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C-5 sterol desaturase
C-5 sterol desaturase (also known as sterol C-5 desaturase and C5SD) is an enzyme that is highly conserved among eukaryotes and catalyzes the dehydrogenation of a C-5(6) bond in a sterol intermediate compound as a step in the biosynthesis of major sterols. The precise structure of the enzyme’s substrate varies by species. For example, the human C-5 sterol desaturase (also known as lathosterol oxidase) oxidizes lathosterol, while its ortholog in the yeast ''Saccharomyces cerevisiae'' oxidizes episterol.〔〔
==Mechanism==
C-5 sterol desaturase couples sterol oxidation to the oxidation of NAD(P)H and the reduction of molecular oxygen. Either NADH or NADPH can be used; in the model plant species ''Arabidopsis thaliana'' C-5 sterol desaturase catalyzes the reaction twice as fast with NADH while in ''S. cerevisiae'' the enzyme has little preference. The precise details of the reaction have been thought to vary between mammals and yeast.〔 However, the enzymes do share a conserved cluster of histidine resdiues, which when mutated (in ''A. thaliana'') dramatically reduce or eliminate enzyme activity, suggesting the involvement of a coordinated iron cation in the mechanism.〔 Mutagenesis studies suggest that in ''A. thaliana'' threonine 114 (which is a serine in humans, mice, and yeast) may help to stabilize the enzyme-substrate complex.〔 Rahier has proposed a reaction mechanism in which an iron-coordinated oxygen abstracts a hydrogen from the substrate leading to a radical intermediate.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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