Enzymatic binding isotope effects and the interaction of glucose with hexokinase

Reaction rates in both chemistry and enzymology are subject to the isotopic composition of the reactants.1 - 3 The physical basis for isotope effects are well understood and derive from normalmode force constant changes as a reactant progresses to product. Accurate measurements of the relative reaction rates of isotopically-substituted compounds can provide unique information about the structure of a chemical transition state or even the identity of a rate-limiting step. The most common applications of these kinetic isotope effects in enzymology today are the measure of ^D(k_cat/K_m), ^T(k_cat/K_m), and ^Dk_cat. Interest in enzymatic transition states is growing, as high affinity inhibitors may be designed from knowledge of their properties. Although binding equilibrium isotope effects have been less studied, they are critical to the goal of understanding enzyme - substrate interactions at each step of catalysis. A transition state may be modeled from kinetic isotope effects (KIE) without knowledge of equilibrium binding isotope effects (BIE), but equilibrium BIE reveal the atomic interactions at the first step - substrate binding - in these molecular machines. This chapter provides a general consideration of KIE including isotopic sensitivity in prebinding, binding, and internal nonchemical steps. We will dissect the equations defining equilibrium and kinetic isotope effects and describe some of the molecular forces responsible for force constant changes. We present some computational analysis of these effects meant to aid the interpretation of both BIE and KIE, and we conclude with a summary of recent findings in the binding of glucose to human brain hexokinase.