A glycyl radical enzyme enables hydrogen sulfide production by the human intestinal bacteriumBilophila wadsworthia

Abstract

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 inBilophila wadsworthia3.1.6 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 inDesulfovibrio desulfuricansDSM642 andDesulfovibrio alaskensisG20, which use isethionate but not taurine; corresponding knockout mutants ofD. alaskensisG20 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 ofB. wadsworthia. This GRE is widely distributed in gut bacterial genomes and may represent a novel target for control of intestinal H2S production.