Figure 2.

Hydrogen sulfide production pathways. The biosynthesis of hydrogen sulfide (H₂S) involves the three enzymes, cystathionine β-synthetase (CBS), cystathionine γ-lyase (CSE), and 3-mercaptopyruvate sulfurtransferase (MPST). The process involving H₂S production via CBS and CSE uses homocysteine and cysteine as substrates. The production of H₂S via cysteine transaminase (CAT) and MPST uses cysteine and a keto-acid as substrates to form 3-mercaptopyruvate in the first step via CAT, followed by the synthesis of H₂S from 3-mercaptopyruvate via MPST.