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IUBMB CommentsThis plant enzyme catalyses a step in the biosynthesis of sterols. It acts on the methyl group of the 4alpha-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 3beta-hydroxysteroid-4alpha-carboxylate 3-dehydrogenase (decarboxylating). Unlike the fungal/animal enzyme EC 1.14.18.9, 4alpha-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-4alpha-methyl-monooxygenase.
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24-ethylidenelophenol,ferrocytochrome-b5:oxygen oxidoreductase (C-4alpha-methyl-hydroxylating)
This plant enzyme catalyses a step in the biosynthesis of sterols. It acts on the methyl group of the 4alpha-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 3beta-hydroxysteroid-4alpha-carboxylate 3-dehydrogenase (decarboxylating). Unlike the fungal/animal enzyme EC 1.14.18.9, 4alpha-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-4alpha-methyl-monooxygenase.
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3beta-hydroxy-4beta-methyl-5alpha-cholest-7-ene-4alpha-carbaldehyde + a ferrocytochrome + O2
3beta-hydroxy-4beta-methyl-5alpha-cholest-7-ene-4alpha-carboxylate + a ferricytochrome + H2O
step 3 of 3 of the overall reaction
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4,4-dimethyl-5alpha-cholest-7-en-3beta-ol + a ferrocytochrome + O2
4beta-hydroxymethyl-4alpha-methyl-5alpha-cholest-7-en-3beta-ol + a ferricytochrome + H2O
step 1 of 3 of the overall reaction
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4beta-hydroxymethyl-4alpha-methyl-5alpha-cholest-7-en-3beta-ol + a ferrocytochrome + O2
3beta-hydroxy-4beta-methyl-5alpha-cholest-7-ene-4alpha-carbaldehyde + a ferricytochrome + H2O
step 2 of 3 of the overall reaction
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(3alpha,4alpha,5alpha,10alpha,13alpha,20S)-4-methyl-9,19-cycloergosta-7,24(28)-dien-3-ol + 6 ferrocytochrome b5 + 3 O2 + 6 H+
(3alpha,4alpha,5alpha,10alpha,13alpha,20S)-3-hydroxy-9,19-cycloergosta-7,24(28)-diene-4-carboxylic acid + 6 ferricytochrome b5 + 4 H2O
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(3alpha,4alpha,5alpha,10alpha,13alpha,20S)-4-methylcholest-7-en-3-ol + 6 ferrocytochrome b5 + 3 O2 + 6 H+
(3alpha,4alpha,5alpha,10alpha,13alpha,20S)-3-hydroxycholest-7-ene-4-carboxylic acid + 6 ferricytochrome b5 + 4 H2O
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ir
24-ethylidenelophenol + 6 ferrocytochrome b5 + 3 O2 + 6 H+
3beta,4alpha,5alpha-3-hydroxystigmasta-7,24(28)-diene-4-carboxylate + 6 ferricytochrome b5 + 4 H2O
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24-methylidenelophenol + 6 ferrocytochrome b5 + 3 O2 + 6 H+
3beta-hydroxyergosta-7,24(241)-dien-4alpha-carboxylate + 6 ferricytochrome b5 + 4 H2O
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ir
3beta-hydroxy-4beta-methyl-5alpha-cholest-7-ene-4alpha-carbaldehyde + a ferrocytochrome + O2
3beta-hydroxy-4beta-methyl-5alpha-cholest-7-ene-4alpha-carboxylate + a ferricytochrome + H2O
step 3 of 3 of the overall reaction
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4,4-dimethyl-5alpha-cholest-7-en-3beta-ol + a ferrocytochrome + O2
4beta-hydroxymethyl-4alpha-methyl-5alpha-cholest-7-en-3beta-ol + a ferricytochrome + H2O
step 1 of 3 of the overall reaction
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4beta-hydroxymethyl-4alpha-methyl-5alpha-cholest-7-en-3beta-ol + a ferrocytochrome + O2
3beta-hydroxy-4beta-methyl-5alpha-cholest-7-ene-4alpha-carbaldehyde + a ferricytochrome + H2O
step 2 of 3 of the overall reaction
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additional information
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additional information
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conversion of cycloartenol into functional phytosterol requires the removal of 2 methyl groups at C4 by the enzymic complex including the sterol 4alpha-methyl oxidase SMO
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additional information
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conversion of cycloartenol into functional phytosterol requires the removal of 2 methyl groups at C4 by the enzymic complex including the sterol 4alpha-methyl oxidase SMO
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additional information
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conversion of cycloartenol into functional phytosterol requires the removal of 2 methyl groups at C4 by the enzymic complex including the sterol 4alpha-methyl oxidase SMO
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additional information
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conversion of cycloartenol into functional phytosterol requires the removal of 2 methyl groups at C4 by the enzymic complex including the sterol 4alpha-methyl oxidase SMO
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Darnet, S.; Rahier, A.
Plant sterol biosynthesis: identification of two distinct families of sterol 4alpha-methyl oxidases
Biochem. J.
378
889-898
2004
Arabidopsis thaliana (Q8VWZ8), Arabidopsis thaliana (Q9ZW22), Nicotiana benthamiana (Q5J954), Nicotiana tabacum (Q6YAU6)
brenda
Darnet, S.; Bard, M.; Rahier, A.
Functional identification of sterol-4alpha-methyl oxidase cDNAs from Arabidopsis thaliana by complementation of a yeast erg25 mutant lacking sterol-4alpha-methyl oxidation
FEBS Lett.
508
39-43
2001
Arabidopsis thaliana (Q9ZW22)
brenda
Rahier, A.
Dissecting the sterol C-4 demethylation process in higher plants. From structures and genes to catalytic mechanism
Steroids
76
340-352
2011
Arabidopsis thaliana
brenda