Flavoprotein phenolic hydroxylases

Recently, Anderson and co-workers have suggested that radical intermediates may be formed in the mechanism of the flavoprotein phenolic hydroxylases 244-246). Their approach has centered on comparing the absorption spectra of inter-... 

PHBH is the protype of the flavoprotein aromatic hydroxylases. Each subunit of this dimeric enzyme contains two active sites which, during catalysis, are alternately visited by the isoalloxazine ring of the FAD cofactor (31). Catalysis is iiutiated by reduction of the flavin in the exterior active site. The reduced flavin then moves to the interior active site where the reactions with oxygen occur. A similar conformational flexibility of the FAD cofactor has been observed in the crystal structures of phenol hydroxylase (EC 1.14.13.7) and 3-hydroxybenzoate 4-hydroxylase (EC 1.14.13.23). PHBH obeys the following kinetic mechanism ... 

The flavoprotein phenol-2-hydroxylase (EC 1.14.13.7) and the o-phenol-splitting catechol-1,2-oxygenase (EC 1.13.1.1) are prepared from... 

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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