Reaction: 24-methylidenelophenol + 6 ferrocytochrome b5 + 3 O2 + 6 H+ = 3β-hydroxyergosta-7,24(241)-diene-4α-carboxylate + 6 ferricytochrome b5 + 4 H2O (overall reaction)
(1a) 24-methylidenelophenol + 2 ferrocytochrome b5 + O2 + 2 H+ = 4α-(hydroxymethyl)ergosta-7,24(241)-dien-3β-ol + 2 ferricytochrome b5 + H2O
(1b) 4α-(hydroxymethyl)ergosta-7,24(241)-dien-3β-ol + 2 ferrocytochrome b5 + O2 + 2 H+ = 4α-formylergosta-7,24(241)-dien-3β-ol + 2 ferricytochrome b5 + 2 H2O
(1c) 4α-formylergosta-7,24(241)-dien-3β-ol + 2 ferrocytochrome b5 + O2 + 2 H+ = 3β-hydroxyergosta-7,24(241)-diene-4α-carboxylate + 2 ferricytochrome b5 + H2O
Glossary: 24-methylidenelophenol = 4α-methyl-5α-ergosta-7,24-dien-3β-ol
Other name(s): SMO2 (gene name)
Systematic name: 24-ethylidenelophenol,ferrocytochrome-b5:oxygen oxidoreductase (C-4α-methyl-hydroxylating)
Comments: This plant enzyme catalyses a step in the biosynthesis of sterols. It acts on the methyl group of the 4α-methylated intermediates 24-ethylidenelophenol and 24-methylidenelophenol and catalyses three successive oxidations, turning it into a carboxyl group. The carboxylate is subsequently removed by EC 1.1.1.418, plant 3β-hydroxysteroid-4α-carboxylate 3-dehydrogenase (decarboxylating). Unlike the fungal/animal enzyme EC 1.14.18.9, 4α-methylsterol monooxygenase, this enzyme is not able to act on 4,4-dimethylated substrates. That activity, which occurs earlier in the pathway, is performed in plants by a second enzyme, EC 1.14.18.10, plant 4,4-dimethylsterol C-4α-methyl-monooxygenase.
Links to other databases: BRENDA, EXPASY, KEGG, MetaCyc, CAS registry number:
References:
1. Pascal, S., Taton, M. and Rahier, A. Plant sterol biosynthesis. Identification and characterization of two distinct microsomal oxidative enzymatic systems involved in sterol C4-demethylation. J. Biol. Chem. 268 (1993) 11639-11654. [PMID: 8505296]
2. Rahier, A., Smith, M. and Taton, M. The role of cytochrome b5 in 4α-methyl-oxidation and C5(6) desaturation of plant sterol precursors. Biochem. Biophys. Res. Commun. 236 (1997) 434-437. [PMID: 9240456]
3. Darnet, S., Bard, M. and Rahier, A. Functional identification of sterol-4α-methyl oxidase cDNAs from Arabidopsis thaliana by complementation of a yeast erg25 mutant lacking sterol-4α-methyl oxidation. FEBS Lett. 508 (2001) 39-43. [PMID: 11707264]
4. Darnet, S. and Rahier, A. Plant sterol biosynthesis: identification of two distinct families of sterol 4α-methyl oxidases. Biochem. J. 378 (2004) 889-898. [PMID: 14653780]