Figure 2.

Catalytic versatility of the H₂S-synthesizing enzymes cystathionine β-synthase (CBS) and cystathionine γ-lyase (CSE). Chemical structure of the metabolites and reaction schemes involved in H₂S synthesis by CBS and CSE. Top panel, “canonical” reactions catalyzed by CBS and CSE as part of the transsulfuration branch of the methionine cycle. Center-bottom panel, alternative H₂S-generating reactions. Chemical structures labeled as α-ketobutyrate and pyruvate are represented as the α-ketobutyric and pyruvic acid, respectively. ^∗Serine and homoserine have not been detected as intermediates in the CSE-catalyzed reactions 3 and 5, respectively, only their downstream products α-ketobutyric and pyruvic acids (resp.).