Steroid degradation in Comamonas testosteroni TA441: Identification of metabolites and the genes involved in the reactions necessary before D-ring cleavage

Abstract

Bacterial steroid degradation has been studied mainly with Rhodococcus equi (Nocardia restrictus) and Comamonas testosteroni as representative steroid degradation bacteria for more than 50 years. The primary purpose was to obtain materials for steroid drugs, but recent studies showed that many genera of bacteria (Mycobacterium, Rhodococcus, Pseudomonas, etc.) degrade steroids and that steroid-degrading bacteria are globally distributed and found particularly in wastewater treatment plants, the soil, plant rhizospheres, and the marine environment. The role of bacterial steroid degradation in the environment is, however, yet to be revealed. To uncover the whole steroid degradation process in a representative steroid-degrading bacterium, C. testosteroni, to provide basic information for further studies on the role of bacterial steroid degradation, we elucidated the two indispensable oxidative reactions and hydration before D-ring cleavage in C. testosteroni TA441. In bacterial oxidative steroid degradation, A- and B-rings of steroids are cleaved to produce 2-hydroxyhexa-2,4-dienoic acid and 9,17-dioxo-1,2,3,4,10,19-hexanorandrostan-5-oic acid. The latter compound was revealed to be degraded to the coenzyme A (CoA) ester of 9α-hydroxy-17-oxo-1,2,3,4,5,6,10,19-octanorandrostan-7-oic acid, which is converted to the CoA ester of 9,17-dioxo-1,2,3,4,5,6,10,19-octanorandrostan-7-oic acid by ORF31- encoded hydroxylacyl dehydrogenase (ScdG), followed by conversion to the CoA ester of 9,17-dioxo-1,2,3,4,5,6,10,19-octanorandrost-8(14)-en-7-oic acid by ORF4-encoded acyl- CoA dehydrogenase (ScdK). Then, a water molecule is added by the ORF5-encoded enoyl-CoA hydratase (ScdY), which leads to the cleavage of the D-ring. The conversion by ScdG is presumed to be a reversible reaction. The elucidated pathway in C. testosteroni TA441 is different from the corresponding pathways in Mycobacterium tuberculosis H37Rv.