Catalytic cycle of mammalian FMOs (Homo sapiens)
Flavin-containing monooxygenases are a group of enzymes that catalyze the oxygenation of substrates, mostly soft nucleophiles via the cofactor flavin. In the catalytic cycle, FMO binds to NADPH and to FAD, causing the reduction of FAD to FADH2. Next, molecular oxygen binds to the complex and is reduced to a hydroperoxide form, called 4a-hydroperoxyflavin. This complex is stable in the absence of a substrate. When a substrate is present, the distal O-atom of the complex is transferred to the substrate yielding an oxygenated product and leaving the flavincomplex 4a-hydroxyflavin that breaks down releasing water. At the end of the cycle, NADP+ is released resulting in FAD as the flavin form to start a next cycle. In contrast to cytochrome P450 enzymes, FMOs are generally not induced or inhibited by xenobiotic substances. The five human FMOs are tissue specific: FMO1 is present in the human fetal liver and the adult kidney, FMO2 is present in the lung and FMO3 is present in the adult liver.
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