On the role of Brønsted catalysis in Pseudomonas fluorescens mannitol 2-dehydrogenase

Abstract

X-ray structure of the Pseudomonas fluorescens mannitol 2-dehydrogenase ternary complex with NAD+ and D-mannitol suggests that Lys-295 provides catalytic base assistance to secondary alcohol group oxidation. We have replaced Lys-295 by site-directed mutagenesis with alanine or methionine and evaluated the catalytic significance of side-chain substitution by kinetic analysis of restoration of activity with external amines, and from pH and solvent isotope effects on the reaction catalysed by K295A (Lys-295 → Ala mutant). K295A and K295M (Lys-295 → Met mutants) show 3 × 10 4- and 2 × 106-fold lower turnover numbers respectively for D-mannitol oxidation (kcatO) at pH 10.0 than the wild-type. The second-order rate constant for non-covalent rescue of activity (kB) by free methylamine base is 31 M-1 · s -1 for K295A, but only 0.021 M-1 · s-1 for K295M. A Brønsted relationship of log kB (corrected for molecular size effects) and pKa of the external amine is linear (slope β= 0.66 ± 0.16; r2 = 0.99) for K295A-catalysed D-mannitol oxidation at pH 10.0. The kcatO values of K295A in H 2O and 2H2O are linearly dependent on [OL -] in the pL range 7.5-10.5 (where L is 1H or 2H). The solvent isotope effect on kcatO is 0.69. The time course of D-fructose reduction by K295A at pH 8.2 displays a pre-steady-state burst of NADH consumption. These data support a mechanism in which the ε-NH2 group of Lys-295 participates in an obligatory pH-dependent, pre-catalytic equilibrium which may control alcohol/alkoxide equilibration of enzyme-bound D-mannitol and activates the C2 atom for subsequent catalytic oxidation by NAD+.