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Flavoprotein monooxygenases, found in species ranging from microorganisms to mammals, transfer one oxygen atom derived from O₂ to a substrate, oxidizing it. In this chapter, we review the enzymes in Groups A and B, which accomplish all their chemistry with just one protein. The catalytic cycles of both groups are roughly similar. NAD (P)H reduces the enzyme-bound flavin, which then reacts with oxygen to form a flavin C4a-(hydro)peroxide - the key oxygenating intermediate. The terminal oxygen of the (hydro)peroxide is transferred to the substrate, leaving the hydroxyflavin, which eliminates water to form oxidized enzyme. Catalysis in both groups is strictly regulated, but in very different ways, to limit NAD(P)H oxidase activity. Group A enzymes only allow the fast reaction of NAD(P)H when the substrate to be oxygenated is bound. In contrast, Group B monooxygenases do not require substrate to be present for rapid flavin reduction, but after the flavin is reduced, NAD(P) remains bound, stabilizing the flavin (hydro)peroxide until it encounters the substrate to be oxygenated. The enzymes in Group A are aromatic hydroxylases; they add oxygen to an activated aromatic ring by electrophilic substitution. The most studied flavoprotein monooxygenase, p-hydroxybenzoate hydroxylase, belongs to this group and will be discussed in detail. The enzymes in Group B catalyze nucleophilic and electrophilic oxygenations. Their substrates include aldehydes, ketones, amines, thiols, boronates, selenides, and thioethers. Conformational changes are important for controlling catalysis in both Group A and Group B monooxygenases. |
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