The importance of loop region residues 40-46 in human dihydrofolate reductase as revealed by site-directed mutagenesis.

Site-directed mutagenesis has been used to delete 2 residues (Gly45-Lys46) from a flexible "loop" region between residues 40 and 46 of human dihydrolate reductase. Steady-state kinetic studies show that the Km values for the deletion mutant enzyme for both dihydrofolate and nicotinamide adenine dinucleotide phosphate (reduced) (NADPH) as well as the pH rate profile are virtually identical to that of the wild type. In contrast, the Vmax value of the mutant enzyme is decreased 2.5-fold. The results suggest that the loop region may play a role in the catalytic efficiency but not necessarily in the binding of substrates. Agents such as KCl, urea, and organomercurials at concentrations which show activating effects on the wild-type human dihydrofolate reductase have little or no effect on the deletion mutant. Competitive enzyme-linked immunosorbent assay experiments using peptide-specific antibodies against cyanogen bromide fragments generated from human dihydrofolate reductase show that the binding of folate, NADPH, and methotrexate, either in binary or in ternary complexes with the wild-type enzyme, causes a striking reduction in the binding of the antibodies. Compared with wild type, the binding of these ligands with the deletion mutant enzyme causes much less inhibition (2-16-fold less) in the binding of all three antibodies. The altered properties of the mutant enzyme can be explained on the basis of a need for the flexible loop 40-46 for reversible protein unfolding during activation and also for conformational changes induced by ligand binding, thus "communicating" the effects of ligand binding.