Dihydrofolate reductase: control of the mode of substrate binding by aspartate 26.

The complex of Lactobacillus casei dihydrofolate reductase with the substrate folate and the coenzyme NADP+ has been shown to exist in solution as a mixture of three slowly interconverting conformations whose proportions are pH-dependent and which differ in the orientation of the pteridine ring of the substrate in the binding site. The Asp26----Asn mutant of L. casei dihydrofolate reductase has been prepared by oligonucleotide-directed mutagenesis and studied by one- and two-dimensional 1H-NMR spectroscopy. NMR studies of the mutant enzyme--folate--NADP+ complex show that this exists to greater than 90% in a single conformation over the pH* range 5-7.1. The single conformation observed corresponds to conformation I (the 'methotrexate-like' conformation) of the wild-type enzyme--folate--NADP+ complex. These observations demonstrate that Asp26 is the ionizable group controlling the pH-dependence of the conformational equilibrium seen in the wild-type enzyme.