 Homocysteine

Homocysteine is a sulfur-containing, non-proteinogenic amino acid with the side chain $-\mathrm{C}{\mathrm{H}}_2\mathrm{C}{\mathrm{H}}_2\mathrm{SH}$. Its neutral-form molecular formula is ${\mathrm{C}}_4{\mathrm{H}}_9\mathrm{N}{\mathrm{O}}_2\mathrm{S}$. The terminal $-\mathrm{SH}$ is a thiol group. The name without a stereochemical qualifier does not itself distinguish the possible configurations; L-homocysteine specifies the biologically important L form.^[ref]

A concrete metabolic connection is the reaction catalyzed by adenosylhomocysteinase:Thus homocysteine is not another name for S-adenosylhomocysteine: hydrolysis separates the sulfur-linked adenosyl portion from the amino-acid portion. The accepted IUBMB entry is now EC 3.13.2.1.^[ref]

An atom-inventory check makes the separation clear: the 14 carbons in S-adenosylhomocysteine partition into the four-carbon homocysteine portion and ten-carbon adenosine portion, $14=4+10$. This checks one part of the stoichiometry; it does not establish the equilibrium position or net reaction direction in a living cell.