BaiJ and BaiB are key enzymes in the chenodeoxycholic acid 7α-dehydroxylation pathway in the gut microbe Clostridium scindens ATCC 35704.

Bile acid transformation is a common gut microbiome activity that produces secondary bile acids, some of which are important for human health. One such process, 7α-dehydroxylation, converts the primary bile acids, cholic acid and chenodeoxycholic acid, to deoxycholic acid and lithocholic acid, respectively. This transformation requires a number of enzymes, generally encoded in a bile acid-inducible ( bai ) operon and consists of multiple steps. Some 7α-dehydroxylating bacteria also harbor additional genes that encode enzymes with potential roles in this pathway, but little is known about their functions. Here, we purified 11 enzymes originating either from the bai operon or encoded at other locations in the genome of Clostridium scindens strain ATCC 35704. Enzyme activity was probed in vitro under anoxic conditions to characterize the biochemical pathway of chenodeoxycholic acid 7α-dehydroxylation. We found that more than one combination of enzymes can support the process and that a set of five enzymes, including BaiJ that is encoded outside the bai operon, is sufficient to achieve the transformation. We found that BaiJ, an oxidoreductase, exhibits an activity that is not harbored by the homologous enzyme from another C. scindens strain. Furthermore, ligation of bile acids to coenzyme A (CoA) was shown to impact the product of the transformation. These results point to differences in the 7α-dehydroxylation pathway among microorganisms and the crucial role of CoA ligation in the process.