Hypotheses are proposed as solutions to one of the most prominent stereochemical problems in enzymology: why some NAD(P)+-dependent dehydrogenases transfer exclusively the pro-R (A) hydrogen of NAD(P)H while others transfer exclusively the pro-S (B) hydrogen of NAD(P)H. The explanation for the stereoselectivities observed is based on a combination of proposed structural and thermodynamic properties of nicotinamide cofactors and the enzymes that use them, and suggests new ideas for understanding stereoselectivity, catalysis, and evolution in enzymes in general. This proposal seeks funding to test these hypotheses, and will use the hypotheses as a guide to the study of the stereochemical and thermodynamic properties of nicotinamide cofactors and dehydrogenases. Physical techniques (nmr, thermodynamic measurements), enzymological methods (kinetics, stereochemical studies), and combinations of the two will be used. The understanding of enzymes and their catalytic abilities that this work should produce is fundamental for understanding biological systems and disease processes.
