Information on EC 1.1.1.170 - 3beta-hydroxysteroid-4alpha-carboxylate 3-dehydrogenase (decarboxylating)

The enzyme participates in the biosynthesis of several important sterols such as ergosterol and cholesterol. It is part of a three enzyme system that removes methyl groups from the C-4 position of steroid molecules. The first enzyme, EC 1.14.18.9, 4alpha-methylsterol monooxygenase, catalyses three successive oxidations of the methyl group, resulting in a carboxyl group; the second enzyme, EC 1.1.1.170, catalyses an oxidative decarboxylation that results in a reduction of the 3beta-hydroxy group at the C-3 carbon to an oxo group; and the last enzyme, EC 1.1.1.270, 3beta-hydroxysteroid 3-dehydrogenase, reduces the 3-oxo group back to a 3beta-hydroxyl. If a second methyl group remains at the C-4 position, this enzyme also catalyses its epimerization from 4beta to 4alpha orientation, so it could serve as a substrate for a second round of demethylation. cf. EC 1.1.1.418, plant 3beta-hydroxysteroid-4alpha-carboxylate 3-dehydrogenase (decarboxylating).