Question

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

Short answer

Tryptophan is the precursor amino acid for both serotonin and melatonin. The biosynthesis of serotonin involves two main steps: hydroxylation through the tryptophan hydroxylase reaction, converting tryptophan to 5-hydroxytryptophan (5-HTP), and decarboxylation by the aromatic amino acid decarboxylase (AAAD) enzyme, converting 5-HTP to serotonin. Melatonin biosynthesis involves three main steps: serotonin conversion by aralkylamine N-acetyltransferase (AANAT) to N-acetylserotonin, hydroxylation by hydroxyindole O-methyltransferase (HIOMT) to 5-methoxy-N-acetylserotonin, and decarboxylation by acetylserotonin O-methyltransferase (ASMT) to melatonin. The types of reactions involved in their biosynthesis include hydroxylation, decarboxylation, acetylation, and methylation.

Step-by-step solution

Identify the precursor amino acid for serotonin and melatonin

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

Serotonin biosynthesis steps

The synthesis of serotonin (also known as 5-hydroxytryptamine, or 5-HT) involves two main steps: 1. Tryptophan Hydroxylase Reaction: Tryptophan is converted to 5-hydroxytryptophan (5-HTP) by the enzyme tryptophan hydroxylase. \[ \text{Tryptophan} + \text{O}_{2} + \text{BH}_{4} \xrightarrow{\text{Tryptophan hydroxylase}} \text{5-hydroxytryptophan} + \text{H}_{2}\text{O} + \text{BH}_{2} \] 2. Aromatic Amino Acid Decarboxylase (AAAD) Reaction: 5-hydroxytryptophan (5-HTP) is then converted to serotonin (5-HT) by the enzyme aromatic amino acid decarboxylase (AAAD). \[ \text{5-hydroxytryptophan} \xrightarrow{\text{Aromatic amino acid decarboxylase}} \text{Serotonin} + \text{CO}_{2} \]

Melatonin biosynthesis steps

The synthesis of melatonin involves three main steps: 1. Serotonin Conversion: Serotonin is converted to N-acetylserotonin by the enzyme aralkylamine N-acetyltransferase (AANAT). \[ \text{Serotonin} + \text{Acetyl-CoA} \xrightarrow{\text{Aralkylamine N-acetyltransferase}} \text{N-acetylserotonin} + \text{CoA} \] 2. N-acetylserotonin Hydroxylation: N-acetylserotonin is converted to 5-methoxy-N-acetylserotonin by the enzyme hydroxyindole O-methyltransferase (HIOMT), which adds a methyl group from S-adenosylmethionine (SAMe) to the hydroxyl group. \[ \text{N-acetylserotonin} + \text{SAMe} \xrightarrow{\text{Hydroxyindole O-methyltransferase}} \text{5-methoxy-N-acetylserotonin} + \text{SAH} \] 3. Decarboxylation: 5-methoxy-N-acetylserotonin is converted to melatonin by the enzyme acetylserotonin O-methyltransferase (ASMT); this enzyme is also known as hydroxyindole O-methyltransferase (HIOMT). \[ \text{5-methoxy-N-acetylserotonin} + \text{SAMe} \xrightarrow{\text{Acetylserotonin O-methyltransferase}} \text{Melatonin} + \text{SAH} \]

Types of reactions involved

Biosynthesis of serotonin and melatonin involves the following types of reactions: 1. Hydroxylation: addition of a hydroxyl group to tryptophan in the synthesis of serotonin. 2. Decarboxylation: removal of a carboxyl group. 3. Acetylation: the addition of an acetyl group. 4. Methylation: the addition of a methyl group.