The binding of NAD+, NADH, and ADP-ribose to horse liver alcohol dehydrogenase has been studied calorimetrically as a function of pH at 25 degrees C. The enthalpy of NADH binding is 0 +/- 0.5 kcal mol-1 in the pH range 6 to 8.6. The enthalpy of NAD+ binding, however, varies with pH in a sigmoidal fashion and is -4.0 kcal mol(NAD)-1 at pH 6.0 and +4.5 kcal mol(NAD)-1 at pH 8.6 with an apparent pKa of 7.6 +/- 0.2. The enthalpy of proton ionization of the group on the enzyme is calculated to be in the range 8.8 to 9.8 kcal mol(H+)-1. In conjunction with the available thermodynamic data on the ionization of zinc-bound water in model compounds, it is concluded that the group with a pKa of 9.8 in the free enzyme and 7.6 in the enzyme . NAD+ binary complex is, most likely, the zinc-bound water molecule. Our studies with zinc-free enzyme provide further evidence for this conclusion. Therefore, the processes involving a conformational change of the enzyme upon NAD+ binding and the suggested mechanism of subsequent quenching of the fluorescence of Trp-314 implicating the participation of an ionized tyrosine group must be re-evaluated in the light of this thermodynamic study.