Kinetic Study of CO2 Hydration by Small-Molecule Catalysts with A Second Coordination Sphere that Mimic the Effect of the Thr-199 Residue of Carbonic Anhydrase

Abstract

Zinc complexes were synthesized as catalysts that mimic the ability of carbonic anhydrase (CA) for the CO₂ hydration reaction (H₂O + CO₂ → H⁺ + HCO₃^-). For these complexes, a tris(2-pyridylmethyl)amine (TPA) ligand mimicking only the active site, and a 6-((bis(pyridin-2-ylmethyl)amino)methyl)pyridin-2-ol (TPA-OH) ligand mimicking the hydrogen-bonding network of the secondary coordination sphere of CA were used. Potentiometric pH titration was used to determine the deprotonation ability of the Zn complexes, and their pK_a values were found to be 8.0 and 6.8, respectively. Stopped-flow spectrophotometry was used to confirm the CO₂ hydration rate. The rate constants were measured to be 648.4 and 730.6 M^-1s^-1, respectively. The low pK_a value was attributed to the hydrogen-bonding network of the secondary coordination sphere of the catalyst that mimics the behavior of CA, and this was found to increase the CO₂ hydration rate of the catalyst.

Keywords: CO2 hydration; Thr-199; carbonic anhydrase; kinetics; stopped-flow spectrophotometer.