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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-4¦Â methyl group, turning it into a carboxylate group; the second enzyme, SdmB, is a bifunctional enzyme that catalyses two different activities. As EC 1.1.1.417, 3¦Â-hydroxysteroid-4¦Â-carboxylate 3-dehydrogenase (decarboxylating), it catalyses an oxidative decarboxylation that results in reduction of the 3¦Â-hydroxy group at the C-3 carbon to an oxo group. As EC 1.1.1.270, 3¦Â-hydroxysteroid 3-dehydrogenase, it reduces the 3-oxo group back to a 3¦Â-hydroxyl. Unlike the animal/fungal enzyme EC 1.14.18.9, 4¦Á-methylsterol monooxygenase, and the plant enzymes EC 1.14.18.10, plant 4,4-dimethylsterol C-4¦Á-methyl-monooxygenase, and EC 1.14.18.11, plant 4¦Á-monomethylsterol monooxygenase, this enzyme acts preferentially on the 4¦Â-methyl group. Since no epimerization of the remaining C-4¦Á methyl group occurs, the enzyme can only remove one methyl group, leaving a 4¦Á-monomethylated product. Known substrates include 4,4-dimethyl-5¦Á-cholest-8-en-3¦Â-ol and 14-demethyllanosterol. |
