Metabolism of Aromatic Amino Acids

METABOLISM OF AROMATIC AMINO ACID

• Phenylalanines, Tyrosine, Tryptophan are aromatic amino acids.
• Phenylalanine is an essential amino acid.
• Tyrosine is a non- essential amino acid.
• Phenylalanine– conversion to tyrosine can reduce the dietary requirement of phenylalanine and the phenomenon is called as sparing action of tyrosine on phenylalanine.
• It is partly glucogenic and partly ketogenic.
• It is hydrophobic amino acid.
• Tyrosine- is synthesized from phenylalanine.
• It is partly glucogenic and partly ketogenic.
• Tyrosine is involved in the synthesis of-

1. epinephrine
2. norepinephrine
3. Dopamine
4. Thyroid hormones (thyroxine and triidothryonine)
5. Melanin pigment. 

• Conversion of phenylalanine to tyrosine– Phenylalanine is hydroxylated by phenylalanine hydroxylase (in liver) to produce tyrosine.
• It needs NADPH, NADH and tetrahydrobiopterine as conenzymes.
• It is an irreversible reaction.
• Catabolism of Tyrosine-
• Phenylalanine is converted to tyrosine.
• Enzymes are-

1. Tyrosine transaminase- PLP is the coenzyme for this reaction.
2. Para hydroxyl phenylpyruvate Hydroxylase- this enzyme belongs to Dioxygenase. Cofactor for this reaction is Copper. Ascorbic acid is also present.
3. Homogentisate oxidase- belongs to dioxygenase. Contains Iron also.
4. Maleyl acetoacetate cis-trans isomerase- belongs to isomerase. Need glutathione as cofactor.

• Specialized Products from Tyrosine- Melanin, Catecholamines, Thyroxine.

Synthesis of Melanin–

• Melanin is the pigment of skin, hair and eye.
• It takes place in the melanosome of melanocyte present in the deeper layers of epidermis.
• Tyrosine is the precursor for melanin and only one enzyme, tyrosinase.
• It is a rate limiting step and Copper is the cofactor.
• DOPA (Dihydroxyphenylalanine) can act as a cofactor for tyrosinase.
• Tyrosinase present in melaonocytes converts tyrosine to DOPA.
• The presence of moles on the body represents a localised severe hyperpigmentation due to hyperactivity of melonocytes.

Defects of Melanin-

• Localized absence or degeneration of melanocytes results in white patch on the skin is known as leucoderma.
• Albinism is an inborn error with generalised lack of melanin synthesis. (tyrosine is absent)
• Malignant Malenoma- malenoblasts increased resulting in malignant malenoma
• Graying of hair – due to absence of malenocytes in the roots of hair.

Synthesis of Catecholamines-

• The amine derivative of catechol is called catecholamines.
• Catecholamines are derived from tyrosine.
• Catecholamines includes-

1. Epinephrine
2. Norepinephrine
3. Dopamine

• Conversion of tyrosine to catecholamines takes place in adrenal medulla (epinephrine) and sympathetic ganglia (norepinephrine).
• Conversion of tyrosine to catecholamines takes place in 4 steps-

1. Ring hydroxylation
2. Decarboxylation
3. Side chain hydroxylation
4. Methylation

• Tyrosine hydroxylase – hydroxylates tyrosine to 3,4 dihydroxyphenylalanine.
• It catalyzes rate limiting reactions and requires tetrahydrobioprotein as coenzyme. 
• DOPA is decarboxylated to form Dopamine by DOPA decarboxylase.
• PLP is the coenzyme for this reaction. 
• Dopamine is an inhibitor of prolactin secretion and important neurotransmitter in extrapyramidal tract, substantia nigra and striatal tract. 
• Dopamine is hydroxylated to form norepinephrine. 
• Methylation of norepinephrine by S- adenosylmethionine gives epinephrine. Adrenaline and epinephrine same name for hormone. 

Functions of  catecholamines-

• Epinephrine and norepinephrine increases the blood pressure.
• Adrenaline increases the rate and force of myocardial contraction.
• Dopamine and norepinephrine acts as neurotransmitters in the brain and ANS.
• Adrenalin is anti-insulin in nature. 

Degradation of Catecholamines-

Catecholamines	Enzyme	Degradation
Epinephrine and Norepinephrine	Catechol O methyltransferase & mono amino oxidase	Vanillyl Mandelic acid
Dopamine		Homovanillic acid

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