Deuterium kinetic isotope effect and stopped-flow kinetic studies of the quinoprotein methylamine dehydrogenase.

Stopped-flow kinetic studies of the reductive half-reaction of methylamine dehydrogenase from Paracoccus denitrificans yielded kinetic constants for the reversible formation of the imine intermediate formed between the substrate and the tryptophan tryptophylquinone (TTQ) prosthetic group and for the hydrogen abstraction step which occurs concomitantly with TTQ reduction. When CD3NH2 was used as a substrate, deuterium kinetic isotope effects of 4.2 and 3.8, respectively, were measured for the rate constants that correspond to the formation and dissociation of the iminoquinone intermediate. A deuterium kinetic isotope effect of 17.2 was measured for the hydrogen abstraction step. The maximum deuterium kinetic isotope effect which was measured in steady-state kinetic experiments was 3.0. These data are discussed in relation to the reaction mechanism of methylamine dehydrogenase and the similar large deuterium kinetic isotope effect for hydrogen abstraction which has been observed for another quinoprotein, plasma amine oxidase.