Flavin monooxygenases oxidizing species

Tertiary amines such as trimethylamine and dimethylamine had long been known to be metabolized to A -oxides by a microsomal amine oxidase that was not dependent on CYP. This enzyme, now known as the microsomal flavin-containing monooxygenase (FMO), is also dependent on NADPH and 02, and has been purified to homogeneity from a number of species. Isolation and characterization of the enzyme from liver and lung samples provided evidence of clearly distinct physicochemical properties and substrate specificities suggesting the presence of at least two different isoforms. Subsequent studies have verified the presence of multiple forms of the enzyme. 

Flavoprotein monooxygenases mainly use NAD(P)H as electron donor and insert one atom of molecular oxygen into then-substrates. Oxygen activation of flavoprotein monooxygenases involves the (transient) stabilization of a flavin C4a-(hydro) peroxide. This species performs either a nucleophilic or electrophilic attack on the substrate (Fig. 6). Oxygenation reactions catalyzed by flavoprotein monooxygenases include hydroxyla-tions, epoxidations, Baeyer-Villiger oxidations, and sulfoxida-tions (43). 

The mechanism for the hydroxylation of aromatic substrates by flavoprotein monooxygenases has been the subject of signiflcant research interest and controversy over the past decade. These enzymes (p-hydroxybenzoate hydroxylase, phenol hydroxylase, and melilotate hydroxylase) catalyze the initial step in the )8-ketoadipic acid pathway, the hydroxylation of substituted phenols into catechols (Scheme 55). Oxygen is required as cosubstrate, which is activated by the reduced FAD cofactor. The complex mechanism for the oxidative half-reaction is thought to consist of at least four steps and three intermediates 239-242) and to involve a controversial 4a,5-ring-opened flavin 242, 249, 250) (Scheme 56). The flavin C4a-hydroperoxy intermediate 64 and flavin C4a-hydroxy intermediate 65 have been assigned the structures shown in Scheme 56 based on the UV absorbance spectra of various model compounds compared with that of the modified enzyme cofactor alkylated at N(5) 243). However, evidence for the intermediacy of various ring-opened flavin species has been tentative at best, as model compounds and model reactions do not support such an intermediate 242). 

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