Fig. 1.

Sources of cysteine and cysteine-generated H₂S in bacteria. Cysteine in bacteria is produced from—(i) l-serine via classical de novo biosynthetic pathway (ii) l-methionine via reverse transsulfuration pathway that involves conversion of Met to S-adenosyl-methionine, followed by generation of S-adenosyl-homocysteine (SAH) which is cleaved to l-homocysteine either by SAH-hydrolase or by successive action of SAH-nucleosidase and S-ribosyl-homocysteinase (LuxS). Subsequently, PLP-dependent enzyme, cystathionine β-synthase (CBS) catalyses the condensation of homocysteine and serine to form l-cystathionine which is converted by cystathionine γ-lyase (CGL) to cysteine, α-ketobutyrate (α-KB) and ammonia (Thomas and Surdin-Kerjan 1997; Guédon et al. 2006) (iii) peptides (cystathione) and (iv) direct intake of extracellular cystine via ABC transporters. Enzymatic biogenesis of H₂S mainly involves CBS, CGL, and 3-MGT/CAT routes. The schematic figure depicts all enzymes as monomers without any relevance to the biologically active oligomeric form. Thin arrows in the figure represent release of a by-product. Inset depicts formation of CSC under varying cellular availability of OAS. In absence, CysK and CysE form CSC where the activity of CysK is inhibited and there is no production of cysteine. Once OAS starts accumulating, the complex dissociates to release CysK for cysteine production