Reaction: 4,4-dimethyl-5α-cholest-7-en-3β-ol + 6 ferrocytochrome b5 + 3 O2 + 6 H+ = 3β-hydroxy-4β-methyl-5α-cholest-7-ene-4α-carboxylate + 6 ferricytochrome b5 + 4 H2O (overall reaction)
(1a) 4,4-dimethyl-5α-cholest-7-en-3β-ol + 2 ferrocytochrome b5 + O2 + 2 H+ = 4α-hydroxymethyl-4β-methyl-5α-cholest-7-en-3β-ol + 2 ferricytochrome b5 + H2O
(1b) 4α-hydroxymethyl-4β-methyl-5α-cholest-7-en-3β-ol + 2 ferrocytochrome b5 + O2 + 2 H+ = 3β-hydroxy-4β-methyl-5α-cholest-7-ene-4α-carbaldehyde + 2 ferricytochrome b5 + 2 H2O
(1c) 3β-hydroxy-4β-methyl-5α-cholest-7-ene-4α-carbaldehyde + 2 ferrocytochrome b5 + O2 + 2 H+ = 3β-hydroxy-4β-methyl-5α-cholest-7-ene-4α-carboxylate + 2 ferricytochrome b5 + H2O
For diagram of reaction click here
Other name(s): methylsterol hydroxylase (ambiguous); 4-methylsterol oxidase (ambiguous); 4,4-dimethyl-5α-cholest-7-en-3β-ol,hydrogen-donor:oxygen oxidoreductase (hydroxylating) (ambiguous); methylsterol monooxygenase (ambiguous); ERG25 (gene name); MSMO1 (gene name); 4,4-dimethyl-5α-cholest-7-en-3β-ol,ferrocytochrome-b5:oxygen oxidoreductase (hydroxylating) (ambiguous)
Systematic name: 4,4-dimethyl-5α-cholest-7-en-3β-ol,ferrocytochrome-b5:oxygen oxidoreductase (C4α-methyl-hydroxylating)
Comments: This enzyme is found in fungi and animals and catalyses a step in the biosynthesis of important sterol molecules such as ergosterol and cholesterol, respectively. The enzyme acts on the 4α-methyl group. Subsequent decarboxylation by EC 1.1.1.170, 3β-hydroxysteroid-4α-carboxylate 3-dehydrogenase (decarboxylating), occurs concomitantly with epimerization of the remaining 4β-methyl into the 4α position, thus making it a suitable substrate for a second round of catalysis. cf. EC 1.14.13.246, 4β-methylsterol monooxygenase; EC 1.14.18.10, plant 4,4-dimethylsterol C-4α-methyl-monooxygenase; and EC 1.14.18.11, plant 4α-monomethylsterol monooxygenase.
Links to other databases: BRENDA, EXPASY, KEGG, MetaCyc, CAS registry number: 37256-80-7
References:
1. Miller, W.L., Kalafer, M.E., Gaylor, J.L. and Delwicke, C.V. Investigation of the component reactions of oxidative sterol demethylation. Study of the aerobic and anaerobic processes. Biochemistry 6 (1967) 2673-2678. [PMID: 4383278]
2. Gaylor, J.L. and Mason, H.S. Investigation of the component reactions of oxidative sterol demethylation. Evidence against participation of cytochrome P-450. J. Biol. Chem. 243 (1968) 4966-4972. [PMID: 4234469]
3. Sharpless, K.B., Snyder, T.E., Spencer, T.A., Maheshwari, K.K. and Nelson, J.A. Biological demethylation of 4,4-dimethyl sterols, Evidence for enzymic epimerization of the 4β-methyl group prior to its oxidative removal. J. Am. Chem. Soc. 91 (1969) 3394-3396. [PMID: 5791927]
4. Brady, D.R., Crowder, R.D. and Hayes, W.J. Mixed function oxidases in sterol metabolism. Source of reducing equivalents. J. Biol. Chem. 255 (1980) 10624-10629. [PMID: 7430141]
5. Fukushima, H., Grinstead, G.F. and Gaylor, J.L. Total enzymic synthesis of cholesterol from lanosterol. Cytochrome b5-dependence of 4-methyl sterol oxidase. J. Biol. Chem. 256 (1981) 4822-4826. [PMID: 7228857]
6. Kawata, S., Trzaskos, J.M. and Gaylor, J.L. Affinity chromatography of microsomal enzymes on immobilized detergent-solubilized cytochrome b5. J. Biol. Chem. 261 (1986) 3790-3799. [PMID: 3949790]