Temperature dependence of kinetic parameters for hyperthermophilic glutamate dehydrogenase from Aeropyrum pernix K1

The temperature dependence of the steady-state kinetic parameters for a glutamate dehydrogenase from Aeropyrum pernix K1 was investigated. The enzyme showed a biphasic kinetic characteristic for L-glutamate and a monophasic one for NADP at 50-90 degrees C. At low concentrations of L-glutamate the Km decreased from 2.02 to 0.56 mM and the catalytic efficiency (Vmax/Km) markedly increased (4-150 micromol x mg(-1) x mM(-1)) along with the increase of temperature from 50 to 90 degrees C. At high concentrations of the substrate the Km was fairly high and approximately constant (around 225 mM), and the catalytic efficiency was low and its temperature-dependent change was small. The Km (0.039 mM) for NADP did not change with the increase of temperature. In the reductive amination, the Kms for 2-oxoglutarate (1.81 and 9.37 mM at low and high levels of ammonia, respectively) were independent on temperature, but the Kms for ammonia and NADPH rose from 86 to 185 mM and 0.050 to 0.175 mM, respectively.