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Evidence for environmentally coupled hydrogen tunneling during dihydrofolate reductase catalysis.

Hydride transfer during catalysis by dihydrofolate reductase from Thermotoga maritima has been studied by stopped flow spectroscopy. The reduction of dihydrofolate by NADPH showed a biphasic temperature dependence of the deuterium kinetic isotope effect. At temperatures above 25 degrees C the KIE was temperature independent, while the reaction rates were strongly temperature dependent. Below 25 degrees C the KIE becomes dependent on temperature, and the ratio of the preexponential factors is inverse, suggesting a greater role for active dynamics that modulate the tunneling distance.