�ݺ�ߣ

AROMATIC AMINO ACIDS
? Aromatic amino acids (AAA)- are amino acids that include an aromatic ring.
? Examples include:
?Among 20 standard amino acids:
? phenylalanine (phe)
? tryptophan (trp)
? histidine (His)
? tyrosine (tyr)
? All plants and micro-organisms must synthesize their aromatic amino acids through
the shikimate pathway in order to make proteins, unlike animals, which obtain
them through their diet.
?Others:
? thyroxine (Phe �� tyr �� thyroxine)
? 5-hydroxytryptophan (derived from trp)
? L-DOPA (Phe �� tyr �� L-DOPA)

PROPERTIES: AROMATIC AMINO ACIDS
? Relatively nonpolar
? Absorb ultraviolet light to different degrees
? Precursors of many important biological compounds, such as
neurotransmitters and hormones, in the human organism
? Phe, Tyr and Trp are central molecules in plant metabolism & function
as building blocks of proteins
? The three AAA serve as precursors for a variety of plant hormones, such
as auxin and salicylate, as well as for a very wide range of aromatic
secondary metabolites with multiple biological functions

ANABOLISM OF AAA: THE SHIKIMATE
PATHWAY
? A seven step metabolic route
? Used by bacteria, fungi, algae, parasites
and plants for the biosynthesis of
chorismate and the aromatic amino acids.
? The first enzyme involved is the shikimate
kinase that catalyzes the ATP-dependent
phosphorylation of shikimate to form
shikimate 3-phosphate.
? Shikimate 3-phosphate is then coupled with
phosphoenol pyruvate to give 5-
enolpyruvylshikimate-3-phosphate

via the enzyme 5-enolpyruvylshikimate-3-phosphate (EPSP) synthase.
? Then 5-enolpyruvylshikimate-3-phosphate is transformed into chorismate by
a chorismate synthase.
NADPH + H+
NADP+
33

THE SHIKIMATE PATHWAY (EXTENDED)
?Prephenic acid is then synthesized by a Claisen rearrangement of chorismate by
Chorismate mutase. Which can then be utilized for:
?Tyrosine biosynthesis:
?Prephenate is oxidatively decarboxylated with retention of the hydroxyl group to give p-
hydroxyphenylpyruvate, which is transaminated using glutamate as the nitrogen source to
give tyrosine and ��-ketoglutarate.
?Phenylalanine biosynthesis:
?Prephenate is converted to phenylpyruvate by prephenate dehydratase.
?Then phenylpyruvate is transaminated to give phenylalanine by phenylalanine
aminotransferase.
?Tryptophan biosynthesis:
?Chorismate produces anthranilate which condenses with phosphoribosylpyrophosphate
(PRPP) to make N-5-Phosphoribosyl anthranilate.
?After ring opening of the ribose moiety and following reductive decarboxylation, indole-
3-glycerolphosphate is produced, which in turn is transformed into indole.
?In the last step, tryptophan synthase catalyzes the formation of tryptophan from indole
and the amino acid serine.

CATABOLISM OF HISTIDINE
? The amino acid is a precursor for histamine and carnosine biosynthesis.
? The enzyme histidine ammonia-lyase converts histidine into ammonia and urocanic
acid.
? A deficiency in this enzyme is present in the rare metabolic disorder histidinemia.
? In Actinobacteria and filamentous fungi, such as Neurospora crassa, histidine can
be converted into the antioxidant ergothioneine.
Fig: Conversion of histidine to histamine by histidine decarboxylase

PHENYLALANINE (PHE)
? An essential ��-amino acid with the formula
C6H5CH2CH(NH2)COOH.
? Classified as neutral and nonpolar because of the inert and
hydrophobic nature of the benzyl side chain.
? The codons for L-phe are UUU and UUC.
Sources:
? Naturally found in - breast milk of mammals.
? Good sources - are eggs, chicken, liver, beef, milk, and
soybeans.
? Other sources - spinach and leafy greens, tofu, amaranth
leaves, and lupin seeds.

PHENYLALANINE (PHE)
Roles & Uses:
? Used in the manufacture of food and drink products.
? Sold as a nutritional supplement for its reputed analgesic and antidepressant
effects.
? The L-isomer is used to biochemically form proteins, coded for by DNA. The
codons for L-phenylalanine are UUU and UUC.
? Is a precursor for tyrosine, dopamine, norepinephrine (noradrenaline), and
epinephrine (adrenaline), and the skin pigment melanin.
? Starting compound used in the flavonoid biosynthesis.
? The genetic disorder phenylketonuria (PKU) is the inability to metabolize
phenylalanine

TRYPTOPHAN (TRP)
? One of the 23 standard amino acids and an essential amino acid in the human
diet.
? Encoded in the standard genetic code as the codon UGG & Contains an indole
functional group.
? Only L-stereoisomer of trp is used in structural or enzyme proteins.
? The D-stereoisomer is occasionally found in naturally produced peptides (e.g.-the
marine venom peptide contryphan).
Dietary sources:
? A routine constituent of most protein-based foods or dietary proteins.
? Particularly plentiful in chocolate, oats, dried dates, milk, yogurt, cottage
cheese, red meat, eggs, fish, poultry, sesame, chickpeas, almonds, sunflower
seeds, pumpkin seeds, spirulina, bananas, and peanuts.
? Found at a high level in turkey meat.

TRYPTOPHAN (TRP)
Roles & uses:
?An essential amino acid & act as building blocks in protein
biosynthesis
?Functions as a biochemical precursor for the compounds - Serotonin
(a neurotransmitter), Niacin (a vitamin), Auxin (a phytohormone).
?Sold over the counter in the US, Canada, and the UK as a dietary
supplement for use as an antidepressant, anxiolytic, and sleep aid.
?Also marketed for the indication of major depression under various
trade names.
?Available in health food stores as a dietary supplement.
?Potential side effects of trp include nausea, diarrhea, drowsiness,
headache, dry mouth, blurred vision, euphoria, and nystagmus
(involuntary eye movements) etc.

TYROSINE (TYR)
? Tyr or 4-hydroxyphenylalanine (from Greek tyros,
meaning cheese), is one of the 23 amino acids that are
used by cells to synthesize proteins.
? A non-essential amino acid with a polar side group.
? Its codons are UAC and UAU.
Dietary sources:
? Found in many high-protein food products such as
chicken, turkey, fish, milk, yogurt, cottage cheese,
cheese, peanuts, almonds, pumpkin seeds, sesame seeds,
soy products, lima beans, avocados, and bananas.
? Can also be synthesized in the body from phenylalanine
by the enzyme phenylalanine hydroxylase.

TYROSINE (TYR)
Roles & Uses:
? Precursor to neurotransmitters and hormones, alkaloids, natural
phenols, pigments (melanin).
? Tyrosine (or its precursor phenylalanine) is needed to synthesize the
benzoquinone structure which forms part of coenzyme Q10.
? Useful during conditions of stress, cold, fatigue, prolonged work and
sleep deprivation.
? Reductions in stress hormone levels & stress-induced weight loss
seen in animal trials.
? Increases plasma neurotransmitter levels (particularly dopamine and
norepinephrine).

HISTIDINE (HIS OR H)
? An essential, ��-amino acid with an imidazole functional
group.
? One of the 23 proteinogenic amino acids.
? Its codons are CAU and CAC.
? First isolated by German physician Albrecht Kossel in
1896
? The conjugate acid (protonated form) of the imidazole
side chain in histidine has a pKa of approximately 6.0
Source:
? Found in soya beans, chicken breast, beef, wheat
germs, raw salmon
? Found in breast milk.

HISTIDINE (HIS OR H)
Metabolism:
? The condensation of ATP and 5-phosphoribosyl 1-pyrophosphate (PRPP) to form N9-59-
phosphoribosyl-ATP (PRATP) catalyzed by the N9-59-phosphoribosyl-ATP transferase.
ATP + ?-D-RIBOSE-5-PHOSPHATE ? PRPP + AMP ? (PRATP) ? ? His
Then 10 steps of reaction produces His.
? His is a precursor for histamine and carnosine biosynthesis.
Conversion of histidine to histamine by histidine decarboxylase
? Can be converted to Histamine, Glutamate, Haemoglobin.
? Building block of many Fe containing molecules e.g.-ferritin.
? Helps in detoxification of heavy metals inside body.
? Indirectly provides sufficient energy & oxygen supply to organs & tissues.
? Regulation of growth, natural repair mechanism, pH of blood.
Roles:

�ݺ�ߣ

Aromatic amino acids in brief

More Related Content

Aromatic amino acids in brief