Abstract
The essential amino acid methionine is converted by two methionine adenosyltransferases (MAT I/III and MATII) to S-adenosylmethionine (SAM). The methyl group of SAM is used in numerous biologically important methylation reactions, yielding S-adenosylhomocysteine (SAH); excess SAM is removed from the cycle by glycine N-methyltransferase (GNMT).
SAH is cleaved by of S-adenosylhomocysteine hydrolase (SAHH) to homocysteine and adenosine, which is further metabolized by adenosine kinase (ADK). Homocysteine can be converted back to methionine by the remethylation pathway or using betaine as a methyl-group donor, in patients treated with this drug.
Alternatively, homocysteine is irreversibly metabolized to sulfate by the transsulfuration pathway. Homocysteine is condensed with serine to form cystathionine, which is subsequently cleaved to form cysteine and α-ketobutyrate; these reactions are catalysed by cystathionine β-synthase (CBS) and cystathionine γ-lyase (CTH), respectively, which can also use cysteine and/or homocysteine to synthesize hydrogen sulfide.
Cysteine can be further converted in a series of reactions into taurine, or via the mitochondrial enzymes, aspartate aminotransferase (AST) and 3-mercaptopyruvate sulfurtransferase (MPST), to pyruvate and hydrogen sulfide. Mitochondrial oxidation of hydrogen sulfide involves several steps yielding thiosulfate, sulfite and finally sulfate; the figure only shows sulfur dioxygenase (ETHE1) and sulfite oxidase (SUOX), which requires the molybdenum cofactor, produced by enzymes encoded by molybdenum cofactor synthesis 1 and 2 genes (MOCS1 and 2) and by gephyrin (GPHN).