A patchwork pathway for oxygenase-independent degradation of side chain containing steroids

The denitrifying betaproteobacterium Sterolibacterium denitrificans serves as model organism for studying the oxygen-independent degradation of cholesterol. Here, we demonstrate its capability of degrading various globally abundant side chain containing zoo-, phyto- and mycosterols. We provide the complete genome that empowered an integrated genomics/proteomics/metabolomics approach, accompanied by the characterization of a characteristic enzyme of steroid side chain degradation. The results indicate that individual molybdopterin-containing steroid dehydrogenases are involved in C25-hydroxylations of steroids with different isoprenoid side chains, followed by the unusual conversion to C26-oic acids. Side chain degradation to androsta-1,4-diene-3,17-dione (ADD) via aldolytic C-C bond cleavages involves acyl-CoA synthetases/dehydrogenases specific for the respective 26-, 24- and 22-oic acids/-oyl-CoAs and promiscuous MaoC-like enoyl-CoA hydratases, aldolases and aldehyde dehydrogenases. Degradation of rings A and B depends on gene products uniquely found in anaerobic steroid degraders, which after hydrolytic cleavage of ring A, again involves CoA-ester intermediates. The degradation of the remaining CD rings via hydrolytic cleavage appears to be highly similar in aerobic and anaerobic bacteria. Anaerobic cholesterol degradation employs a composite repertoire of more than 40 genes partially known from aerobic degradation in gammaproteobacteria/actinobacteria, supplemented by unique genes that are required to circumvent oxygenase-dependent reactions.