Homocysteine (HCy)

Homocysteine is an amino acid with the formula HSCH₂CH₂CH(NH₂)CO₂H. It is a homologue of the amino acid cysteine, differing by an additional methylene (-CH₂-) group. It is biosynthesized from methionine by the removal of its terminal C^ε methyl group. Homocysteine can be recycled into methionine or converted into cysteine with the aid of B-vitamins.

Homocysteine is not obtained from the diet. Instead, it is biosynthesized from methionine via a multi-step process. First, methionine receives an adenosine group from ATP, a reaction catalyzed by S-Adenosyl-methionine synthetase, to give S-adenosyl methionine ("SAM"). SAM then transfers the methyl group to an acceptor molecule. The adenosine is then hydrolyzed to yield L-homocysteine. L-homocysteine has two primary fates: conversion via tetrahydrofolate (THF) back into L-methionine or conversion to L-cysteine.