Squalene Epoxidase

Squalene monooxygenase (also called squalene epoxidase) is an enzyme that uses NADPH and molecular oxygen to oxidize squalene to 2,3-oxidosqualene (squalene epoxide). Squalene epoxidase catalyzes the first oxygenation step in sterol biosynthesis and is thought to be one of the rate-limiting enzymes in this pathway. In humans, squalene epoxidase is encoded by the SQLE gene.[6] Several eukaryote genomes lack a squalene monooxygenase encoding gene, but instead encode an alternative squalene epoxidase that catalyzes the oxidation of squalene. The canonical squalene monooxygenase is a flavoprotein monooxygenase. Flavoprotein monooxygenase form flavin hydroperoxides at the enzyme active site, which then transfer the terminal oxygen atom of the hydroperoxide to the substrate. Squalene monooxygenase differs from other flavin monooxygenases in that the oxygen is inserted as an epoxide rather than as a hydroxyl group. Squalene monooxygenase contains a loosely bound FAD flavin and obtains electrons from NADPH-cytochrome P450 reductase, rather than binding the nicotinamide cofactor NADPH directly. The alternative squalene epoxidase belongs to the fatty acid hydroxylase superfamily and obtains electrons from cytochrome b5.