Question

From which amino acid are serotonin and melatonin synthesized? What types of reactions are involved in their biosynthesis?

Short answer

Answer: The amino acid precursor for serotonin and melatonin synthesis is tryptophan. Serotonin biosynthesis involves two main reactions: hydroxylation of tryptophan to form 5-hydroxytryptophan (5-HTP) and decarboxylation of 5-HTP to form serotonin. Melatonin biosynthesis involves two main reactions as well: N-acetylation of serotonin to form N-acetylserotonin and O-methylation of N-acetylserotonin to form melatonin.

Step-by-step solution

Identify the amino acid precursor for serotonin and melatonin

The amino acid from which serotonin and melatonin are synthesized is tryptophan.

Describe the biosynthesis of serotonin from tryptophan

The biosynthesis of serotonin from tryptophan involves two main reactions: 1. Hydroxylation: In the first step, tryptophan is hydroxylated to form 5-hydroxytryptophan (5-HTP) by the enzyme tryptophan hydroxylase. Tryptophan + O2 -> 5-HTP This is a monooxygenase reaction, which means that only one of the two oxygens from the molecular oxygen (O2) is incorporated into the product, and the other is reduced to water. 2. Decarboxylation: In the second step, 5-HTP is decarboxylated to form serotonin (5-hydroxytryptamine or 5-HT) by the enzyme aromatic L-amino acid decarboxylase. 5-HTP -> Serotonin + CO2 This is a decarboxylation reaction, which means that a carboxyl group is removed from the molecule and released as carbon dioxide (CO2).

Describe the biosynthesis of melatonin from serotonin

The biosynthesis of melatonin from serotonin involves two main reactions: 1. N-acetylation: In the first step, serotonin is N-acetylated to form N-acetylserotonin by the enzyme aralkylamine N-acetyltransferase (AANAT) in the presence of acetyl-CoA as an acetyl donor. Serotonin + Acetyl-CoA -> N-acetylserotonin + CoA This is an acetylation reaction, which means that an acetyl group is added to the amine group of the molecule. 2. O-methylation: In the second step, N-acetylserotonin is O-methylated to form melatonin by the enzyme hydroxyindole O-methyltransferase (HIOMT) in the presence of S-adenosylmethionine (SAM) as a methyl donor. N-acetylserotonin + SAM -> Melatonin + S-adenosylhomocysteine (SAH) This is a methylation reaction, which means that a methyl group is added to the hydroxyl group of the molecule.