Graciously awarded Best Offered Talk

Abstract

Bile acids (BAs), the dominant chemicals in bile, play key roles in nutrient absorption, cell signaling, and microbiome regulation. Host derived BAs cholate (CA) and chenodeoxycholate (CDCA) are conjugated with glycine or taurine in the liver prior to undergoing enterohepatic circulation. Approximately 5% of BAs are not reabsorbed, instead entering the colon and undergoing metabolism by our gut microbiota. Deconjugation, the “gateway reaction” of BA metabolism, is performed by bile salt hydrolase (BSH) as it liberates glycine or taurine from host-conjugated BAs. Here, we show that BSH from Clostridium perfringens and other gut bacteria can also ligate amino acids to BAs, expanding the diversity of the BA pool. Interestingly, different gut bacteria produce highly varied amino acid conjugation profiles. Comparisons between BSH protein sequences suggested that Asn82 plays a role in dictating amino acid ligand diversity and overall rates of reconjugation. Indeed, site-directed mutation and expression of bsh^N82Y in E. coli resulted in reduced overall MCBA production with enrichment of glycine conjugates compared to WT. We sought to determine if a BA-enzyme intermediate forms at residue Cys2, similar to deconjugation, and expression of bsh^C2A results in complete ablation of production. Further analysis suggests an optimal pH of 5.3 for reconjugation, slightly more basic than the pH 4.7 optimum for deconjugation. Together, our findings suggest that BSH, one of the most highly studied enzymes involved in microbial BA metabolism, plays a much broader role than previously thought in shaping the human bile acid pool and affecting gut or liver function.