Kinetic Study of the Catalytic Mechanism of Mannitol Dehydrogenase from Pseudomonas fluorescens

To characterize catalysis by NAD-dependent long-chain mannitol 2-dehydrogenases (MDHs), the recombinant wild-type MDH from Pseudomonas fluorescens was overexpressed in Escherichia coli and purified. The enzyme is a functional monomer of 54 kDa, which does not contain Zn2+ and has B-type stereospecificity with respect to hydride transfer from NADH. Analysis of initial velocity patterns together with product and substrate inhibition patterns and comparison of primary deuterium isotope effects on the apparent kinetic parameters, D k cat, D(k cat/K NADH), and D(k cat/K fructose), show that MDH has an ordered kinetic mechanism at pH 8.2 in which NADH adds before d-fructose, and d-mannitol and NAD are released in that order. Isomerization of E−NAD to a form which interacts with d-mannitol nonproductively or dissociation of NAD from the binary complex after isomerization is the slowest step (≥110 s-1) in d-fructose reduction at pH 8.2. Release of NADH from E−NADH (32 s-1) is the major rate-limiting step in mannitol oxidation at this pH. At the pH optimum for d-fructose reduction (pH 7.0), the rate of hydride transfer contributes significantly to rate limitation of the catalytic cascade and the overall reaction. D(k cat/K fructose) decreases from 2.57 at pH 7.0 to a value of ≤1 above pH 9.6, corresponding to the pK of 9.34 observed in the pH profile of k cat/K fructose. Therefore, hydride transfer is not pH-dependent, and d-fructose is not sticky at pH 7.0. A comparison of the kinetic data of MDH and mammalian sorbitol dehydrogenase, presumably involved in detoxification metabolism, is used to point out a physiological function of MDH in the oxidation of d-mannitol with high specificity and fluxional efficiency under prevailing reaction conditions in vivo.