Fig. 1.

Cysteine biosynthesis via methionine or cystine. Reactions 1 to 4 describe the methionine cycle whereby methionine is converted to S-adenosylmethionine by methionine adenosyltransferase (1). S-Adenosylmethionine is in turn converted to S-adenosylhomocysteine by methyltransferases (2) that donate the methyl group of S-adenosylmethionine to various acceptors. The adenosine moiety of S-adenosylhomocysteine is then cleaved from homocysteine by adenosylhomocysteinase (3). Finally, methylation of homocysteine by methionine synthase yields methionine to complete the cycle (4). Folate metabolism provides the methyl donor for the latter reaction in the form of N.⁵-methyltetrahydrofolate. Homocysteine can also be methylated to methionine by betaine-homocysteine methyltransferase using glycine betaine as the methyl donor (not shown). Reactions 5 to 6 describe transsulfuration in which cystathionine β-synthase condenses homocysteine and serine to form cystathionine (5) which CGL then cleaves into cysteine and 2-oxobutanoate (6). CGL also cleaves cystine to yield cysteine persulfide (7) which undergoes reduction (e.g., thiols: RSH) to produce H₂S and the corresponding disulfides (RSSR) (8)