The effects of hydrogen ion concentration on the simplest steady-state enzyme systems

Abstract

Laidler (1955) showed that consideration of the effect of pH on enzymic mechanisms that obey steady-state kinetics leads to the inclusion in the equations of a ‘perturbation term’ that can introduce curvature into the Lineweaver–Burk plots. He also stated conditions in which this term vanishes. This term can lead to apparent activation by substrate. Further, several cases are shown in which simplification, but not disappearance, of the perturbation term can lead to linearity of Lineweaver–Burk plots. These cases arise when the ionization of groups at the active site either is unaffected or is completely prevented when the enzyme–substrate complex is formed. It is also shown that V_(app.) can vary with pH without a concomitant change in K_m(app.) in certain cases that obey steady-state kinetics without implying that K_m=K_s. When the perturbation term is significant, Dixon's (1953) rules for the calculation of pK values will not always apply.