Thermodynamics of carbonic anhydrase catalysis. A comparison between human isoenzymes B and C.

摘要: The CO2 hydration and HCO3- dehydration activities of human red cell carbonic anhydrase isozymes B C (HCAB HCAC) have been studied as a function temperature from 0 degrees to 37 C. Arrhenius plots ln kcat versus 1/T are linear for both in reactions, indicating that the rate-determining steps remain unchanged over this range. kcat, at pH 7.5, is 13 X 10(5) s-1 isozyme 0.71 B. Km, hydration, 10 mM 5 B, invariant with temperature. uncatalyzed reactions significantly affected by temperature, 30- 40-fold rate enhancements being observed enzyme-catalyzed processes much less sensitive 2- 3-fold HCAB 5- 6-fold HCAC These observations consistent significant lowering free energy activation isozymes. This effect greater accounting its higher catalytic power. enthalpy activation, 7.5 8.2, rate-limiting step considerably enzyme compared is, however, more than offset large negative entropy case HCAB. observation indicates either mechanistic difference events or structural organizations active sites two isozymes, both.