Cryo-kinetics Reveal Dynamic Effects on the Chemistry of Human Dihydrofolate Reductase.

Effects of isotopic substitution on the rate constants of human dihydrofolate reductase (HsDHFR), an important target for anti-cancer drugs, have not previously been characterized due to its complex fast kinetics. Here, we report the results of cryo-measurements of the kinetics of the HsDHFR catalyzed reaction and the effects of protein motion on catalysis. Isotopic enzyme labeling revealed an enzyme KIE (kH LE /kH HE ) close to unity above 0 °C; however, the enzyme KIE was increased to 1.72±0.15 at -20 °C, indicating that the coupling of protein motions to the chemical step is minimized under optimal conditions but enhanced at non-physiological temperatures. The presented cryogenic approach provides an opportunity to probe the kinetics of mammalian DHFRs, thereby laying the foundation for characterizing their transition state structure.