METHOTREXATE-RESISTANT VARIANTS OF HUMAN DIHYDROFOLATE-REDUCTASE - EFFECTS OF PHE31 SUBSTITUTIONS

Substitution of glycine or alanine for phenylalanine 31 in human dihyd rofolate reductase produces variants that are inhibited less by methot rexate (MTX) than the previously reported serine variant. The 100 time s decrease in MTX affinity for the glycine variant is due to slower bi nding, and to inability of the initial complex to isomerize to a nondi ssociating conformer. A polar group at position 31 is unnecessary for resistance, but residues larger than serine confer no resistance. The glycine variant best fulfills criteria for gene therapy: low K(m) for H-2folate, high k(cat), and good stability. Although k(cat) is unalter ed by these mutations, the rate of hydride transfer is greatly decreas ed. Presteady-state measurements have enabled a complete catalytic sch eme to be constructed for the glycine variant that predicts observed s teady-state behavior. The crystal structures of inhibitor complexes of the serine, alanine, and glycine mutants and of the wild-type enzyme show that the mutations cause little perturbation of the protein backb one, of side chains of residues at the active site, or of the bound in hibitor. A molecule of bound water occupies the space vacated by the p henyl group.