Structural and Kinetic Insight into the Biosynthesis of H2S and l‑Lanthionine from l‑Cysteine by a Pyridoxal l‑Phosphate-Dependent Enzyme from Fusobacterium nucleatum

Fusobacterium nucleatum is a common oral bacterium and a major producer of H2S, a toxic gas linked to the pathogenesis of periodontal disease. The bacterium encodes a fold type II pyridoxal l-phosphate (PLP)-dependent enzyme, Fn1220 or lanthionine synthase (LS), that generates H2S and l-lanthionine (a component of the peptidoglycan layer) through β-replacement of l-cysteine by a second molecule of l-cysteine. Herein, we show through detailed kinetic analysis that LS elicits catalytic promiscuity as demonstrated for other fold type II PLP-dependent homologues, namely, O-acetylserine sulfhydrylase (OASS) and cystathionine β-synthase (CBS). Like OASS, LS can assimilate H2S by catalyzing the β-replacement of O-acetyl-l-serine by sulfide to form l-cysteine. However, the turnover for this reaction in LS is slower than that of other studied OASS enzymes due to slower conversion to the α-aminoacrylate intermediate. Similar to yeast and human CBS, LS can generate H2S and l-cystathionine through β-replacement of l-cysteine by a second molecule of l-homocysteine; however, whereas this is the main H2S-forming reaction in CBS, it is not for LS. LS shows a marked preference for forming H2S and l-lanthionine through the condensation of 2 equiv of l-cysteine. Sequence alignment of LS with other CBS and OASS enzymes and inspection of the LS crystal structure in the external aldimine state with l-lanthionine reveal that LS possesses a unique loop that engages in hydrogen-bond contact with the product, providing a structural rationale for the enzyme’s catalytic preference for H2S and l-lanthionine biosynthesis.