KINETICS AND MECHANISM OF THE HYDRATION OF CO2 AND DEHYDRATION OF HCO3- CATALYZED BY A ZN(II) COMPLEX OF 1,5,9-TRIAZACYCLODODECANE AS A MODEL FOR CARBONIC-ANHYDRASE

A detailed kinetic study of the hydration of CO2 and dehydration of HCO3- was performed in the absence and presence of a model Zn(II) complex for carbonic anhydrase. The Zn(II) complex of 1,5,9-triazacyclododecane ([12]aneN(3)) was found to catalyze both the hydration of CO2 and dehydration of HCO3-. The pH dependence of these reactions clearly demonstrates that it is only the hydroxo form of the complex that catalyzes the hydration reaction via CO2 uptake by coordinated hydroxide, whereas it is only the aqua complex that catalyzes the dehydration of HCO3- via a ligand substitution process. The kinetic data reveal acid dissociation constants (expressed as pK, values) of 7.45 +/- 0.10 and 7.29 +/- 0.13 from the hydration and dehydration reactions, respectively, which are in excellent agreement with the thermodynamically determined value of 7.5 for the model Zn(II) complex. The second-order rate constants for the catalytic reaction paths are 581 +/- 64 and 4.8 +/- 0.7 M(-1) s(-1) at 25 degrees C for the hydration of CO2 and dehydration of HCO3-, respectively. The results are discussed in reference to data available for other model systems and carbonic anhydrase itself.