Flavoproteins reaction with oxygen

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 ... 

Cytochrome Reductase of Yeast. A third yeast flavoprotein was isolated by Haas and co-workers. This was called cytochrome reductase because its rate of reaction with cytochrome c is over 150,000 times as great as the rate of cytochrome c reduction by old yellow enzyme. Its rate of reaction with oxygen is only 8 per cent that of old yellow enzyme. The prosthetic group is FMN. Reduction occurs only with TPNH. Another enzyme, DPN-cytochrome reductase, was partially purified in Hogness laboratory. 

The oxygen reactivity of flavohydroquinone bound to apoflavoprotein dehydrogenases can vary considerably from fast (flavodoxins), moderate (xanthine oxidase) to nil (succinate dehydrogenase) Most, but not all, flavoprotein dehydrogenases contain one or more types of metal prosthetic groups, e.g. xanthine oxidase contains also Fe and Mo. Since these metal ions are involved in electron flux, their possible participation in the reaction with O2 cannot be excluded. Much evidence, however, indicates that the flavin is involved in the one-electron reduction of Oj, as shown in Equation (5). 

Metalloenzymes of at least three different types catalyze the destruction of superoxide radicals that arise from reactions of oxygen with heme proteins, reduced flavoproteins, and other metalloenzymes. These superoxide dismutases (SODs) convert superoxide anion radicals 02 into H202 and 02 (Eq. 16-26). The H202 can then be destroyed by catalase (Eq. 16-8). 

Figure 6 General mechanism for flavoprotein monooxygenases. With Baeyer-Villiger monooxygenases (nucleophilic oxygenation), NADP+ stays bound during the entire reaction cycle.

Another important flavoprotein reaction occurs during the stimulation of neutrophils, which leads to a flux of superoxide radicals via the reduction of oxygen by a membrane-bound flavoprotein. Spin-trapping experiments with DMPO [149,150] have confirmed that superoxide radicals are released into the medium. Both O2 and DH (formed by secondary reactions of Oj) have been spin-trapped in neutrophil systems. Spin-trapped radicals are not observed in the presence of superoxide dismutase. 

Most of the microsomal reactions can be classified as oxidations by what are referred to as mixed-function oxidases utilizing molecular oxygen and cofactors. The key enzyme is an iron-hemecytochrome P-450, a flavoprotein dependent in its reduction and reoxidation on the NADPH to NADP reaction. The 450 notation is based on the 450 nm absorption peak the enzyme exhibits on reaction with carbon monoxide. Thus, drug interactions with this enzyme system can be evaluated by measuring absorption spectra changes. 

Until recently, the amino acid oxidase reaction has been studied only in the direction of ammonia and a-keto acid formation. In the presence of air the reaction proceeds to completion and is essentially irreversible because of the reoxidation of the reduced flavoprotein by molecular oxygen [reaction (5)]. Meister and his associates 11, 12) have provided a clear demonstration of the reversibility of the amino acid oxidase reaction with D-amino acid oxidase (from sheep kidney) and L-amino acid oxidase (from snake venom). When an amino acid, ammonia, and the a-keto acid analog of a second amino acid are incubated with either amino acid oxidase under anaerobic conditions, the formation of the second amino acid is observed ... 

In flavin-dependent monooxygenases, a flavin-oxygen intermediate reacts with the substrate, also producing water in a second step, and requiring cofactors for regeneration of the flavin moiety. The unusual flavoprotein vanillyl-alcohol oxidase (EC 1.1.3.38), in which the flavin moiety is covalently bound, catalyzes the oxidation of p-substituted phenols as well as deamination, hydroxylation and dehydrogenation reactions [10]. 

Anderson, R. F. Flavin-oxygen complex formed on the reaction of superoxide ions with flavo-semiquinone radicals. In Flavins and flavoproteins (Massey, V., Williams, C. H. eds.) pp. 278-283, New York, Elsevier North Holland 1982... 

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