The causes of sharply bent or discontinuous Arrhenius plots for enzyme-catalysed reactions

Two models are proposed to account for the abrupt changes in the enthalpy of activation (delta H*) that are sometimes observed, without corresponding abrupt changes in the maximum velocity (V), at apparent transition temperatures in the Arrhenius plots of enzyme-catalyzed reactions. Both models predict that a small but observable discontinuity in V will usually occur, which distinguishes such plots from the sharply bent but continuous plots that result when the individual rate constants contributing to V have widely different temperature dependencies. One model applies to membrane-bound enzymes only, and predicts that delta H* will always be smaller above the transition. The other applies to both soluble and membrane-bound enzymes, and can account for either an increase or a decrease in delta H* as the temperature is raised.