pH-dependence of catalytic constants of the enzyme reaction--some remarks.

The influence of the hydrogen ion concentration on catalytic constants (kcat, KM and kcat/KM) was investigated under steady-state conditions where all steps of the Scheme, are taken into the consideration. The relative value of the rate constants has an influence on pH dependence of catalytic constants. The pH of intersection points of the tangents at the portion of functions pKM, Ig kcat, lg kcat/KM = f(pH) with different slopes are the values of true pKa of catalytically important ionizable groups in the free enzyme or in the enzyme-substrate complex(es), when the conditions k'-1, k'1, k'2 = 0 are valid. If e.g. k'-1, k'1 not equal to 0 the pH of the intersection points of the tangents is sometimes not the true value of the pKa. In this connection it is possible that more than one intersection point exists (in the plot lg kcat, pKM or lg kcat/KM versus pH) when only one ionizing group in the active site of an enzyme is present.