Methylsterol monooxygenase (, methylsterol hydroxylase, 4-methylsterol oxidase, 4,4-dimethyl-5alpha-cholest-7-en-3beta-ol,hydrogen-donor:oxygen oxidoreductase (hydroxylating)) is an enzyme with systematic name 4,4-dimethyl-5alpha-cholest-7-en-3beta-ol,NAD(P)H:oxygen oxidoreductase (hydroxylating). This enzyme catalyses the following chemical reaction
4,4-dimethyl-5alpha-cholest-7-en-3beta-ol + 3 NAD(P)H + 3 H<sup>+</sup> + 3 O<sub>2</sub> 3beta-hydroxy-4beta-methyl-5alpha-cholest-7-ene-4alpha-carboxylate + 3 NAD(P)<sup>+</sup> + 4 H<sub>2</sub>O (overall reaction)
(1a) 4,4-dimethyl-5alpha-cholest-7-en-3beta-ol + NAD(P)H + H<sup>+</sup> + O<sub>2</sub> 4beta-hydroxymethyl-4alpha-methyl-5alpha-cholest-7-en-3beta-ol + NAD(P)<sup>+</sup> + H<sub>2</sub>O
(1b) 4beta-hydroxymethyl-4alpha-methyl-5alpha-cholest-7-en-3beta-ol + NAD(P)H + H<sup>+</sup> + O<sub>2</sub> 3beta-hydroxy-4beta-methyl-5alpha-cholest-7-ene-4alpha-carbaldehyde + NAD(P)<sup>+</sup> + 2 H<sub>2</sub>O
(1c) 3beta-hydroxy-4beta-methyl-5alpha-cholest-7-ene-4alpha-carbaldehyde + NAD(P)H + H<sup>+</sup> + O<sub>2</sub> 3beta-hydroxy-4beta-methyl-5alpha-cholest-7-ene-4alpha-carboxylate + NAD(P)<sup>+</sup> + H<sub>2</sub>O
Methylsterol monooxygenase requires cytochrome b5.
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