This bacterial enzyme participates in the biosynthesis of bacterial sterols. Together with EC 1.14.13.246, 4beta-methylsterol monooxygenase (SdmA) it forms an enzyme system that removes one methyl group from the C-4 position of 4,4-dimethylated steroid molecules. SdmA catalyses three successive oxidations of the C-4beta methyl group, turning it into a carboxylate group; SdmB is a bifunctional enzyme that catalyses two different activities. As EC 1.1.1.417 it catalyses an oxidative decarboxylation that results in reduction of the 3beta-hydroxy group at the C-3 carbon to an oxo group. As EC 1.1.1.270, 3beta-hydroxysteroid 3-dehydrogenase, it reduces the 3-oxo group back to a 3beta-hydroxyl. Since the remaining methyl group at C-4 is in an alpha orientation, it cannot serve as a substrate for a second round of demethylation by this system.
