Conservation of mechanism in flavoprotein-catalyzed amine oxidation

The goals of this presentation are to summarize the present understanding of the mechanism of amine oxidation by flavoproteins and to examine the possibility that a member of the monoamine oxidase family catalyzes oxidation of a carbon-carbon bond. In the discussion of mechanism, the emphasis is on the protonation state of the amine substrate, since the once-controversial mechanism of oxidation appears to be resolved. The argument will be made that flavoproteins catalyzing amine oxidation preferentially bind the form of the substrate in which the reacting nitrogen is uncharged. The reaction of a member of L-6-hydroxynicotine oxidase, which has been proposed to oxidize a carbon-carbon bond in its substrate during nicotine catabolism, is then discussed. Analysis of the reaction product establishes that the enzyme catalyzes oxidation of a carbon-nitrogen. The effects of site-directed mutagenesis and analysis of the substrate specificity identify the key residues for substrate binding.