Evidence for a functional role of the dynamics of glycine-121 of Escherichia coli dihydrofolate reductase obtained from kinetic analysis of a site-directed mutant

Two-dimensional heteronuclear (¹H-¹⁵N) nuclear magnetic relaxation studies of dihydrofolate reductase (DHFR) from Escherichia coli have demonstrated that glycine-121 which is 19 Å from the catalytic center of the enzyme has large-amplitude backbone motions on the nanosecond time Scale [Epstein, D. M., Benkovic, S. J., and Wright, P. E. (1995) Biochemistry 34, 11037-11048]. In order to probe the dynamic-function relationships of this residue, we constructed a mutant enzyme in which this glycine was changed to valine. Equilibrium binding studies indicated that the Val-121 mutant retained wild-type binding properties with respect to dihydrofolate and tetrahydrofolate; however, binding to NADPH and NADP⁺ was decreased by 40- fold and 2-fold, respectively, relative to wild-type DHFR. Single-turnover experiments indicated that hydride transfer was reduced by 200-fold to a rate of 1.3 s^-1 and was the rate-limiting step in the steady state. Interestingly, pre-steady-state kinetic analysis of the Val-121 mutant revealed a conformational change which preceded chemistry that occurred at a rate of 3.5 s^-1. If this step exists in the kinetic mechanism of the wild- type enzyme, then it would be predicted to occur at a rate of approximately 2000 s^-1. Glycine-121 was also changed to alanine, serine, leucine, and proline. While the Ala-121 and Ser-121 mutants behaved similar to wild-type DHFR, the Leu-121 and Pro-121 mutants behaved like Val-121 DHFR in that hydride transfer was the rate-limiting step in the steady state and a conformational change preceding chemistry was observed. Finally, insertion of a glycine or valine between amino acids 121 and 122 produced mutant enzymes with properties similar to wild-type or Val-121 DHFRs, respectively. Taken together, these results provide compelling evidence for dynamic coupling of a remote residue to kinetic events at the active site of DHFR.