The Enzyme Database

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Accepted name: 4β-methylsterol monooxygenase
Reaction: a 3β-hydroxy-4,4-dimethylsteroid + 3 NADH + 3 H+ + 3 O2 = a 3β-hydroxy-4α-methylsteroid-4β-carboxylate + 3 NAD+ + 4 H2O (overall reaction)
(1a) a 3β-hydroxy-4,4-dimethylsteroid + NADH + H+ + O2 = a 3β-hydroxy-4β-hydroxymethyl-4α-methylsteroid + NAD+ + H2O
(1b) a 3β-hydroxy-4β-hydroxymethyl-4α-methylsteroid + NADH + H+ + O2 = a 3β-hydroxy-4β-formyl-4α-methylsteroid + NAD+ + 2 H2O
(1c) a 3β-hydroxy-4β-formyl-4α-methylsteroid + NADH + H+ + O2 = a 3β-hydroxy-4α-methylsteroid-4β-carboxylate + NAD+ + H2O
Other name(s): sdmA (gene name)
Systematic name: 3β-hydroxy-4,4-dimethylsteroid,NADH:oxygen oxidoreductase (C-4mβ-hydroxylating)
Comments: 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-4β methyl group, turning it into a carboxylate group; the second enzyme, SdmB, is a bifunctional enzyme that catalyses two different activities. As EC, 3β-hydroxysteroid-4β-carboxylate 3-dehydrogenase (decarboxylating), 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, 3β-hydroxysteroid 3-dehydrogenase, it reduces the 3-oxo group back to a 3β-hydroxyl. Unlike the animal/fungal enzyme EC, 4α-methylsterol monooxygenase, and the plant enzymes EC, plant 4,4-dimethylsterol C-4α-methyl-monooxygenase, and EC, plant 4α-monomethylsterol monooxygenase, this enzyme acts preferentially on the 4β-methyl group. Since no epimerization of the remaining C-4α methyl group occurs, the enzyme can only remove one methyl group, leaving a 4α-monomethylated product. Known substrates include 4,4-dimethyl-5α-cholest-8-en-3β-ol and 14-demethyllanosterol.
Links to other databases: BRENDA, EXPASY, KEGG, MetaCyc
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]
[EC created 2019]

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