Steady-state kinetic analysis of the quinoprotein methylamine dehydrogenase from Paracoccus denitrificans

A steady-state kinetic analysis was performed of the reaction of methylamine and phenazine ethosulphate (PES) with the quinoprotein methylamine dehydrogenase from Paracoccus denitrificans. Experiments with methylamine and PES as varied-concentration substrates produced a series of parallel reciprocal plots, and when the concentrations of these substrates were varied in a constant ratio a linear reciprocal plot of initial velocity against PES concentration was obtained. Nearly identical values of V/Km of PES were obtained with four different n-alkylamines. These data suggest that this reaction proceeds by a ping-pong type of mechanism. The enzyme reacted with a variety of n-alkylamines but not with secondary, tertiary or aromatic amines or amino acids. The substrate specificity was dictated primarily by the Km value exhibited by the particular amine. A deuterium kinetic isotope effect was observed with deuterated methylamine as a substrate. The enzyme exhibited a pH optimum for V at pH 7.5. The absorbance spectrum of the pyrroloquinoline quinone prosthetic group of this enzyme was also effected by pH at values greater than 7.5. The enzyme was relatively insensitive to changes in ionic strength, and exhibited a linear Arrhenius plot over a range of temperatures from 10 degrees C to 50 degrees C with an energy of activation 46 kJ/mol (11 kcal/mol).