METABOLISM OF AMINOACIDS. DIGESTION OF
PROTEINS.
GENERAL AND SPECIFIC PATHWAYS OF AMINO ACIDS TRANSFORMATION.
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 functions in body
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.
Newborn
Newborns of mammals are exceptional in protein digestion and assimilation in that they can absorb
intact proteins at the small intestine. This enables passive
immunity from milk.
DIGESTION
OF PROTEIN
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. To learn more, keep reading.
There is a
process by which the body converts the ingested foods into its simpler
constituents that can be easily absorbed and assimilated. This above mentioned
process gives us an idea of
what is
digestion. Proteins are defined as the group
of complex organic macromolecules containing carbon, oxygen, hydrogen, nitrogen
and sulfur and are composed of one or more amino acid chains. Proteins are
components of enzymes, hormones and antibodies, and therefore are very
important for an organism's survival. The present article focuses its
discussion on how protein is digested inside a human's body. The digestion of
proteins takes place in two organs, stomach and small intestine. Let's learn
what happens in each of them during protein digestion.
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. 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.
Function
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.
Mechanism
The enzymatic mechanism is similar to that of other serine
proteases. These enzymes contain a catalytic
triad consisting of histidine-57, aspartate-102,
and serine-195.
These three
residues form a charge relay that serves to make the active site
serine
nucleophilic. This is achieved by modifying the electrostatic environment of
the serine. The enzymatic reaction that trypsins catalyze is thermodynamically
favorable but requires significant activation
energy (it is "kinetically
unfavorable"). In addition, trypsin contains an "oxyanion hole"
formed by the backbone amide hydrogen atoms of Gly-193 and Ser-195,
which serves to stabilize the developing negative charge on the carbonyl oxygen
atom of the cleaved amides.
The aspartate residue (Asp 189) located in the catalytic pocket
(S1) of trypsins is responsible for attracting and stabilizing positively
charged lysine
and/or arginine,
and is, thus, responsible for the specificity of the enzyme. This means that
trypsin predominantly cleaves proteins at the carboxyl side (or "C-terminal
side") of the amino acids lysine and arginine
except when either is bound to a C-terminal proline.,
although large-scale mass spectrometry data suggest cleavage occurs even with
proline. Trypsins are considered endopeptidases,
i.e., the cleavage occurs within the polypeptide
chain rather than at the terminal amino acids located at the ends of polypeptides.
Properties
Trypsins has an optimal operating pH of about 7.5-8.5 and
optimal operating temperature of about
The activity of trypsins is not affected by the inhibitor
tosyl phenylalanyl chloromethyl ketone, TPCK, which deactivates chymotrypsin.
This is important because, in some applications, like mass
spectrometry, the specificity of cleavage is important.
Trypsins should be stored at very cold temperatures
(between −20°C and −80°C) to prevent autolysis,
which may also be impeded by storage of trypsins at pH 3 or by using trypsin modified
by reductive methylation.
When the pH is adjusted back to pH 8, activity returns.
Clinical significance
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.
Role in disease
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.
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.
Adenine
Deamination of adenine
results in the formation of hypoxanthine. Hypoxanthine, in a manner analogous
to the imine tautomer of adenine, selectively base pairs with cytosine
instead of thymine.
This results in a post-replicative transition mutation, where the original A-T
base pair transforms into a G-C base pair.
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.
In Figure, only the first
half-reaction is shown, since the second half-reaction is the exact reversal of
the first one; this also implies that the entire reaction is reversible.
Overall, the mechanism consists in the first substrate arriving and leaving
before the second substrate enters and leaves; this is dubbed a Ping Pong Bi
Bi reaction (Figure).1
While two different substrates must be used for the the reaction to have a net
effect, it is of course possible for amino acid 1 and amino acid 2 to be
identical—the reaction will work just fine but achieve no net turnover.
The reaction mechanism revolves
around the coenzyme pyridoxal phosphate (PLP):
1. At the outset of the reaction, PLP is
bound as a Schiff base to the ε-amino group of a lysine residue in the
active site (Figure).
2. The bond between PLP and the enzyme
is separated, and PLP forms a Schiff base with the amino acid substrate instead
(Figure, steps 1 and 2).
3. The liberated lysine residue
abstracts the α hydrogen as a proton (step 3), and the electron left
behind travels all the way down the PLP ring. PLP is often said to act as an
'electron sink'. This has the effect of turning the bond between the α
carbon and the α nitrogen into a Schiff base.
4. The Schiff base is hydrolyzed to
yield the α-keto acid and the amino derivative of the PLP (called
pyridoxamine phosphate; steps 4 and 5).
The PLP in its various forms stays
within the the active site throughout, even when not bound to the enzyme
covalently. As stated above, the second half reaction is the exact reversal of
the first, and you might want to draw the individual steps for yourself.
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.
The term "decarboxylation" literally means
removal of the COOH (carboxyl group) and its replacement with a proton. The
term simply relates the state of the reactant and product. Decarboxylation is
one of the oldest organic reactions, since it often entails simple pyrolysis,
and volatile products distill from the reactor. Heating is required because the
reaction is less favorable at low temperatures. Yields are highly sensitive to
conditions. In retrosynthesis, decarboxylation reactions can
be considered the opposite of homologation reactions, in that the chain
length becomes one carbon shorter. Metals, especially copper compounds, are
usually required. Such reactions proceed via the intermediacy of metal
carboxylate complexes.
Decarboxylation of aryl carboxylates can generate the equivalent
of the corresponding aryl anion, which in turn can undergo cross coupling reactions.
Alkylcarboxylic acids and their salts do not always
undergo decarboxylation readily. Exceptions are the decarboxylation of beta-keto acids,
α,β-unsaturated acids, and α-phenyl, α-nitro, and
α-cyanoacids. Such reactions are accelerated due to the formation of a
zwitterionic tautomer in which the carbonyl is protonated and the carboxyl
group is deprotonated. Typically fatty acids do not decarboxylate readily.
Reactivity of an acid towards decarboxylation depends upon stability of
carbanion intermediate formed in above mechanism. Many reactions have been
named after early workers in organic chemistry. The Barton decarboxylation, Kolbe electrolysis, Kochi
reaction and Hunsdiecker reaction are radical
reactions. The Krapcho decarboxylation is a related
decarboxylation of an ester. In ketonic decarboxylation a carboxylic acid
is converted to a ketone.
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