S-adenosylhomocysteine

S-adenosylhomocysteine is the sulfur-linked adenosyl derivative of homocysteine. The L-homocysteine form recorded by ChEBI has neutral molecular formula ${\mathrm{C}}_{14}{\mathrm{H}}_{20}{\mathrm{N}}_6{\mathrm{O}}_5\mathrm{S}$. Specifying connectivity, stereochemistry and protonation is more informative than the abbreviation SAH alone.^[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.^[ref]

Adenosylhomocysteinase catalyses the reversible hydrolysis connecting S-adenosylhomocysteine and water with homocysteine and adenosine.^[ref] Stoichiometrically, complete hydrolysis of $2\mu \mathrm{mol}$ S-adenosylhomocysteine would yield $2\mu \mathrm{mol}$ 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.