Examination of the reduced affinity of the thymidylate synthase G52S mutation for FdUMP by ab initio and semi-empirical studies

BACKGROUND: The G52S mutation in the Arg50 loop of thymidylate synthase leads to decreased binding of FdUMP. It has been suggested that the mutation affects the Arg50 residue (within the Arg50 loop) responsible for binding the phosphate of FdUMP. The binding of the methylguanidinium moiety as a model for Arg50 to a methylphosphate entity as a model for FdUMP was investigated with theoretical calculations, as well as the structure of the Arg50-Thr51-Gly52 tripeptide in comparison with the Arg50-Thr51-Ser52 tripeptide. METHODS: Gaussian-98 and PC Spartan programs were used to perform Hartree-Fock and Post-Hartree-Fock quantum chemical calculations as well as MNDO (semi-empirical calculations). RESULTS: It was found that the strongest binding occurs between the negative methylphosphate ion and methylguanidine. The replacement of Gly52 by Ser52 leads to a significant displacement of Arg50, which may be responsible for the decreased binding to FdUMP. CONCLUSION: The arginine-phosphate binding appears to be geometry dependent. Thus, the displacement of the Arg50 residue, as observed in these calculated models, upon mutation of Gly52 to Ser may contribute to decreased binding of FdUMP to mTS (G52S).