A glycyl radical enzyme enables hydrogen sulfide production by the human intestinal bacterium Bilophila wadsworthia.

Hydrogen sulfide (H2S) production in the intestinal microbiota has many contributions to human health and disease. An important source of H2S in the human gut is anaerobic respiration of sulfite released from the abundant dietary and host-derived organic sulfonate substrate in the gut, taurine (2-aminoethanesulfonate). However, the enzymes that allow intestinal bacteria to access sulfite from taurine have not yet been identified. Here we decipher the complete taurine desulfonation pathway in Bilophila wadsworthia 3.1.6 using differential proteomics, in vitro reconstruction with heterologously produced enzymes, and identification of critical intermediates. An initial deamination of taurine to sulfoacetaldehyde by a known taurine:pyruvate aminotransferase is followed, unexpectedly, by reduction of sulfoacetaldehyde to isethionate (2-hydroxyethanesulfonate) by an NADH-dependent reductase. Isethionate is then cleaved to sulfite and acetaldehyde by a previously uncharacterized glycyl radical enzyme (GRE), isethionate sulfite-lyase (IslA). The acetaldehyde produced is oxidized to acetyl-CoA by a dehydrogenase, and the sulfite is reduced to H2S by dissimilatory sulfite reductase. This unique GRE is also found in Desulfovibrio desulfuricans DSM642 and Desulfovibrio alaskensis G20, which use isethionate but not taurine; corresponding knockout mutants of D. alaskensis G20 did not grow with isethionate as the terminal electron acceptor. In conclusion, the novel radical-based C-S bond-cleavage reaction catalyzed by IslA diversifies the known repertoire of GRE superfamily enzymes and enables the energy metabolism of B. wadsworthia This GRE is widely distributed in gut bacterial genomes and may represent a novel target for control of intestinal H2S production.

Deciphering Human Gut Microbiota-Nutrient Interactions: A Role for Biochemistry.

The human gut contains trillions of microorganisms that play a central role in many aspects of host biology, including the provision of key nutrients from the diet. However, our appreciation of how gut microbes and their extensive metabolic capabilities affect the nutritional status of the human host is in its infancy. In this Perspective, we highlight how recent efforts to elucidate the biochemical basis for gut microbial metabolism of dietary components are reshaping our view of these organisms' roles in host nutrition. Gaining a molecular understanding of gut microbe-nutrient interactions will enhance our knowledge of how diet affects host health and disease, ultimately enabling personalized nutrition and therapeutics.

Molecular basis of C-S bond cleavage in the glycyl radical enzyme isethionate sulfite-lyase.

Desulfonation of isethionate by the bacterial glycyl radical enzyme (GRE) isethionate sulfite-lyase (IslA) generates sulfite, a substrate for respiration that in turn produces the disease-associated metabolite hydrogen sulfide. Here, we present a 2.7 Å resolution X-ray structure of wild-type IslA from Bilophila wadsworthia with isethionate bound. In comparison with other GREs, alternate positioning of the active site ß 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 14 IslA variants and computational analyses, 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.