Mechanism of the Methylamine Dehydrogenase Reductive Half Reaction

Summary

Bacterial W3A1 methyl ami ne dehydrogenase (MADH) catalyzes the reductive deamination of primary amines. With no electron acceptor present, the amine reduces the enzyme bound methoxatin (PQQ) to a “hydroquinone” species, and product aldehyde, but no ammonia, is released. In stopped flow experiments, we observe 2 kinetically significant intermediates before and 1 after the functionally irreversible PQQ reduction, a reduction that displays an exceptionally large H/D kie. We observe no other substantial H/D kie in the reductive half-reaction. The reaction that follows PQQ reduction, the release of aldehyde from the enzyme, is either fully or partially rate limiting in the catalytic turnover of methylamine and propylamine with phenazine methosulfate as the 1-electron acceptor. Finally, we have been able to reconstruct visible absorption spectra of two proposed catalytic intermediates. Our data suggest that the MADH reductive half reaction may have a mechanism similar to that proposed by Eckert and Bruice (J. Am. Chem. Soc. 105 4431, 1983) for amine reduction of model o-quinones.