The Enzyme Database

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Accepted name: plant 4,4-dimethylsterol C-4α-methyl-monooxygenase
Reaction: 24-methylidenecycloartanol + 6 ferrocytochrome b5 + 3 O2 + 6 H+ = 3β-hydroxy-4β,14α-dimethyl-9β,19-cyclo-5α-ergost-24(241)-en-4α-carboxylate + 6 ferricytochrome b5 + 4 H2O (overall reaction)
(1a) 24-methylidenecycloartanol + 2 ferrocytochrome b5 + O2 + 2 H+ = 4α-(hydroxymethyl)-4β,14α-dimethyl-9β,19-cyclo-5α-ergost-24(241)-en-3β-ol + 2 ferricytochrome b5 + H2O
(1b) 4α-(hydroxymethyl)-4β,14α-dimethyl-9β,19-cyclo-5α-ergost-24(241)-en-3β-ol + 2 ferrocytochrome b5 + O2 + 2 H+ = 4α-formyl-4β,14α-dimethyl-9β,19-cyclo-5α-ergost-24(241)-en-3β-ol + 2 ferricytochrome b5 + 2 H2O
(1c) 4α-formyl-4β,14α-dimethyl-9β,19-cyclo-5α-ergost-24(241)-en-3β-ol + 2 ferrocytochrome b5 + O2 + 2 H+ = 3β-hydroxy-4β,14α-dimethyl-9β,19-cyclo-5α-ergost-24(241)-en-4α-carboxylate + 2 ferricytochrome b5 + H2O
Glossary: 24-methylidenecycloartanol = 4α,4β,14α-trimethyl-9β,19-cyclo-5α-ergost-24(241)-en-3β-ol
Other name(s): SMO1 (gene name)
Systematic name: 24-methylidenecycloartanol,ferrocytochrome-b5:oxygen oxidoreductase (C-4α-methyl-hydroxylating)
Comments: This plant enzyme catalyses a step in the biosynthesis of sterols. It acts on the 4α-methyl group of the 4,4-dimethylated intermediate 24-methylidenecycloartanol and catalyses three successive oxidations, turning it into a carboxyl group. The carboxylate is subsequently removed by EC, plant 3β-hydroxysteroid-4α-carboxylate 3-dehydrogenase (decarboxylating), which also catalyses the epimerization of the remaining 4β-methyl into the 4α position. Unlike the fungal/animal enzyme EC, 4α-methylsterol monooxygenase, this enzyme is not able to remove the methyl group from C-4-monomethylated substrates. That activity is performed in plants by a second enzyme, EC, plant 4α-monomethylsterol monooxygenase.
Links to other databases: BRENDA, EXPASY, KEGG, MetaCyc
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. [DOI] [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]
[EC created 2019]

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