Molecular Basis of C–S Bond Cleavage in the Glycyl Radical Enzyme Isethionate Sulfite-Lyase

Abstract

Desulfonation of isethionate by the bacterial glycyl radical enzyme (GRE) isethionate sulfite-lyase (IslA) generates sulfite, a substrate for sulfite respiration that in turn produces the disease-associated metabolite hydrogen sulfide. Here, we present a 2.7 A resolution X-ray structure of wild-type IslA from Bilophila wadsworthia with isethionate bound. In comparison to other GREs, alternate positioning of the active site β strands allows for distinct residue positions to contribute to substrate binding. These structural differences combined with sequence variations create a highly tailored active site for the binding of the negatively charged isethionate substrate. Through the kinetic analysis of fourteen IslA variants, we probe the mechanism by which radical chemistry is used for C–S bond cleavage. This work further elucidates the structural basis of chemistry within the GRE superfamily and will inform structure-based inhibitor design of IsIA and thus of microbial hydrogen sulfide production.