Hydrogen Tunneling in Glucose Oxidation by the Archaeon Thermoplasma acidophilum

Abstract

Abstract The thermophilic archaeon Thermoplasma acidophilum, with an optimal growth temperature in the region of 60 °C, has evolved a D-glucose dehydrogenase, dependent on NADP+ and accepting only the β-anomer of D-glucose, that exhibits a temperature dependence of the rate constants k cat/K mβ for 1-h-β-D-glucose and 1-d-β-D-glucose that indicate two modes of quantum tunneling in the hydride-transfer reaction from substrate to NADP+. Near the optimal temperature for the organism, tunneling seems to occur in a prepared configuration that has most logically been designed by molecular evolution. At lower temperatures, a discontinuity in the temperature dependence of the catalytic rate constant is observed and is thought to arise from a protein structural transition. Below the transition temperature, tunneling appears to occur by a mechanism involving sequential configurational searches for a tunneling state, as is more commonly observed in non-enzymic reactions.