Carbanion versus hydride transfer mechanisms in flavoprotein-catalyzed dehydrogenations

The present understanding of the mechanisms by which flavoproteins oxidize amino acid or hydroxy acids to the respective imino or keto acids is reviewed. The observation that many of these enzymes catalyze the elimination of HBr or HCl from the appropriate β-halogenated substrate was long considered evidence for a carbanion intermediate. Recent structural and mechanistic studies are not compatible with the intermediacy of carbanions in the reactions catalyzed by D-amino acid oxidase and flavocytochrome b₂. In contrast, the data are most consistent with mechanisms involving direct hydride transfer.