Dihydrofolate reductase as a model for biological hydmde transfer

The mechanisms of catalysis and inhibition of dihydrofolate reductase have been evaluated with Poisson-Boltzmann electrostatic and quantum chemical (alculations. and the results have been used as a basis for experimental design in order to test the putative catalytic mechanism. The studies indicate that the catalytically important residue Asp 27 is ionized in the apoenzyme and that the experimentally observed pK of 6.5 associated with the catalytic chemical step is due to the formation of the enol tautomer of the substrate's pterin ring. The tautomer is induced to form as a result of substrate binding, in which the substrata desolvates the active site and binds to the carboxylate of Asp 27. Although the binding reaction is favorable, desolvation and burial of the negative charge on Asp 27 is not. Protonation of Asp 27 occurs, concerted with tautomerization of the substrate, resulting in active site electrical neutrality and activation of the substrata for catalysis. The data presented here support the proposal that correlation of general acid, general base behavior between roups in an enzyme active ite is an important aspect of enzymatic catalysis.