IpdE1-IpdE2 Is a Heterotetrameric Acyl Coenzyme A Dehydrogenase That Is Widely Distributed in Steroid-Degrading Bacteria.

Steroid-degrading bacteria, including (), utilize an architecturally distinct subfamily of acyl coenzyme A dehydrogenases (ACADs) for steroid catabolism. These ACADs are αβ heterotetramers that are usually encoded by adjacent like genes. In mycobacteria, and (formerly and ) occur in divergently transcribed operons associated with the catabolism of 3aα--4α(3'-propanoate)-7aβ-methylhexahydro-1,5-indanedione (HIP), a steroid metabolite. In , Δ and Δ mutants had similar phenotypes, showing impaired growth on cholesterol and accumulating 5-OH HIP in the culture supernatant. Bioinformatic analyses revealed that IpdE1 and IpdE2 share many of the features of the α- and β-subunits, respectively, of heterotetrameric ACADs that are encoded by adjacent genes in many steroid-degrading proteobacteria. When coproduced in a rhodococcal strain, IpdE1 and IpdE2 of formed a complex that catalyzed the dehydrogenation of 5OH-HIP coenzyme A (5OH-HIP-CoA) to 5OH-3aα--4α(3'-prop-1-enoate)-7aβ-methylhexa-hydro-1,5-indanedione coenzyme A (()5OH-HIPE-CoA). This corresponds to the initial step in the pathway that leads to degradation of steroid C and D rings via β-oxidation. Small-angle X-ray scattering revealed that the IpdE1-IpdE2 complex was an αβ heterotetramer typical of other ACADs involved in steroid catabolism. These results provide insight into an important class of steroid catabolic enzymes and a potential virulence determinant in .