Estimated free-energy profiles for the CO2-hydration reaction catalyzed by CA II. The energy states of native CA II (black) are from a previous study (Behravan et al., 1990). The energy states of V143I CA II (red) are qualitatively estimated with respect to the native form by considering the structural information and the variations in the reaction rate constants. Note that the energy level of [EZnH2O + HCO3−] in the V143I variant is assumed to be the same as that in native CA II. The depicted energy gaps are not to scale.