The intrinsic pKa-values of functional groups in enzymes: improper deductions from the pH-dependence of steady-state parameters.

The assumptions implicit in the deductions made from the pH-dependence of rate measurements of enzyme-catalyzed reactions are summarized, and the limitations of such determinations are discussed nonalgebraically. The following types of pH-profile are considered (in order of increasing utility): pH-"activity" curves at fixed [S] o;pH-dependences of kcat, Km, kcat/Km, and Ki; pH-dependences of kmodification (by specific reagents) and of competitive labeling (by nonspecific reagents); pH-dependences of elementary steps; and the direct observation of a titrating group.