Contains a Rieske [2Fe-2S] iron-sulfur cluster. This bacterial enzyme (SdmA) participates in the biosynthesis of bacterial sterols. Together with SdmB 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; the second enzyme, SdmB, is a bifunctional enzyme that catalyses two different activities. As EC 126.96.36.1997
, 3beta-hydroxysteroid-4beta-carboxylate 3-dehydrogenase (decarboxylating), 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 188.8.131.520
, 3beta-hydroxysteroid 3-dehydrogenase, it reduces the 3-oxo group back to a 3beta-hydroxyl. Unlike the animal/fungal enzyme EC 184.108.40.206
, 4alpha-methylsterol monooxygenase, and the plant enzymes EC 220.127.116.11
, plant 4,4-dimethylsterol C-4alpha-methyl-monooxygenase, and EC 18.104.22.168
, plant 4alpha-monomethylsterol monooxygenase, this enzyme acts preferentially on the 4beta-methyl group. Since no epimerization of the remaining C-4alpha methyl group occurs, the enzyme can only remove one methyl group, leaving a 4alpha-monomethylated product. Known substrates include 4,4-dimethyl-5alpha-cholest-8-en-3beta-ol and 14-demethyllanosterol.