Question

Both norepinephrine and epinephrine are synthesized from the same protein- derived amino acid. From which amino acid are they synthesized? What types of reactions are involved in their biosynthesis?

Short answer

Answer: Norepinephrine and epinephrine are synthesized from the amino acid tyrosine. Their biosynthesis involves hydroxylation, decarboxylation, and methylation reactions.

Step-by-step solution

Identify the amino acid

Both norepinephrine and epinephrine are synthesized from the amino acid tyrosine. Tyrosine is a protein-derived amino acid, which acts as a precursor for the synthesis of catecholamines, including norepinephrine and epinephrine.

Formation of DOPA

The first reaction in the biosynthesis of norepinephrine and epinephrine involves the conversion of tyrosine to dihydroxyphenylalanine (DOPA) by the enzyme tyrosine hydroxylase. This is a hydroxylation reaction, in which a hydroxyl group is added to the tyrosine moiety in a rate-limiting step: \[Tyrosine \xrightarrow{tyrosine\ hydroxylase} DOPA\]

Formation of dopamine

The next step in the pathway is the conversion of DOPA to dopamine through a decarboxylation reaction. This is catalyzed by the enzyme DOPA decarboxylase (also known as aromatic L-amino acid decarboxylase) which removes a carboxyl group from DOPA: \[DOPA \xrightarrow{DOPA\ decarboxylase} Dopamine\]

Formation of norepinephrine

The third step in the biosynthesis of norepinephrine and epinephrine involves the conversion of dopamine to norepinephrine. This is a hydroxylation reaction, in which a hydroxyl group is added to the dopamine molecule by the enzyme dopamine β-hydroxylase: \[Dopamine \xrightarrow{dopamine\ \beta -hydroxylase} Norepinephrine \]

Formation of epinephrine

The final step in the biosynthesis of epinephrine from norepinephrine is catalyzed by the enzyme phenylethanolamine N-methyltransferase (PNMT). In this reaction, a methyl group is transferred from S-adenosylmethionine (SAM) to the norepinephrine molecule, converting it into epinephrine: \[Norepinephrine \xrightarrow{PNMT} Epinephrine\] In summary, norepinephrine and epinephrine are synthesized from the amino acid tyrosine through a series of reactions involving hydroxylation, decarboxylation, and methylation steps.