Electrostatic effects of bound NADH and NAD⁺ on ionizing groups in liver alcohol dehydrogenase

A simple point charge model has been used to estimate the effect of electrostatic interactions of bound NADH and NAD+ on ionizing groups in liver alcohol dehydrogenase. The results provide clear evidence that the much-discussed effects of pH on coenzyme binding in the pH range 7-10 are predominantly of electrostatic origin and attributable to ionization of the water molecule which is bound at the active-site zinc ion. Ionization of Lys-228 is proposed to account for the proton-dependent destabilization of the enzyme X NADH and enzyme X NAD+ complexes which has been reported to occur above a pKa of 10.4.