Kinetics of substrate, coenzyme, and inhibitor binding to Escherichia coli dihydrofolate reductase.

Reduced nicotinamide adenine dinucleotide phosphate (NADPH), folate, dihydrofolate, and the inhibitors trimethoprim and methotrexate bind rapidly and reversibly to both dihydrofolate reductase isoenzymes isolated from Escherichia coli RT500. The coenzyme and substrates appear to bind to only one of the mixture of two forms of the isoenzyme present at equilibrium, while the inhibitors bind to both forms. The proportions of the two forms are different for the two isoenzymes and are pH dependent in each case. The measured association rate constants for substrates and inhibitors lie in the range (1--2) x 10(-7) M-1 s-1 at 25 degrees C but are unlikely to be diffusion controlled. The rate constant for NADPH binding is 2 x 10(6) M-1 s-1. The formation of binary complexes takes place through a multistep mechanism. A minimum of three steps is required to explain the kinetic results. An equilibrium between two or more forms of the enzyme--ligand complex governs the overall dissociation. The stability of this equilibrium is largely responsible for the tighter binding of inhibitors relative to substrate or coenzyme and also for the different binding strengths of inhibitors to the isoenzymes.