= S-adenosylhomocysteine
{c}
<S-adenosylhomocysteine> is the sulfur-linked adenosyl derivative of <homocysteine>. The L-homocysteine form recorded by ChEBI has neutral <molecular formula> $\mathrm{C_{14}H_{20}N_6O_5S}$. Specifying connectivity, <stereochemistry> and protonation is more informative than the abbreviation SAH alone.<S-adenosyl-L-homocysteine in ChEBI>[CHEBI:16680, Definition, Formula, Net Charge and structure]{ref}
<S-adenosylhomocysteine> is a product of <methyl group> transfer from <S-adenosylmethionine>. For a concrete example, <acetylserotonin O-methyltransferase> transfers a <methyl group> to <N-acetylserotonin>, producing <melatonin> and <S-adenosylhomocysteine>. One donor molecule therefore corresponds to one methylation event in this reaction; it is not a freely reusable donor after losing that group.<Enzyme Nomenclature by the IUBMB>[EC2.1.1.4, accepted reaction]{ref}
<Adenosylhomocysteinase> catalyses the reversible hydrolysis connecting <S-adenosylhomocysteine> and <water> with <homocysteine> and <adenosine>.<Enzyme Nomenclature by the IUBMB>[EC3.13.2.1, accepted name and reaction]{ref} Stoichiometrically, complete hydrolysis of $2\,\mathrm{\mu mol}$ <S-adenosylhomocysteine> would yield $2\,\mathrm{\mu mol}$ of each organic product. The conditional calculation gives a maximum under complete conversion; it does not specify the actual equilibrium position or the net direction in a living <biological cell>.
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