Identification of a gatekeeper residue that prevents dehydrogenases from acting as oxidases

The oxygen reactivity of flavoproteins is poorly understood. Here we show that a single Ala to Gly substitution in l-galactono-gamma-lactone dehydrogenase (GALDH) turns the enzyme into a catalytically competent oxidase. GALDH is an aldonolactone oxidoreductase with a vanillyl-alcohol oxidase (VAO) fold. We found that nearly all oxidases in the VAO family contain either a Gly or a Pro at a structurally conserved position near the C4a locus of the isoalloxazine moiety of the flavin, whereas dehydrogenases prefer another residue at this position. Mutation of the corresponding residue in GALDH (Ala-113 --> Gly) resulted in a striking 400-fold increase in oxygen reactivity, whereas the cytochrome c reductase activity is retained. The activity of the A113G variant shows a linear dependence on oxygen concentration (k(ox) = 3.5 x 10(5) m(-1) s(-1)), similar to most other flavoprotein oxidases. The Ala-113 --> Gly replacement does not change the reduction potential of the flavin but creates space for molecular oxygen to react with the reduced flavin. In the wild-type enzyme, Ala-113 acts as a gatekeeper, preventing oxygen from accessing the isoalloxazine nucleus. The presence of such an oxygen access gate seems to be a key factor for the prevention of oxidase activity within the VAO family and is absent in members that act as oxidases.