Metabolism of Aromatic Amino Acids

Metabolism of Aromatic Amino Acids


  • Phenylalanines, Tyrosine, Tryptophan are aromatic amino acids.
  • Phenylalanine is an essential amino acid.
  • Tyrosine is a non- essential amino acid.
  • Phenylalanineconversion 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

Exam Important

  • Specialized Products from Tyrosine-
    • Melanin
    • Catecholamines
    • Thyroxine
  • Epinephrine and norepinephrine is catabolized by Catechol O Methyl then by Monoamino Oxidase.
  • The major end product of epinephrine and norepinephrine is Vanillyl Mandelic Acid.
  • Diagnosis of Phenylketonuria is by Guthries test.
Don’t Forget to Solve all the previous Year Question asked on Metabolism of Aromatic Amino Acids

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