= Homocysteine
\b[Homocysteine] is a sulfur-containing, non-proteinogenic <amino acid> with the side chain $\mathrm{-CH_2CH_2SH}$. Its neutral-form <molecular formula> is $\mathrm{C_4H_9NO_2S}$. 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.<Homocysteine in ChEBI>[CHEBI:17230, Definition, Structure, Formula and Ontology; CHEBI:17588 for the L form.]{ref}
A concrete metabolic connection is the reaction catalyzed by <adenosylhomocysteinase>:
$$
\text{S-adenosyl-L-homocysteine}+\mathrm{H_2O}
\rightleftharpoons\text{L-homocysteine}+\text{adenosine}.
$$
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.<Enzyme Nomenclature by the IUBMB>[EC 3.13.2.1, Reaction; entry transferred from EC 3.3.1.1 in 2022.]{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>.
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