Long-range structural effects in a second-site revertant of a mutant dihydrofolate reductase.Brown, K.A., Howell, E.E., Kraut, J.
(1993) Proc Natl Acad Sci U S A 90: 11753-11756
- PubMed: 8265622
- DOI: https://doi.org/10.1073/pnas.90.24.11753
- Primary Citation of Related Structures:
- PubMed Abstract:
X-ray crystal structures have been determined for a second-site revertant (Asp-27-->Ser, Phe-137-->Ser; D27S/F137S) and both component single-site mutants of Escherichia coli dihydrofolate reductase. The primary D27S mutation, located in the substrate binding pocket, greatly reduces catalytic activity as compared to the wild-type enzyme. The additional F137S mutation, which partially restores catalytic activity, is located on the surface of the molecule, well outside of the catalytic center and approximately 15 A from residue 27. Comparison of kinetic data for the single-site F137S mutant, specifically constructed as a control, and for the double-mutant enzymes indicates that the effects of the F137S and D27S mutations on catalysis are nonadditive. This result suggests that the second-site mutation might mediate its effects through a structural perturbation propagated along the polypeptide backbone. To investigate the mechanism by which the F137S substitution elevates the catalytic activity of D27S we have determined the structure of the D27S/F137S double mutant. We also present a rerefined structure for the original D27S mutant and a preliminary structural interpretation for the F137S single-site mutant. We find that while either single mutant shows little more than a simple side-chain substitution, the double mutant undergoes an extended structural perturbation, which is propagated between these two widely separated sites via the helix alpha B.
Department of Chemistry, University of California at San Diego, La Jolla 92093.