochloric acid activates pepsinogen into the
enzyme
pepsin,
which then helps digestion by breaking the bonds linking amino acids,
a process known as proteolysis. In addition, many microorganisms
have their growth inhibited by such an acidic environment, which is helpful to
prevent infection.
In hypochlorhydria and achlorhydria,
there is low or no gastric acid in the stomach, potentially leading to problems
as the disinfectant
properties of the gastric lumen are decreased. In such conditions, there is
greater risk of infections of the digestive
tract (such as infection with Vibrio or Helicobacter
bacteria).
In Zollinger–Ellison syndrome and hypercalcemia,
there are increased gastrin levels, leading to excess gastric acid production,
which can cause gastric ulcers.
In diseases featuring excess vomiting, patients develop hypochloremic
metabolic alkalosis (decreased blood acidity by
H+
and chlorine
depletion).
Absorption of Amino Acids and Peptides
Dietary proteins are, with very few exceptions, not absorbed. Rather, they
must be digested into amino acids or di- and tripeptides first. In previous
sections, we've seen two sources secrete proteolytic enzymes into the lumen of
the digestive tube:
·
the stomach
secretes pepsinogen, which is converted to the active protease
pepsin by the action of acid.
·
the pancreas
secretes a group of potent proteases, chief among them trypsin,
chymotrypsin and carboxypeptidases.
Through the action of these gastric and pancreatic proteases, dietary
proteins are hydrolyzed within the lumen of the small intestine predominantly
into medium and small peptides (oligopeptides).
The brush border of the small intestine is equipped with a family of
peptidases. Like lactase and maltase, these peptidases are integral membrane
proteins rather than soluble enzymes. They function to further the hydrolysis
of lumenal peptides, converting them to free amino acids and very small
peptides. These endproducts of digestion, formed on the surface of the
enterocyte, are ready for absorption.
Absorption of Amino Acids
The mechanism by which amino acids are absorbed is conceptually
identical to that of monosaccharides. The lumenal plasma membrane of
the absorptive cell bears at least four sodium-dependent amino acid
transporters - one each for acidic, basic, neutral and amino acids. These
transporters bind amino acids only after binding sodium. The fully loaded
transporter then undergoes a conformational change that dumps sodium and the
amino acid into the cytoplasm, followed by its reorientation back to the
original form.
Thus, absorption of amino acids is also absolutely dependent on the
electrochemical gradient of sodium across the epithelium. Further, absorption
of amino acids, like that of monosaccharides, contributes to generating the
osmotic gradient that drives water absorption.
The basolateral membrane of the enterocyte contains additional transporters
which export amino acids from the cell into blood. These are not dependent on
sodium gradients.
Absorption of Peptides
There is virtually no absorption of peptides longer than four amino acids.
However, there is abundant absorption of di- and tripeptides in the small
intestine. These small peptides are absorbed into the small intestinal
epithelial cell by cotransport with H+ ions via a transporter called
PepT1.
Once inside the enterocyte, the vast bulk of absorbed di- and tripeptides
are digested into amino acids by cytoplasmic peptidases and exported from the
cell into blood. Only a very small number of these small peptides enter blood
intact.
Absorption of Intact Proteins
As emphasized, absorption of intact proteins occurs only in a few
circumstances. In the first place, very few proteins get through the gauntlet
of soluble and membrane-bound proteases intact. Second, "normal"
enterocytes do not have transporters to carry proteins across the plasma
membrane and they certainly cannot permeate tight junctions.
One important exception to these general statements is that for a very few
days after birth, neonates have the ability to absorb intact proteins. This
ability, which is rapidly lost, is of immense importance because it allows the
newborn animal to acquire passive immunity by absorbing immunoglobulins in
colostral milk.
In constrast to humans and rodents, there is no significant transfer of
antibodies across the placenta
in many animals (cattle, sheep, horses and pigs to name a few), and the young
are born without circulating antibodies. If fed colostrum during the first day
or so after birth, they absorb large quantities of immunoglobulins and acquire
a temporary immune system that provides protection until they generate their
own immune responses.
The small intestine rapidly loses the capacity to absorb intact proteins -
a process called closure - and consequently, animals that do not receive
colostrum within the first few days after birth will likely die due to
opportunistic infections.
General
pathways of amino acids transformation
Deamination of amino acids, it kinds.
The role of vitamins in deamination of amino acids.
Removal of an amino group from a
molecule.
the elimination of an amino group (NH2)
from organic compounds. Deamination is accompanied by the substitution of some
other group, such as H, OH, OR, or Hal, for the amino group or by the formation
of a double bond. In particular, deamination is brought about by the action of
nitrous acid on primary amines. In this reaction, acyclic amines yield alcohols
(I) and olefins (II), for example:
The deamination of alicyclic amines is accompanied by
ring expansion or contraction. In the presence of strong inorganic acids,
aromatic amines and nitrous acid yield diazonium salts. Such reactions as
hydrolysis, hydrogenolysis, decomposition of quaternary ammonium salts, and
pyrolytic reactions also result in deamination.
Deamination plays an important part in the life processes
of animals, plants, and microorganisms. Oxidative deamination, with the
formation of ammonia and α-keto acids, is characteristic of d-amino
acids. Amines also undergo oxidative deamination. Except for glutamate
dehydrogenase, which deaminates L-glutamic acid, oxidases of natural amino
acids are not very active in animal tissues. Therefore, most L-amino acids
undergo indirect deamination by means of prior transamination, with the
formation of glutamic acid, which then undergoes oxidative deamination or other
transformations. Other types of deamination are reductive, hydrolytic
(deamination of amino derivatives of purines, pyrimidines, and sugars), and
intramolecular (histidine deamination), which occur mainly in microorganisms.
Oxidative Deamination
Reaction
Deamination is also an oxidative reaction that occurs
under aerobic conditions in all tissues but especially the liver. During oxidative
deamination, an amino acid is converted into the corresponding keto acid by
the removal of the amine functional group as ammonia and the amine functional
group is replaced by the ketone group. The ammonia eventually goes into the
urea cycle.
Oxidative deamination occurs primarily on glutamic acid
because glutamic acid was the end product of many transamination reactions.
The glutamate dehydrogenase is allosterically controlled
by ATP and ADP. ATP acts as an inhibitor whereas ADP is an activator.
Central Role
for Glutamic Acid:
Apparently
most amino acids may be deaminated but this is a significant reaction only for
glutamic acid. If this is true, then how are the other amino acids deaminated?
The answer is that a combination of transamination and deamination of glutamic
acid occurs which is a recycling type of reaction for glutamic acid. The
original amino acid loses its amine group in the process. The general reaction
sequence is shown on the left.
Synthesis of
New Amino Acids:
Transamination of
amino acids, mechanism, role of enzymes and coenzymes.
In the degradation of most standard
amino acids, an early step in degradation consists in transamination, which is the transfer of the α-amino group from the
amino acid to an α-keto acid. There are several different
aminotransferases, each of which is specific for an
individual amino acid or for a group of chemically similar ones, such as the
branched amino acids leucine, isoleucine, and valine. The α-keto acid that
accepts the amino group is always α-ketoglutarate (Figure). Transamination
is freely reversible; therefore, both glutamate and α-ketoglutarate are
substrates of every single transaminase. If amino groups are to be transferred
between two amino acids other than glutamate, this will still occur by
transient formation of glutamate (Figure).
Transamination reactions. a: Glutamate pyruvate transaminase (also called
alanine amino transferase) transfers the α-amino group from alanine to
α-ketoglutarate, which yields glutamate and pyruvate. b: All transaminases
have α-ketoglutarate as one of their substrates. Transfer of amino groups
between arbitrary amino and α-keto acids (here: alanine and oxaloacetate)
occurs by transient transfer to α-ketoglutarate.
The
mechanism of transamination is depicted in Figure for alanine, yet is the same with all
transaminases. The reaction occurs in two stages:
1.
Transfer of the amino group from alanine
to the enzyme, which releases pyruvate, and
2.
Transfer of the amino group from the
enzyme to α-ketoglutarate, which releases glutamate.
Decarboxylisation of amino
acids, role of enzymes and co-enzymes.
Decarboxylation is a chemical
reaction that removes a carboxyl group and releases carbon
dioxide (CO2). Usually, decarboxylation refers to a
reaction of carboxylic acids, removing a carbon atom from a
carbon chain. The reverse process, which is the first chemical step in photosynthesis,
is called carboxylation, the addition of CO2
to a compound. Enzymes that catalyze decarboxylations are called decarboxylases
or, the more formal term, carboxy-lyases.
Amino acid Amine Function
serine → ethanolamine → Conversion
of phosphatidyl
serine
to phosphatidyl
ethanolamine in bacteria
lysine → cadaverine
arginine/ornithine → putrescine→ leads to spermine and
spermidine
S-adenosylmethionine → aminopropane donor→decarboxylase
contains pyruvate in place of PLP; leads to spermine and spermidine
histidine→histamine→vasodilator, inflammatory agent, stimulates
acid secretion in stomach; formed and stored for secretion in granulocytes,
e.g. mast cells
glutamate →gamma-aminobutyrate,
GABA →important
neurotransmitter in brain
3,4-dihydroxyphenylalanine (Dopa) →dopamine, hydroxytyramine→
inhibitory neurotransmitter in brain, precursor of catecholamines, melanin
5-hydroxytryptophan →serotonin →precursor of melatonin
phenylalanine→phenylethylamine → antidepressant, mild
amphetamine-like stimulant, present in chocolate
Key
Concepts:
Histamine
mediates a wide variety of physiological and pathological responses, such as inflammation,
gastric acid secretion, neurotransmission and immune modulation.
Histamine
is synthesised through decarboxylation of l‐histidine by histidine decarboxylase
and is catabolised through oxidative deamination or methylation.
Tissue
histamine levels are transiently increased by degranulation of mast cells and
basophils while they are upregulated by de novo synthesis in gastric
enterochromaffin‐like cells, and neurons.
Histamine
exerts its functions by acting on its specific receptors consisting of H1, H2,
H3 and H4 receptors.
Histamine
is a paracrine mediator, of which actions are generally limited in the local
microenvironment.
Histamine
H1 receptor antagonists have brought successful therapeutic approaches for
immediate allergy, because histamine evokes vasodilation and increased vascular
permeability by acting on the H1 receptor.
In
the central nervous system, histamine is involved in awakening, appetite,
maintenance of circadian rhythm, learning and memory, which are regulated by
the H1 and H3 receptors.
Histamine
stimulates parietal cells to induce gastric acid secretion by acting on the H2
receptor, of which antagonists drastically improved therapeutic approaches for
peptic ulcer.
Pre‐synaptic
histamine H3 receptor regulates release of various neurotransmitters including
histamine itself, and is expected as a potential drug target for the treatment
of cognitive dysfunctions.
Histamine
H4 receptor is expressed exclusively in blood cells and mediates their
chemotaxis in response to histamine.
Gamma-aminobutyric acid (GABA), a major inhibitory
neurotransmitter in the mammalian central nervous system, is produced from
glutamic acid in a reaction catalysed by glutamic acid decarboxylase. The
sequential actions of GABA-transaminase (converting GABA to succinic
semialdehyde) and succinic semialdehyde dehydrogenase (oxidizing succinic
semialdehyde to succinic acid) allow oxidative metabolism of GABA through the
tricarboxylic acid cycle.
Glutamate decarboxylase or glutamic acid
decarboxylase (GAD) is an enzyme that catalyzes the decarboxylation of
glutamate to GABA and CO2. GAD uses PLP as a cofactor. The reaction proceeds as
follows:
5-Hydroxytryptophan is decarboxylated to serotonin
(5-hydroxytryptamine or 5-HT) by the enzyme aromatic-L-amino-acid decarboxylase
with the help of vitamin B6. This reaction occurs both in nervous tissue and in
the liver. 5-HTP crosses the blood–brain barrier, while 5-HT does not. Excess
5-HTP, especially when administered with Vitamin B6, is thought to be
metabolized and excreted.
The main functions of serotonin are:
the regulation of mood, appetite, sleep, muscle
contraction, and 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.
Dopamine is synthesized in the body from within cells
(mainly by neurons and cells in the medulla of the adrenal glands) and can be
created from any one of the following three amino acids:
L-Phenylalanine (PHE)
L-Tyrosine (L-4-hydroxyphenylalanine; TYR)
L-DOPA (L-3,4-dihydroxyphenylalanine; DOPA)
Diagnostic
role of determination of AlAT and AsAT
Aspartate transaminase (AST),
also called aspartate aminotransferase (AspAT/ASAT/AAT) or serum
glutamic oxaloacetic transaminase (SGOT), is a pyridoxal phosphate (PLP)-dependent transaminase
enzyme. AST catalyzes the reversible transfer of an α-amino group between
aspartate and glutamate and, as such, is an important enzyme in amino acid
metabolism. AST is found in the liver, heart, skeletal muscle, kidneys,
brain,
and red blood cells, and it is commonly measured clinically as a marker for
liver health.
Two isoenzymes are present in a wide variety of
eukaryotes. In humans:
·
GOT1/cAST, the cytosolic
isoenzyme derives mainly from red blood
cells and heart.
·
GOT2/mAST, the mitochondrial
isoenzyme is present predominantly in liver.
These isoenzymes are thought to have evolved from a common ancestral AST
via gene duplication, and they share a sequence homology of approximately 45%.
AST has also been found in a number of microorganisms,
including E. coli,
H.
mediterranei, and T. thermophilus. In E. coli, the
enzyme is encoded by the aspCgene and has also been shown to exhibit the
activity of an aromatic-amino-acid transaminase.
AST is similar to alanine transaminase (ALT) in that both
enzymes are associated with liver parenchymal
cells. The difference is that ALT is found predominantly in the liver, with
clinically negligible quantities found in the kidneys, heart, and skeletal
muscle, while AST is found in the liver, heart (cardiac
muscle), skeletal muscle, kidneys, brain, and red blood cells. As a
result, ALT is a more specific indicator of liver inflammation
than AST, as AST may be elevated also in diseases affecting other organs, such
as myocardial infarction, acute pancreatitis, acute hemolytic
anemia, severe burns, acute renal
disease, musculoskeletal diseases, and trauma.
AST was defined as a biochemical marker
for the diagnosis of acute myocardial infarction in 1954. However, the use of
AST for such a diagnosis is now redundant and has been superseded by the cardiac
troponins.
AST (SGOT) is commonly measured
clinically as a part of diagnostic liver function tests, to determine liver
health.
Alanine transaminase
Alanine transaminase or ALT
is a transaminase
It is also called serum glutamic pyruvic transaminase (SGPT) or alanine
aminotransferase (ALAT).
ALT is found in serum
and in various bodily tissues, but is most commonly associated with the liver. It catalyzes the
two parts of the alanine cycle.
It is
commonly measured clinically as a part of a diagnostic evaluation of hepatocellular injury, to determine liver
health. When used in diagnostics, it is almost always measured in international
units/liter (U/L). While sources vary on specific normal range values for
patients, 10-40 U/L is the standard normal range for experimental studies.
Alanine transaminase shows a marked diurnal
variation.
Significantly elevated levels of
ALT(SGPT) often suggest the existence of other medical problems such as viral hepatitis,
diabetes,
congestive heart failure, liver damage, bile duct
problems, infectious mononucleosis, or myopathy.
For this reason, ALT is commonly used as a way of screening for liver problems.
Elevated ALT may also be caused by dietary choline deficiency. However,
elevated levels of ALT do not automatically mean that medical problems exist.
Fluctuation of ALT levels is normal over the course of the day, and ALT levels
can also increase in response to strenuous physical exercise.
Investigation of detoxification processes and
biosynthesis of urine. Hormonal adjusting and pathologies of proteins
metabolism.
Ammonia the highly toxic product of protein
catabolism, is rapidly inactivated by a variety of reactions. Some product of
these reactions are utilized for other purposes (thus salvaging a portion of
the amino nitrogen), while others are excreted. The excreted form varies quite
widely among vertebrate and invertebrate animals. The development of a pathway
for nitrogen disposal in a species appears to depend chiefly on the
availability of water.
Humans are totally dependent on other organisms for converting
atmospheric nitrogen into forms available to the body. Nitrogen fixation is
carried out by bacterial nitrogenases forming reduced nitrogen, NH4+ which can
then be used by all organisms to form amino acids.
Overview of the flow of nitrogen in the biosphere. Nitrogen, nitrites
and nitrates are acted upon by bacteria (nitrogen fixation) and plants and we
assimilate these compounds as protein in our diets. Ammonia incorporation in
animals occurs through the actions of glutamate dehydrogenase and glutamine
synthase. Glutamate plays the central role in mammalian nitrogen flow, serving
as both a nitrogen donor and nitrogen acceptor.
Reduced nitrogen enters the human body as dietary free amino acids,
protein, and the ammonia produced by intestinal tract bacteria. A pair of
principal enzymes, glutamate dehydrogenase and glutamine synthatase, are found
in all organisms and effect the conversion of ammonia into the amino acids
glutamate and glutamine, respectively. Amino and amide groups from these 2
substances are freely transferred to other carbon skeletons by transamination
and transamidation reactions.
Aminotransferases exist for all amino acids except threonine and lysine.
The most common compounds involved as a donor/acceptor pair in transamination
reactions are glutamate and α-KG, which participate in reactions with many
different aminotransferases. Serum aminotransferases such as aspartate
aminotransferase, AST and alanine transaminase, ALT have been used as clinical
markers of tissue damage, with increasing serum levels indicating an increased
extent of damage. Alanine transaminase has an important function in the
delivery of skeletal muscle carbon and nitrogen (in the form of alanine) to the
liver. In skeletal muscle, pyruvate is transaminated to alanine, thus affording
an additional route of nitrogen transport from muscle to liver. In the liver,
alanine transaminase transfers the ammonia to α-KG and regenerates
pyruvate. The pyruvate can then be diverted into gluconeogenesis. This process
is referred to as the glucose-alanine cycle.
The glucose-alanine cycle is used primarily as a
mechanism for skeletal muscle to eliminate nitrogen while replenishing its
energy supply. Glucose oxidation produces pyruvate which can undergo
transamination to alanine. This reaction is catalyzed by alanine transaminase,
ALT. Additionally, during periods of fasting, skeletal muscle protein is
degraded for the energy value of the amino acid carbons and alanine is a major
amino acid in protein. The alanine then enters the blood stream and is
transported to the liver. Within the liver alanine is converted back to
pyruvate which is then a source of carbon atoms for gluconeogenesis. The newly
formed glucose can then enter the blood for delivery back to the muscle. The
amino group transported from the muscle to the liver in the form of alanine is
converted to urea in the urea cycle and excreted.
The reaction catalyzed by glutamate dehydrogenase is:
The glutamate dehydrogenase utilizes both
nicotinamide nucleotide cofactors; NAD+ in the direction of nitrogen liberation
and NADP+ for nitrogen incorporation. In the forward reaction as shown above
glutamate dehydrogenase is important in converting free ammonia and α-KG
to glutamate, forming one of the 20 amino acids required for protein synthesis.
However, it should be recognized that the reverse reaction is a key anapleurotic
process linking amino acid metabolism with TCA cycle activity. In the reverse
reaction, glutamate dehydrogenase provides an oxidizable carbon source used for
the production of energy as well as a reduced electron carrier, NADH. As
expected for a branch point enzyme with an important link to energy metabolism,
glutamate dehydrogenase is regulated by the cell energy charge. ATP and GTP are
positive allosteric effectors of the formation of glutamate, whereas ADP and
GDP are positive allosteric effectors of the reverse reaction. Thus, when the
level of ATP is high, conversion of glutamate to α-KG and other TCA cycle
intermediates is limited; when the cellular energy charge is low, glutamate is
converted to ammonia and oxidizable TCA cycle intermediates. Glutamate is also
a principal amino donor to other amino acids in subsequent transamination
reactions. The multiple roles of glutamate in nitrogen balance make it a
gateway between free ammonia and the amino groups of most amino acids.
The Glutamine Synthetase Reaction:
The glutamine synthetase reaction is also important in several respects.
First it produces glutamine, one of the 20 major amino acids. Second, in
animals, glutamine is the major amino acid found in the circulatory system. Its
role there is to carry ammonia to and from various tissues but principally from
peripheral tissues to the kidney, where the amide nitrogen is hydrolyzed by the
enzyme glutaminase (reaction below); this process regenerates glutamate and
free ammonium ion, which is excreted in the urine.
Note that, in this function, ammonia arising in peripheral tissue is
carried in a non-ionizable form which has none of the neurotoxic or
alkalosis-generating properties of free ammonia.
Liver contains both glutamine synthetase and
glutaminase but the enzymes are localized in different cellular segments. This
ensures that the liver is neither a net producer nor consumer of glutamine. The
differences in cellular location of these two enzymes allows the liver to
scavenge ammonia that has not been incorporated into urea. The enzymes of the
urea cycle are located in the same cells as those that contain glutaminase. The
result of the differential distribution of these two hepatic enzymes makes it
possible to control ammonia incorporation into either urea or glutamine, the
latter leads to excretion of ammonia by the kidney.
When acidosis occurs the body will divert more glutamine from the liver
to the kidney. This allows for the conservation of bicarbonate ion since the
incorporation of ammonia into urea requires bicarbonate (see below). When
glutamine enters the kidney, glutaminase releases one mole of ammonia
generating glutamate and then glutamate dehydrogenase releases another mole of
ammonia generating α-KG. The ammonia will ionizes to ammonium ion (NH4+)
which is excreted. The net effect is a reduction in the concentration of
hydrogen ion, [H+], and thus an increase in the pH.
Earlier it was noted that kidney glutaminase was responsible for
converting excess glutamine from the liver to urine ammonium. However, about
80% of the excreted nitrogen is in the form of urea which is also largely made
in the liver, in a series of reactions that are distributed between the
mitochondrial matrix and the cytosol. The series of reactions that form urea is
known as the Urea Cycle or the Krebs-Henseleit Cycle.
Enzymes:
1:
carbamoyl phosphate synthetase-I (CPS-I)
2:
ornithine transcarbamoylase (OTC)
3:
argininosuccinate synthetase
4:
argininosuccinase
5:
arginase
http://www.youtube.com/watch?v=AoBbVu5rnMs&feature=related
The essential features of the urea cycle
reactions and their metabolic regulation are as follows: arginine from the diet
or from protein breakdown is cleaved by the cytosolic enzyme arginase,
generating urea and ornithine. In subsequent reactions of the urea cycle a new
urea residue is built on the ornithine, regenerating arginine and perpetuating
the cycle.
Ornithine arising in the cytosol is transported
to the mitochondrial matrix, where ornithine transcabamoylase catalyzes the
condensation of ornithine with carbamoyl phosphate, producing citrulline. The
energy for the reaction is provided by the high-energy anhydride of carbamoyl
phosphate.
The product, citrulline, is then transported to
the cytosol, where the remaining reactions of the cycle take place. The
synthesis of citrulline requires a prior synthesis of carbamoyl phosphate (CP).
The activation step requires 2 equivalents of ATP
and the mitochondrial matrix enzyme carbamoyl phosphate synthetase-I (CPS-I)
(see reaction mechanism).There are two CP synthetases: a mitochondrial enzyme,
CPS-I, which forms CP destined for inclusion in the urea cycle, and a cytosolic
CP synthatase (CPS-II), which is involved in pyrimidine nucleotide
biosynthesis. CPS-I is positively regulated by the allosteric effector
N-acetyl-glutamate, while the cytosolic enzyme is acetylglutamate independent.
In a 2-step reaction, catalyzed by cytosolic
argininosuccinate synthetase, citrulline is converted to argininosuccinate.
The reaction involves the addition of AMP (from
ATP) to the amido carbonyl of citrulline, forming an activated intermediate on the
enzyme surface (AMP-citrulline), and the subsequent addition of aspartate to
form argininosuccinate.
Arginine and fumarate are produced from
argininosuccinate by the cytosolic enzyme argininosuccinate lyase.
In the final step of the cycle arginase cleaves
urea from aspartate, regenerating cytosolic ornithine, which can be transported
to the mitochondrial matrix for another round of urea synthesis.
Beginning and ending with ornithine, the
reactions of the cycle consumes 3 equivalents of ATP and a total of 4
high-energy nucleotide phosphates. Urea is the only new compound generated by
the cycle; all other intermediates and reactants are recycled.
The energy consumed in the production of urea is
more than recovered by the release of energy formed during the synthesis of the
urea cycle intermediates. Ammonia released during the glutamate dehydrogenase
reaction is coupled to the formation of NADH. In addition, when fumarate is
converted back to aspartate, the malate dehydrogenase reaction used to convert
malate to oxaloacetate generates a mole of NADH. These two moles of NADH, thus,
are oxidized in the mitochondria yielding 6 moles of ATP.
Regulation of
the Urea Cycle
The urea cycle operates only to eliminate excess nitrogen.
On high-protein diets the carbon skeletons of the amino acids are oxidized for
energy or stored as fat and glycogen, but the amino nitrogen must be excreted.
To facilitate this process, enzymes of the urea cycle are controlled at the
gene level. With long-term changes in the quantity of dietary protein, changes
of 20-fold or greater in the concentration of cycle enzymes are observed. When
dietary proteins increase significantly, enzyme concentrations rise. On return
to a balanced diet, enzyme levels decline. Under conditions of starvation,
enzyme levels rise as proteins are degraded and amino acid carbon skeletons are
used to provide energy, thus increasing the quantity of nitrogen that must be
excreted.
Short-term regulation of the cycle occurs principally at CPS-I, which is
relatively inactive in the absence of its allosteric activator
N-acetylglutamate. The steady-state concentration of N-acetylglutamate is set
by the concentration of its components acetyl-CoA and glutamate and by
arginine, which is a positive allosteric effector of N-acetylglutamate
synthetase (glutamate transacylase).
A complete lack of any one of the enzymes of the urea cycle will result
in death shortly after birth. However, deficiencies in each of the enzymes of
the urea cycle, including N-acetylglutamate synthase, have been identified.
These disorders are referred to as urea cycle disorders or UCDs. A common
thread to most UCDs is hyperammonemia leading to ammonia intoxication with the
consequences described below. Deficiencies in arginase do not lead to
symptomatic hyperammonemia as severe or as commonly as in the other UCDs.
Clinical symptoms are most severe when the UCD is at the level of carbamoyl
phosphate synthetase I. Symptoms of UCDs usually arise at birth and encompass,
ataxia, convulsions, lethargy, poor feeding and eventually coma and death if
not recognized and treated properly. In fact, the mortality rate is 100% for
UCDs that are left undiagnosed. Several UCDs manifest with late-onset such as
in adulthood. In these cases the symptoms are hyperactivity, hepatomegaly and
an avoidance of high protein foods.
In general, the treatment of UCDs has as common elements the reduction
of protein in the diet, removal of excess ammonia and replacement of
intermediates missing from the urea cycle. Administration of levulose reduces
ammonia through its action of acidifying the colon. Bacteria metabolize
levulose to acidic byproducts which then promotes excretion of ammonia in the
feces as ammonium ions, NH4+. Antibiotics can be administered to
kill intestinal ammonia producing bacteria. Sodium benzoate and sodium
phenylbutyrate can be administered to covalently bind glycine (forming
hippurate) and glutamine (forming phenylacetylglutamine), respectively. These
latter compounds, which contain the ammonia nitrogen, are excreted in the
feces. Dietary supplementation with arginine or citrulline can increase the
rate of urea production in certain UCDs.
AMINO ACID
BIOSYNTHESIS
Glutamate and
Aspartate
Glutamate and aspartate are synthesized from their widely distributed
α-keto acid precursors by simple transamination reactions. The former
catalyzed by glutamate dehydrogenase and the latter by aspartate aminotransferase,
AST. Aspartate is also derived from asparagine through the action of
asparaginase. The importance of glutamate as a common intracellular amino donor
for transamination reactions and of aspartate as a precursor of ornithine for
the urea cycle.
Alanine and
the Glucose-Alanine Cycle
Aside from its role in protein synthesis, alanine
is second only to glutamine in prominence as a circulating amino acid. In this
capacity it serves a unique role in the transfer of nitrogen from peripheral
tissue to the liver. Alanine is transferred to the circulation by many tissues,
but mainly by muscle, in which alanine is formed from pyruvate at a rate
proportional to intracellular pyruvate levels. Liver accumulates plasma
alanine, reverses the transamination that occurs in muscle, and proportionately
increases urea production. The pyruvate is either oxidized or converted to
glucose via gluconeogenesis. When alanine transfer from muscle to liver is
coupled with glucose transport from liver back to muscle, the process is known
as the glucose-alanine cycle. The key feature of the cycle is that in 1
molecule, alanine, peripheral tissue exports pyruvate and ammonia (which are
potentially rate-limiting for metabolism) to the liver, where the carbon
skeleton is recycled and most nitrogen eliminated. There are 2 main pathways to
production of muscle alanine: directly from protein degradation, and via the
transamination of pyruvate by glutamate-pyruvate aminotransferase.
Cysteine
Biosynthesis
The sulfur for cysteine synthesis comes from the essential amino acid
methionine. A condensation of ATP and methionine catalyzed by methionine
adenosyltransferase S-adenosylmethionine (SAM or AdoMet).
Biosynthesis
of S-adenosylmethionine, SAM
SAM serves as a precurosor for numerous methyl transfer reactions the
conversion of norepinephrine to epinenephrine. The result of methyl transfer is
the conversion of SAM to S-adenosylhomocysteine. S-adenosylhomocysteine is then
cleaved by adenosylhomocyteinase to yield homocysteine and adenosine.
Homocysteine can be converted back to methionine by methionine synthase, a
reaction that occurs under methionine-sparing conditions and requires
N5-methyl-tetrahydrofolate as methyl donor. This reaction was discussed in the
context of vitamin B12-requiring enzymes. Transmethylation reactions employing
SAM are extremely important, but in this case the role of S-adenosylmethionine
in transmethylation is secondary to the production of homocysteine (essentially
a by-product of transmethylase activity). In the production of SAM all
phosphates of an ATP are lost: one as Pi and two as PPi. It is adenosine which
is transferred to methionine and not AMP. In cysteine synthesis, homocysteine
condenses with serine to produce cystathionine, which is subsequently cleaved
by cystathionase to produce cysteine and α-ketobutyrate.
The sum of the latter two reactions is known as trans-sulfuration.
Cysteine is used for protein synthesis and other body needs, while the α-ketobutyrate
is decarboxylated and converted to propionyl-CoA. While cysteine readily
oxidizes in air to form the disulfide cystine, cells contain little if any free
cystine because the ubiquitous reducing agent, glutathione effectively reverses
the formation of cystine by a non-enzymatic reduction reaction.
Utilization
of methionine in the synthesis of cysteine
The 2 key enzymes of this pathway, cystathionine synthase and
cystathionase (cystathionine lyase), both use pyridoxal phosphate as a
cofactor, and both are under regulatory control. Cystathionase is under
negative allosteric control by cysteine, as well, cysteine inhibits the
expression of the cystathionine synthase gene. Genetic defects are known for
both the synthase and the lyase. Missing or impaired cystathionine synthase
leads to homocystinuria and is often associated with mental retardation,
although the complete syndrome is multifaceted and many individuals with this
disease are mentally normal. Some instances of genetic homocystinuria respond
favorably to pyridoxine therapy, suggesting that in these cases the defect in
cystathionine synthase is a decreased affinity for the cofactor. Missing or
impaired cystathionase leads to excretion of cystathionine in the urine but
does not have any other untoward effects. Rare cases are known in which
cystathionase is defective and operates at a low level. This genetic disease
leads to methioninuria with no other consequences.
Tyrosine
Biosynthesis
Tyrosine is produced in cells by hydroxylating
the essential amino acid phenylalanine. This relationship is much like that
between cysteine and methionine. Half of the phenylalanine required goes into the
production of tyrosine; if the diet is rich in tyrosine itself, the
requirements for phenylalanine are reduced by about 50%. Phenylalanine
hydroxylase is a mixed-function oxygenase: one atom of oxygen is incorporated
into water and the other into the hydroxyl of tyrosine. The reductant is the
tetrahydrofolate-related cofactor tetrahydrobiopterin, which is maintained in
the reduced state by the NADH-dependent enzyme dihydropteridine reductase.
Biosynthesis
of tyrosine from phenylalanine
Missing or deficient phenylalanine hydroxylase
leads to the genetic disease known as phenlyketonuria (PKU), which if untreated
leads to severe mental retardation. The mental retardation is caused by the
accumulation of phenylalanine, which becomes a major donor of amino groups in
aminotransferase activity and depletes neural tissue of α-ketoglutarate.
This absence of α-ketoglutarate in the brain shuts down the TCA cycle and
the associated production of aerobic energy, which is essential to normal brain
development.
Ornithine and
Proline Biosynthesis
Glutamate is the precursor of both proline and ornithine, with glutamate
semialdehyde being a branch point intermediate leading to one or the other of
these 2 products. While ornithine is not one of the 20 amino acids used in
protein synthesis, it plays a significant role as the acceptor of carbamoyl
phosphate in the urea cycle. Ornithine serves an additional important role as
the precursor for the synthesis of the polyamines. The production of ornithine
from glutamate is important when dietary arginine, the other principal source
of ornithine, is limited. The fate of glutamate semialdehyde depends on
prevailing cellular conditions. Ornithine production occurs from the
semialdehyde via a simple glutamate-dependent transamination, producing
ornithine.
Ornithine
synthesis from glutamate
Serine
Biosynthesis
The main pathway to serine starts with the glycolytic intermediate
3-phosphoglycerate.
An NADH-linked dehydrogenase converts 3-phosphoglycerate into a keto
acid, 3-phosphopyruvate, suitable for subsequent transamination. Aminotransferase
activity with glutamate as a donor produces 3-phosphoserine, which is converted
to serine by phosphoserine phosphatase.
Glycine
Biosynthesis
The main pathway to glycine is a 1-step reaction catalyzed by serine
hydroxymethyltransferase.
This reaction involves the transfer of the hydroxymethyl group from
serine to the cofactor tetrahydrofolate (THF), producing glycine and
N5,N10-methylene-THF. Glycine produced from serine or from the diet can also be
oxidized by glycine cleavage complex, GCC, to yield a second equivalent of
N5,N10-methylene-tetrahydrofolate as well as ammonia and CO2.
Glycine is involved in many anabolic reactions other than protein
synthesis including the synthesis of purine nucleotides, heme, glutathione,
creatine and serine.
Aspartate/Asparagine
and Glutamate/Glutamine Biosynthesis
Glutamate is synthesized by the reductive amination of
α-ketoglutarate catalyzed by glutamate dehydrogenase; it is thus a
nitrogen-fixing reaction. In addition, glutamate arises by aminotransferase
reactions, with the amino nitrogen being donated by a number of different amino
acids. Thus, glutamate is a general collector of amino nitrogen. Aspartate is
formed in a transamintion reaction catalyzed by aspartate transaminase, AST.
This reaction uses the aspartate α-keto acid analog, oxaloacetate, and glutamate
as the amino donor. Aspartate can also be formed by deamination of asparagine
catalyzed by asparaginase. Asparagine synthetase and glutamine synthetase,
catalyze the production of asparagine and glutamine from their respective
α-amino
acids. Glutamine is produced from glutamate by the direct incorporation of
ammonia; and this can be considered another nitrogen fixing reaction.
Asparagine, however, is formed by an amidotransferase reaction.
Aminotransferase reactions are readily reversible. The direction of any
individual transamination depends principally on the concentration ratio of
reactants and products. By contrast, transamidation reactions, which are
dependent on ATP, are considered irreversible. As a consequence, the
degradation of asparagine and glutamine take place by a hydrolytic pathway
rather than by a reversal of the pathway by which they were formed. As
indicated above, asparagine can be degraded to aspartate.
PATHWAYS OF
AMINO ACID DEGRADATION
There are 20 standard amino acids in proteins,
with a variety of carbon skeletons. Correspondingly, there are 20 different
catabolic pathways for amino acid degradation. In humans, these pathways taken
together normally account for only 10 to 15% of the body's energy production.
Therefore, the individual amino acid degradative pathways are not nearly as
active as glycolysis and fatty acid oxidation. In addition, the activity of the
catabolic pathways can vary greatly from one amino acid to the next, depending
upon the balance between requirements for biosynthetic processes and the
amounts of a given amino acid available. For this reason, we shall not examine
them all in detail. The 20 catabolic pathways converge to form only five
products, all of which enter the citric acid cycle. From here the carbons can
be diverted to gluconeogenesis or ketogenesis, or they can be completely
oxidized to CO2 and H2O.
All or part of the carbon skeletons of ten of the
amino acids are ultimately broken down to yield acetyl-CoA. Five amino acids
are converted into α-ketoglutarate, four into succinyl-CoA, two into
fumarate, and two into oxaloacetate. The individual pathways for the 20 amino
acids will be summarized by means of flow diagrams, each leading to a specific
point of entry into the citric acid cycle. In these diagrams the amino acid
carbon atoms that enter the citric acid cycle are shown in color. Note that
some amino acids appear more than once, reflecting the fact that different
parts of their carbon skeletons have different fates. Some of the enzymatic reactions
in these pathways that are particularly noteworthy for their mechanisms or
their medical significance will be singled out for special discussion.
Arginine
Catabolism
The catabolism of arginine begins within the
context of the urea cycle. It is hydrolyzed to urea and ornithine by
arginase.
The Arginine
Metabolic Pathways: Nitric Oxide Synthase and Arginase
Nitric Oxide
Synthases (NOS)
Is an important cellular-signaling molecule, a potent vasodilatator due
to the smooth muscle relaxation. It also inhibits platelet adherence and
aggregation, reduces adherence of leukocytes to the endothelium. Furthermore,
NO has been shown to inhibit DNA synthesis and mitogenesis, and the
proliferation of vascular smooth muscle cells. These antiproliferative effects
are likely to be mediated by cyclic GMP.
Nitric Oxide Synthases from the biochemical point of view, are a family
of complex enzymes catalyzing the oxidation of L-arginine to form NO and
L-citrulline. The three human NOS isoforms identified to date are: eNOS
(endothelial NOS), nNOS (neuronal NOS), and iNOS (inducible NOS). Their genes
are found on human chromosomes 7, 12, and 17, respectively, and so they were
named for the tissue in which they were first cloned and characterized.
vasculoprotective effect of individual NOS isoforms in human organism is not
sufficiently clarified yet. Endothelial NOS (eNOS) and neuronal NOS (nNOS) are
constitutively expressed, mainly in endothelial cells and nitrergic nerves,
respectively, synthesizing a small amount of NO under basal conditions and on
stimulation by various agonists. By contrast, inducible NOS (iNOS) is expressed
when stimulated by inflammatory stimuli, synthesizing a large amount of NO in a
transient manner. The knowledge of nitric oxide synthases (NOSs) is of extreme
scientific importance, not only for understanding new pathophysiological
mechanisms but also as a target for therapeutic intervention.
The role of NO in regulating vascular tone and
mediating platelet function is attributable to the ongoing activity of eNOS. It
is pharmacologically identical with previously isolated EDRF
(endothelium-derived releasing factor), exprimed by the intact endothelium.
Inactivation of the eNOS pathway limits the contribution of NO to vessel
homeostasis and results in increased vascular tone and platelet adhesion and
aggregation. The activity of eNOS is regulated by the intracellular free
calcium concentration and calcium- calmodulin complexes. Endothelial NOS is a
constitutively expressed protein predominantly associated with the particulate
subcellular fraction, suggesting that the native enzyme is a membrane-bound
protein. A detailed analysis of the membrane association of eNOS showed that
this enzyme is localized to the Golgi apparatus as well as to specific structures
in the plasmalemmal membrane called caveolae. The association of eNOS with a
region of the plasma membrane in which several key signal-transducing complexes
are concentrated (such as G-proteins) is likely to have profound repercussions
on enzyme activity as well as on its accessibility to intracellular mechanisms
of the pathway release, including mechanisms independent of intracellular
calcium release.
Glycine
Catabolism
Glycine is classified as a glucogenic amino acid,
since it can be converted to serine by serine hydroxymethyltransferase, and
serine can be converted back to the glycolytic intermediate, 3-phosphoglycerate
or to pyruvate by serine/threonine dehydratase.
Nevertheless, the main glycine catabolic pathway leads to the production
of CO2, ammonia, and one equivalent of N5,N10-methyleneTHF by the
mitochondrial glycine cleavage complex.
Hyperglycinemia refers to a condition where
glycine is elevated in the blood.
Types include:
-
Propionic acidemia, also known as
"ketotic glycinemia"
-
Glycine encephalopathy, also known as
"non-ketotic hyperglycinemia".
Glycine encephalopathy (also known as non-ketotic hyperglycinemia or
NKH) is a rare autosomal recessive disorder of glycine metabolism. After
phenylketonuria, glycine encephalopathy is the second most common disorder of
amino acid metabolism. The disease is caused by defects in the glycine cleavage
system, an enzyme responsible for glycine catabolism. There are several forms
of the disease, with varying severity of symptoms and time of onset. The
symptoms are exclusively neurological in nature, and clinically this disorder
is characterized by abnormally high levels of the amino acid glycine in bodily
fluids and tissues, especially the cerebral spinal fluid.
Glutamine/Glutamate
and Asparagine/Aspartate Catabolism
Glutaminase is an important kidney tubule enzyme involved in converting
glutamine (from liver and from other tissue) to glutamate and NH3+, with the
NH3+ being excreted in the urine. Glutaminase activity is present in many other
tissues as well, although its activity is not nearly as prominent as in the
kidney. The glutamate produced from glutamine is converted to -ketoglutarate,
making glutamine a glucogenic amino acid. Asparaginase is also widely
distributed within the body, where it converts asparagine into ammonia and
aspartate. Aspartate transaminates to oxaloacetate, which follows the
gluconeogenic pathway to glucose. Glutamate and aspartate are important in
collecting and eliminating amino nitrogen via glutamine synthetase and the urea
cycle, respectively. The catabolic path of the carbon skeletons involves simple
1-step aminotransferase reactions that directly produce net quantities of a TCA
cycle intermediate. The glutamate dehydrogenase reaction operating in the
direction of -ketoglutarate
production provides a second avenue leading from glutamate to gluconeogenesis.
Alanine
Catabolism
Alanine is also important in intertissue nitrogen transport as part of
the glucose-alanine cycle. Alanine's catabolic pathway involves a simple
aminotransferase reaction that directly produces pyruvate. Generally pyruvate
produced by this pathway will result in the formation of oxaloacetate, although
when the energy charge of a cell is low the pyruvate will be oxidized to CO2
and H2O via the PDH complex and the TCA cycle. This makes alanine a glucogenic
amino acid.
Proline
Catabolism
Glutamine is converted to glutamate by
glutaminase or several other enzymes by the removal of the amide nitrogen. Proline
is first converted to a Schiff base and then converted by hydrolysis to
glutamate-5-semialdehyde. All of these changes occur on the same carbon.
Arginine and histidine contain 5adjacent carbons and a sixth carbon attached
through a nitrogen attom. The catabolism of these amino acids is thus slightly
more complicated than glutamine or proline. Arginine is converted to ornithine
and urea. Ornithine is furthere transaminated to produce
glutamate-5-semialdehyde. Glutamate-5-semialdehyde is converted to glutamate.
The enzymes involved in the steps of the histidine pathway are listed in the
box in the lower right corner of the diagram. Tetrahydrofolate is the cofactor
in the final step converting histidine to glutamate. Transamination or
deamination of glutamate produces a-ketoglutarate which feeds into the citric
acid cycle.
Glutamate semialdehyde can serve as the precursor for proline
biosynthesis as described above or it can be converted to glutamate. Proline
catabolism is a reversal of its synthesis process. The glutamate semialdehyde
generated from ornithine and proline catabolism is oxidized to glutamate by an
ATP-independent glutamate semialdehyde dehydrogenase. The glutamate can then be
converted to ketoglutarate
in a transamination reaction. Thus arginine, ornithine and proline, are
glucogenic.
Hyperprolinemia is an excess of a particular
protein building block (amino acid), called proline, in the blood. This
condition generally occurs when proline is not broken down properly by the
body. There are two inherited forms of hyperprolinemia, called type I and type
II.
Valine,
Leucine and Isoleucine Catabolism
This group of essential amino acids are identified as the branched-chain
amino acids, BCAAs. Because this arrangement of carbon atoms cannot be made by
humans, these amino acids are an essential element in the diet. The catabolism
of all three compounds initiates in muscle and yields NADH and FADH2 which can
be utilized for ATP generation. The catabolism of all three of these amino
acids uses the same enzymes in the first two steps. The first step in each case
is a transamination using a single BCAA aminotransferase, with
α-ketoglutarate as amine acceptor. As a result, three different
α-keto acids are produced and are oxidized using a common branched-chain
α-keto acid dehydrogenase, yielding the three different CoA derivatives.
Subsequently the metabolic pathways diverge, producing many intermediates. The
principal product from valine is propionylCoA, the glucogenic precursor of
succinyl-CoA.
Isoleucine catabolism terminates with production
of acetylCoA and propionylCoA; thus isoleucine is both glucogenic and
ketogenic. Leucine gives rise to acetylCoA and acetoacetylCoA, and is thus
classified as strictly ketogenic. There are a number of genetic diseases
associated with faulty catabolism of the BCAAs. The most common defect is in
the branched-chain α-keto acid dehydrogenase.
Since there is only one dehydrogenase enzyme for all three amino acids,
all three α-keto
acids accumulate and are excreted in the urine. The disease is known as Maple
syrup urine disease because of the characteristic odor of the urine in
afflicted individuals. Mental retardation in these cases is extensive.
Unfortunately, since these are essential amino acids, they cannot be heavily
restricted in the diet; ultimately, the life of afflicted individuals is short
and development is abnormal The main neurological problems are due to poor
formation of myelin in the CNS.
Threonine
Catabolism
There are at least 3 pathways for threonine catabolism. One involves a
pathway initiated by threonine dehydrogenase yielding a-amino-b-ketobutyrate.
The α-amino-β-ketobutyrate is either converted to acetyl-CoA and
glycine or spontaneously degrades to aminoacetone which is converted to
pyruvate. The second pathway involves serine/threonine dehydratase yielding
α-ketobutyrate which is further catabolized to propionyl-CoA and finally
the TCA cycle intermediate, succinyl-CoA. The third pathway utilizes threonine
aldolase. The products of this reaction are both ketogenic (acetyl-CoA) and
glucogenic (pyruvate).
Methionine
Catabolism
The principal fates of the essential amino acid methionine are
incorporation into polypeptide chains, and use in the production of
α-ketobutyrate and cysteine via SAM as described above. The
transulfuration reactions that produce cysteine from homocysteine and serine
also produce α-ketobutyrate, the latter being converted to succinyl-CoA.
Regulation of the methionine metabolic pathway is based on the availability of
methionine and cysteine.
If both amino acids are present in adequate quantities, SAM accumulates
and is a positive effector on cystathionine synthase, encouraging the
production of cysteine and α-ketobutyrate (both of which are
glucogenic). However, if methionine is scarce, SAM will form only in small
quantities, thus limiting cystathionine synthase activity. Under these
conditions accumulated homocysteine is remethylated to methionine, using
N5-methylTHF and other compounds as methyl donors.
Cysteine Catabolism
There are several pathways for
cysteine catabolism. The simplest, but least important pathway is catalyzed by
a liver desulfurase and produces hydrogen sulfide, (H2S) and pyruvate.
The more important catabolic pathway is via a cytochrome-P450-coupled enzyme,
cysteine dioxygenase that oxidizes the cysteine sulfhydryl to sulfinate,
producing the intermediate cysteinesulfinate. Cysteinesulfinate can serve as a
biosynthetic intermediate undergoing decarboxylation and oxidation to produce
taurine. Catabolism of cysteinesulfinate proceeds through transamination to
b-sulfinylpyruvate which is in undergoes desulfuration yielding bisulfite, (HSO3-)
and the glucogenic product, pyruvate. The enzyme sulfite oxidase uses O2
and H2O to convert HSO3- to sulfate, (SO4-)
and H2O2. The resultant sulfate is used as a precursor
for the formation of 3'-phosphoadenosine-5'-phosphosulfate,PAPS.
PAPS is used for the transfer
of sulfate to biological molecules such as the sugars of the
glycosphingolipids.Other than protein, the most important product of cysteine
metabolism is the bile salt precursor taurine, which is used to form the bile
acid conjugates taurocholate and taurochenodeoxycholate.The enzyme
cystathionase can also transfer the sulfur from one cysteine to another
generating thiocysteine and pyruvate. Transamination of cysteine yields -mercaptopyruvate
which then reacts with sulfite, (SO32-), to produce thiosulfate, (S2O32-) and
pyruvate. Both thiocysteine and thiosulfate can be used by the enzyme rhodanese
to incorporate sulfur into cyanide, (CN-), thereby detoxifying the cyanide to
thiocyanate.
Phenylalanine and Tyrosine Catabolism
Phenylalanine normally has
only two fates: incorporation into polypeptide chains, and production of
tyrosine via the tetrahydrobiopterin-requiring phenylalanine hydroxylase. Thus,
phenylalanine catabolism always follows the pathway of tyrosine catabolism. The
main pathway for tyrosine degradation involves conversion to fumarate and
acetoacetate, allowing phenylalanine and tyrosine to be classified as both
glucogenic and ketogenic. Tyrosine is equally important for protein biosynthesis
as well as an intermediate in the biosynthesis of several physiologically
important metabolites e.g. dopamine, norepinephrine and epinephrine.
http://www.youtube.com/watch?v=hpaki7F4HR0
http://www.youtube.com/watch?v=CWfrVS4Bm1Y&feature=related
As in phenylketonuria
(deficiency of phenylalanine hydroxylase), deficiency of tyrosine transaminase
leads to urinary excretion of tyrosine and the intermediates between
phenylalanine and tyrosine.
Phenylketonuria
(commonly known as PKU) is an inherited disorder that increases the levels of a
substance called phenylalanine in the blood. Phenylalanine is a building block of
proteins (an amino acid) that is obtained through the diet. It is found in all
proteins and in some artificial sweeteners. If PKU is not treated,
phenylalanine can build up to harmful levels in the body, causing intellectual
disability and other serious health problems.
The signs and
symptoms of PKU vary from mild to severe. The most severe form of this disorder
is known as classic PKU. Infants with classic PKU appear normal until they are
a few months old. Without treatment, these children develop permanent
intellectual disability. Seizures, delayed development, behavioral problems,
and psychiatric disorders are also common. Untreated individuals may have a
musty or mouse-like odor as a side effect of excess phenylalanine in the body.
Children with classic PKU tend to have lighter skin and hair than unaffected
family members and are also likely to have skin disorders such as eczema.
Less severe
forms of this condition, sometimes called variant PKU and non-PKU
hyperphenylalaninemia, have a smaller risk of brain damage. People with very
mild cases may not require treatment with a low-phenylalanine diet.
Babies born to mothers with
PKU and uncontrolled phenylalanine levels (women who no longer follow a
low-phenylalanine diet) have a significant risk of intellectual disability
because they are exposed to very high levels of phenylalanine before birth.
These infants may also have a low birth weight and grow more slowly than other
children. Other characteristic medical problems include heart defects or other heart
problems, an abnormally small head size (microcephaly), and behavioral
problems. Women with PKU and uncontrolled phenylalanine levels also have an
increased risk of pregnancy loss.
The adverse neurological
symptoms are the same for the two diseases. Genetic diseases (such as various
tyrosinemias and alkaptonuria) are also associated with other defective enzymes
of the tyrosine catabolic pathway. The first genetic disease ever recognized,
alcaptonuria, is caused by defective homogentisic acid oxidase. Homogentisic
acid accumulation is relatively innocuous, causing urine to darken on exposure
to air, but no life-threatening effects accompany the disease.
The other genetic deficiencies
lead to more severe symptoms, most of which are associated with abnormal neural
development, mental retardation, and shortened life span.
Albinism is a
congenital disorder characterized by the complete or partial absence of pigment
in the skin, hair and eyes due to absence or defect of tyrosinase, a
copper-containing enzyme involved in the production of melanin. Albinism
results from inheritance of recessive gene alleles and is known to affect all
vertebrates, including humans. While an organism with complete absence of
melanin is called an albino an organism with only a diminished amount of
melanin is described as albinoid.
Albinism is associated with a
number of vision defects, such as photophobia, nystagmus and astigmatism. Lack
of skin pigmentation makes for more susceptibility to sunburn and skin cancers.
In rare cases such as Chédiak–Higashi syndrome, albinism may be
associated with deficiencies in the transportation of melanin granules. This
also affects essential granules present in immune cells leading to increased
susceptibility to infection.
Lysine Catabolism
Lysine catabolism is unusual
in the way that the α-amino group is transferred to α-ketoglutarate
and into the general nitrogen pool. The reaction is a transamination in which
the α-amino group is transferred to the α-keto carbon of α-ketoglutarate
forming the metabolite, saccharopine. Unlike the majority of transamination
reactions, this one does not employ pyridoxal phosphate as a cofactor.
Saccharopine is immediately hydrolyzed by the enzyme α-aminoadipic
semialdehyde synthase in such a way that the amino nitrogen remains with the
α-carbon of α-ketoglutarate, producing glutamate and
α-aminoadipic semialdehyde. Because this transamination reaction is not
reversible, lysine is an essential amino acid. The ultimate end-product of
lysine catabolism is acetoacetyl-CoA Genetic deficiencies in the enzyme
α-aminoadipic semialdehyde synthase have been observed in individuals who
excrete large quantities of urinary lysine and some saccharopine. The lysinemia
and associated lysinuria are benign. Other serious disorders associated with
lysine metabolism are due to failure of the transport system for lysine and the
other dibasic amino acids across the intestinal wall. Lysine is essential for
protein synthesis; a deficiencies of its transport into the body can cause
seriously diminished levels of protein synthesis. Probably more significant
however, is the fact that arginine is transported on the same dibasic amino
acid carrier, and resulting arginine deficiencies limit the quantity of
ornithine available for the urea cycle. The result is severe hyperammonemia
after a meal rich in protein. The addition of citrulline to the diet prevents
the hyperammonemia. Lysine is also important as a precursor for the synthesis
of carnitine, required for the transport of fatty acids into the mitochondria
for oxidation. Free lysine does not serve as the precursor for this reaction,
rather the modified lysine found in certain proteins. Some proteins modify
lysine to trimethyllysine using SAM as the methyl donor to transfer methyl
groups to the α -amino of the lysine side chain. Hydrolysis of proteins
containing trimethyllysine provide the substrate for the subsequent conversion
to carnitine.
SPECIALIZED PRODUCTS OF AMINO ACIDS
Tyrosine-Derived Neurotransmitters
The majority of tyrosine that
does not get incorporated into proteins is catabolized for energy production.
One other significant fate of tyrosine is conversion to the catecholamines. The
catecholamine neurotransmitters are dopamine, norepinephrine, and epinephrine
(see also Biochemistry of Nerve Transmission).Norepinephrine is the principal
neurotransmitter of sympathetic postganglionic endings. Both norepinephrine and
the methylated derivative, epinephrine are stored in synaptic knobs of neurons
that secrete it, however, epinephrine is not a mediator at postganglionic
sympathetic endings.Tyrosine is transported into catecholamine-secreting
neurons and adrenal medullary cells where catechaolamine synthesis takes place.
The first step in the process requires tyrosine hydroxylase, which like
phenylalanine hydroxylase requires tetrahydrobiopterin as cofactor. The
hydroxylation reaction generates DOPA (3,4-dihydrophenylalanine). DOPA
decarboxylase converts DOPA to dopamine, dopamine b-hydroxylase converts
dopamine to norepinephrine and phenylethanolamine N-methyltransferase converts
norepinephrine to epinephrine. This latter reaction is one of several in the
body that uses SAM as a methyl donor generating S-adenosylhomocysteine. Within
the substantia nigra and some other regions of the brain, synthesis proceeds
only to dopamine. Within the adrenal medulla dopamine is converted to
norepinephrine and epinephrine.
Synthesis of the catecholamines from tyrosine
Once synthesized, dopamine,
norepinephrine and epinephrine are packaged in granulated vesicles. Within
these vesicles, norepinephrine and epinephrine are bound to ATP and a protein
called chromogranin A. Metabolism of the catecholemines occurs through the
actions of catecholamine-O-methyltransferase, (COMT) and monoamine oxidase,
(MAO). Both of these enzymes are widley distributed throughout the body.
However, COMT is not found in nerve endings as is MAO.
Tryptophan-Derived Neurotransmitters
Tryptopan serves as the
precursor for the synthesis of serotonin (5-hydroxytryptamine, 5-HT, see also
Biochemistry of Nerve Transmission) and melatonin
(N-acetyl-5-methoxytryptamine).
Serotonin is synthesized
through 2-step process involving a tetrahydrobiopterin-dependent hydroxylation
reaction (catalyzed by tryptophan-5-monooxygenase) and then a decarboxylation
catalyzed by aromatic L-amino acid decarboxylase. The hydroxylase is normally not
saturated and as a result, an increased uptake of tryptophan in the diet will
lead to increased brain serotonin content. Serotonin is present at highest
concentrations in platelets and in the gastrointestinal tract. Lesser amounts
are found in the brain and the retina. Serotonin containing neurons have their
cell bodies in the midline raphe nuclei of the brain stem and project to
portions of the hypothalamus, the limbic system, the neocortex and the spinal
cord. After release from serotonergic neurons, most of the released serotonin
is recaptured by an active reuptake mechanism. The function of the
antidepressant, Prozac is to inhibit this reuptake process, thereby, resulting
in prolonged serotonin presence in the synaptic cleft. The function of serotonin
is exerted upon its interaction with specific receptors. Several serotonin
receptors have been cloned and are identified as 5HT1, 5HT2, 5HT3, 5HT4, 5HT5,
5HT6, and 5HT7. Within the 5HT1 group there are subtypes 5HT1A, 5HT1B, 5HT1D,
5HT1E, and 5HT1F. There are three 5HT2 subtypes, 5HT2A, 5HT2B, and 5HT2C as
well as two 5HT5 subtypes, 5HT5a and 5HT5B. Most of these receptors are coupled
to G-proteins that affect the activities of either adenylate cyclase or
phospholipase C (PLC). The 5HT3 class of receptors are ion channels.Some
serotonin receptors are presynaptic and others postsynaptic. The 5HT2A
receptors mediate platelet aggregation and smooth muscle contraction. The 5HT2C
receptors are suspected in control of food intake as mice lacking this gene
become obese from increased food intake and are also subject to fatal seizures.
The 5HT3 receptors are present in the gastrointestinal tract and are related to
vomiting. Also present in the gastrointestinal tract are 5HT4 receptors where
they function in secretion and peristalsis. The 5HT6 and 5HT7 receptors are
distributed throughout the limbic system of the brain and the 5HT6 receptors
have high affinity for antidepressant drugs. Melatonin is derived from
serotonin within the pineal gland and the retina, where the necessary
N-acetyltransferase enzyme is found. The pineal parenchymal cells secrete
melatonin into the blood and cerebrospinal fluid. Synthesis and secretion of
melatonin increases during the dark period of the day and is maintained at a
low level during daylight hours. This diurnal variation in melatonin synthesis
is brought about by norepinephrine secreted by the postganglionic sympathetic
nerves that innervate the pineal gland. The effects of norepinephrine are
exerted through interaction with b-adrenergic receptors. This leads to
increased levels of cAMP, which in turn activate the N-acetyltransferase
required for melatonin synthesis. Melatonin functions by inhibiting the
synthesis and secretion of other neurotransmitters such as dopamine and GABA.
Creatine Biosynthesis
Creatine is synthesized in the
liver by methylation of guanidoacetate using SAM as the methyl donor.
Guanidoacetate itself is formed in the kidney from the amino acids arginine and
glycine.
Synthesis of creatine and creatinine
Creatine is used as a storage
form of high energy phosphate. The phosphate of ATP is transferred to creatine,
generating creatine phosphate, through the action of creatine phosphokinase.
The reaction is reversible such that when energy demand is high (e.g. during
muscle exertion) creatine phosphate donates its phosphate to ADP to yield
ATP.Both creatine and creatine phosphate are found in muscle, brain and blood.
Creatinine is formed in muscle from creatine phosphate by a nonenzymatic
dehydration and loss of phosphate. The amount of creatinine produced is related
to muscle mass and remains remarkably constant from day to day. Creatinine is
excreted by the kidneys and the level of excretion (creatinine clearance rate)
is a measure of renal function.
Glutathione Functions
Glutathione (abbreviated GSH)
is a tripeptide composed of glutamate, cysteine and glycine that has numerous
important functions within cells. It serves as a reductant, is conjugated to
drugs to make them more water soluble, is involved in amino acid transport
across cell membranes (the γ-glutamyl cycle), is a part of the
peptidoleukotrienes, serves as a cofactor for some enzymatic reactions and as
an aid in the rearrangement of protein disulfide bonds.
The role of GSH as a reductant
is extremely important particularly in the highly oxidizing environment of the
erythrocyte. The sulfhydryl of GSH can be used to reduce peroxides formed
during oxygen transport. The resulting oxidized form of GSH consists of two
molecules disulfide bonded together (abbreviated GSSG).
The enzyme glutathione
reductase utilizes NADPH as a cofactor to reduce GSSG back to two moles of GSH.
Hence, the pentose phosphate pathway is an extremely important pathway of
erythrocytes for the continuing production of the NADPH needed by glutathione
reductase. In fact as much as 10% of glucose consumption, by erythrocytes, may
be mediated by the pentose phosphate pathway.
Several mechanisms exist for
the transport of amino acids across cell membranes. Many are symport or
antiport mechanisms that couple amino acid transport to sodium transport. The
γ-glutamyl cycle is an example of a group transfer mechanism of amino acid
transport. Although this mechanism requires more energy input, it is rapid and
has a high capacity. The cycle functions primarily in the kidney, particularly
renal epithelial cells. The enzyme γ-glutamyl transpeptidase is located in
the cell membrane and shuttles GSH to the cell surface to interact with an
amino acid.
Synthesis of glutathione (GSH) Structure
of GSSG
Reaction with an amino acid
liberates cysteinylglycine and generates a γ-glutamyl-amino acid which is
transported into the cell and hydrolyzed to release the amino acid. Glutamate
is released as 5-oxoproline and the cysteinylglycine is cleaved to its
component amino acids. Regeneration of GSH requires an ATP-dependent conversion
of 5-oxoproline to glutamate and then the 2 additional moles of ATP that are
required during the normal generation of GSH.
. Polyamnine Biosynthesis
One of the earliest signals that cells have
entered their replication cycle is the appearance of elevated levels of mRNA
for ornithine decarboxylase (ODC), and then increased levels of the enzyme,
which is the first enzyme in the pathway to synthesis of the polyamines.
Because of the latter, and because the polyamines are highly cationic and tend
to bind nucleic acids with high affinity, it is believed that the polyamines
are important participants in DNA synthesis, or in the regulation of that
process.
The key features of the
pathway are that it involves putrescine, an ornithine catabolite, and
S-adenosylmethionine (SAM) as a donor of 2 propylamine residues. The first
propylamine conjugation yields spermidine and addition of another to spermidine
yields spermine.The function of ODC is to produce the 4-carbon saturated
diamine, putrescine. At the same time, SAM decarboxylase cleaves the SAM
carboxyl residue, producing decarboxylated SAM
(S-adenosymethylthiopropylamine), which retains the methyl group usually
involved in SAM methyltransferase activity. SAM decarboxylase activity is
regulated by product inhibition and allosterically stimulated by putrescine.
Spermidine synthase catalyzes the condensation reaction, producing spermidine
and 5'-methylthioadenosine. A second propylamine residue is added to spermidine
producing spermine.The signal for regulating ODC activity is unknown, but since
the product of its activity, putrescine, regulates SAM decarboxylase activity,
it appears that polyamine production is principally regulated by ODC
concentration.The butylamino group of spermidine is used in a posttranslational
modification reaction important to the process of translation. A specific
lysine residue in the translational initiation factor eIF-4D is modified.
Following the modification the residue is hydroxylated yielding a residue in
the protein termed hypusine.
Nitric Oxide Synthesis and Function
Vasodilators, such as
acetylcholine, do not exert their effects upon the vascular smooth muscle cell
in the absence of the overlying endothelium. When acetylcholine binds its
receptor on the surface of endothelial cells, a signal cascade, coupled to the
activation phospholipase C (PLC), is initiated. The PLCg-mediated release of
inositol trisphosphate, IP3 (from membrane associated
phosphatidylinositol-4,5-bisphosphate, PIP2), leads to the release of
intracellular stores of Ca2+. In turn, the elevation in Ca2+
leads to the liberation of endothelium-derived relaxing factor (EDRF) which
then diffuses into the adjacent smooth muscle. Within smooth muscle cells, EDRF
reacts with the heme moiety of a soluble guanylyl cyclase, resulting in
activation of the latter and a consequent elevation of intracellular levels of
cGMP. The net effect is the activation of cGMP-responsive enzymes which lead to
smooth muscle cell relaxation. The coronary artery vasodilator, nitroglycerin,
acts to increase intracellular release of EDRF and thus of cGMP. Quite
unexpectedly, EDRF was found to be the free radical diatomic gas, nitric oxide,
NO. NO is formed by the action of NO synthase, (NOS) on the amino acid
arginine.
Nitric oxide is involved in a
number of other important cellular processes in addition to its impact on
vascular smooth muscle cells. Events initiated by NO that are important for
blood coagulation include inhibition of platelet aggregation and adhesion and
inhibition of neutrophil adhesion to platelets and to the vascular endothelium.
NO is also generated by cells of the immune system and as such is involved in
non-specific host defense mechanisms and macrophage-mediated killing. NO also
inhibits the proliferation of tumor cells and microorganisms. Additional
cellular responses to NO include induction of apoptosis (programmed cell
death), DNA breakage and mutation. NOS is a very complex enzyme, employing five
redox cofactors: NADPH, FAD, FMN, heme and tetrahydrobiopterin. NO can also be
formed from nitrite, derived from vasodilators such as glycerin trinitrate
during their metabolism. The half-life of NO is extremely short, lasting only
2-4 seconds. This is because it is a highly reactive free radical and interacts
with oxygen and superoxide. NO is inhibited by hemoglobin and other heme
proteins which bind it tightly.Chemical inhibitors of NOS are available and can
markedly decrease production of NO. The effect is a dramatic increase in blood
pressure due to vasoconstriction. Another important cardiovascular effect of NO
is exerted through the production of cGMP, which acts to inhibit platelet
aggregation.