Figure 1.

Biosynthesis of lanthionine from cysteine. Lanthionine (3,3′-thiodialanine) is a nonproteogenic amino acid, analog of cysteine, consisting of two alanine residues crosslinked on their β-carbon atoms by a thioether linkage. Full box indicates enzymes; open box indicates coenzymes. CBS, cystathionine-β-synthase; CSE, cystathionase. PLP, Pyridoxalphosphate. Lanthionine is stoichiometrically yielded as a stable byproduct of H₂S biosynthesis. Dark background; complete transsulfuration pathway. Clear background; H₂S biosynthesis by CBS and/or CSE independently.