Abstract
Abstract Site-specific mutations on dihydrofolate reductase from Escherichia coli at the Phe-31 site have generated (Try-31)-DHFR and (Val-31)-DHFR mutant enzymes. The pH dependence of logV and logV⁄KDHF for these enzymes suggests that protonation is important for both the interaction of dihydrofolate and the maximum velocity of the reaction. More importantly, a “hollow” is observed for the Tyr-31 mutant in a logV⁄K–pH profile, necessitating a modification of the wild-type kinetic scheme. The intrinsic pKa of 5.8, obtained based on the modified more general kinetic scheme, for the Tyr-31 mutant agrees well with that obtained from inhibition studies by 2,4-diamino-6,7-dimethylpteridine.
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