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 A 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.
Cameron
C E
CE
Department of Chemistry, The Pennsylvania State University, University Park, Pennsylvania 16802, USA.
Benkovic
S J
SJ
D001665:Binding Sites; D002384:Catalysis; D004926:Escherichia coli / Q000201:enzymology*; D005453:Fluorescence; D005998:Glycine / Q000235:genetics / Q000378:metabolism*; D007700:Kinetics; D009682:Magnetic Resonance Spectroscopy; D008958:Models, Molecular; D016297:Mutagenesis, Site-Directed; D009249:NADP / Q000378:metabolism; D011485:Protein Binding; D011487:Protein Conformation; D013762:Tetrahydrofolate Dehydrogenase / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D013816:Thermodynamics; D014633:Valine / Q000235:genetics / Q000378:metabolism
D009249:NADP; D013762:Tetrahydrofolate Dehydrogenase; D014633:Valine; D005998:Glycine