Reaction: a 3β-hydroxy-4α-methylsteroid-4β-carboxylate + NAD(P)+ = a 4α-methyl-3-oxosteroid + NAD(P)H + CO2 + H+
Other name(s): sdmB (gene name)
Systematic name: 3β-hydroxysteroid-4β-carboxylate:NAD(P)+ 3-oxidoreductase (decarboxylating)
Comments: This bacterial enzyme participates in the biosynthesis of bacterial sterols. Together with EC 126.96.36.199, 4β-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-4β methyl group, turning it into a carboxylate group; SdmB is a bifunctional enzyme that catalyses two different activities. As EC 188.8.131.527 it catalyses an oxidative decarboxylation that results in reduction of the 3β-hydroxy group at the C-3 carbon to an oxo group. As EC 184.108.40.2060, 3β-hydroxysteroid 3-dehydrogenase, it reduces the 3-oxo group back to a 3β-hydroxyl. Since the remaining methyl group at C-4 is in an α orientation, it cannot serve as a substrate for a second round of demethylation by this system.
Links to other databases: BRENDA, EXPASY, KEGG, MetaCyc, CAS registry number:
1. Lee, A.K., Banta, A.B., Wei, J.H., Kiemle, D.J., Feng, J., Giner, J.L. and Welander, P.V. C-4 sterol demethylation enzymes distinguish bacterial and eukaryotic sterol synthesis. Proc. Natl Acad. Sci. USA 115 (2018) 5884-5889. [PMID: 29784781]