Equation 1 predicts that, for pH>>pKa, the catalytic parameter will reach a plateau, log10P=log10P0, while, for pH<<pKa, log10P=log10P0+pH-pKa, and the dependence of the logarithm of the catalytic parameter with pH is a line of slope unity. Consequently, according to equation 1 the pKa value should be visually apparent in profiles for the pH-dependence of the logarithm of rate parameter as the approximate pH value at which the change of trend is observed (i.e., the pH value for the "kink" in the profile). The catalytic efficiency versus pH profiles can be in fact adequately described by equation 1 with a perturbed pK of about 7, as shown in the left panel of Figure . On the other hand, the profiles for the pH-dependence of the logarithm catalytic rate constant do not show a plateau but are reasonably well described by straight lines (Figure ). It would seem that the pK of the catalytic base in the Michaelis complex is highly perturbed and clearly above the studied pH range (i.e., above 8.5). However, such interpretation in terms of a single pK value does not hold because the slopes of the log10kcat vs. pH lines are clearly below unity. A more reasonable interpretation is perhaps that the Michaelis complex exist in an ensemble of conformations with different pK values (see the Supplementary Discussion). Of course, there may be other factors that could contribute to the seemingly anomalous of the pH-dependence of kcat, among that the equilibrium of formation of the Michaelis complex is not established. Yet, the contribution of an ensemble of conformations in the Michaelis complex is reasonable and it is supported by our molecular dynamics simulations (see "Computational characterization of the altered catalytic activity of the Kemp eliminase variants").