Convex Arrhenius plots and their interpretation

This paper draws attention to selected experiments on enzyme-catalyzed reac tions that show convex Arrhenius plots, which are very rare, and points out that Tolman's interpretation of the activation energy places a fundamental model-independent constraint on any detailed explanation of these reaction s. The analysis presented here shows that in such systems, the rate coeffic ient as a function of energy is not just increasing more slowly than expect ed, it is actually decreasing. This interpretation of the data provides a c onstraint on proposed microscopic models, i.e., it requires that any succes sful model of a reaction with a convex Arrhenius plot should be consistent with the microcanonical rate coefficient being a decreasing function of ene rgy. The implications and limitations of this analysis to interpreting enzy me mechanisms are discussed, This model-independent conclusion has broad ap plicability to all fields of kinetics, and we also draw attention to an ana logy with diffusion in metastable fluids and glasses.