Flavoprotein oxidase catalysis

Examples of this class of enzymes are glucose oxidase and D-amino acid oxidase The classification of the flavoproteins used here is that originally suggested which has been modified recently . In the author s own view the original classification has the advantage of being simple and yet quite useful whereas the new classification does not add to simplify and classify the rather complex picture of flavoprotein catalysis. Nevertheless, in flavoprotein oxidases, the 1,5-dihydroflavin is very reactive towards Oj. On the other hand, the two-electron reduced form of flavoprotein oxidases reacts slowly with pure one-electron acceptors, e.g. ferricyanide. That the two-electron transition is biologically favoured in these enzymes explains why they can react easily with sulfite... 

Flavin-dependent oxynitrilase has many properties common to flavoprotein oxidases. The enzyme binds sulfite in an N5 adduct, and one-electron reduction produces anionic semiquinone.The sulfite adduct and one- or two-electron reduced enzyme are inactive, as is apoenzyme, suggesting that the flavin is involved in catalysis. When the flavin is replaced by 5-carba-5-deaza FAD, a low level of activity is retained. " " However, when the enzyme containing the artificial flavin is exposed to H2O2, the isoalloxazine ring system is partially degraded to a redox-inactive heterocyclic system. " After the formation of this redox-inactive flavin derivative, enzyme activity actually increases dramatically, suggesting that the flavin plays a structural role. It is possible that the redox state of the flavin serves a regulatory role, since enzyme activity decreases when the natural flavin is reduced however, this has yet to be proven. 

Roth J. P Klinman J. P. Catalysis of electron transfer during activation of 02 by the flavoprotein glucose oxidase. Proc. Nat. Acad. Sci. USA 2003, 100, 62-67. 

L-lactate oxidase (LOX) catalyzes the oxidation of L-lactate to pyruvate with the concomitant reduction of the enzyme-hound FMN. The reduced enzyme is oxidized hy molecular oxygen resulting in oxidized enzyme and H207- LOX has substantial sequence homology to other a-hydroxy acid oxidizing flavoproteins, containing all the residues thought to he important for catalysis in this class of enzymes/... 

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