Toward understanding the activity of cobalt carbonic anhydrase: A comparative study of zinc- and cobalt-cyclen

Density functional theory (DFT) calculations were used to study the mechanism of CO₂ hydrolysis by Co-(1, 4, 7, 10-tetrazacyclododecane), also referred to as cobalt-cyclen, and evaluate the associated thermodynamic and kinetic parameters. A microkinetic model was then built based on the kinetics and thermodynamics derived from first principles. The intrinsic reaction rate constant was calculated to be 10572 M^-1 s^-1, three times larger than that of zinc-cyclen. The high activity was ascribed to a very large pK_a value of 13.3. The monodentate structure of a key intermediate along the reaction coordinate, [Cyclen-Co-HCO₃]⁺, was more stable than the bidentate isomer, due to a hydrogen bond formed between bicarbonate and the ligand. The reaction rate constant decreased significantly over 0-12 ms, which was attributed to the fast decrease of the concentration of the catalytic form, Co-OH^-. The conversion of CO₂ at 1000 ms as a function of pH was calculated to compare the relative activity of zinc-cyclen and cobalt-cyclen, and zinc-cyclen was found to be a much better catalyst. Through calculating the ratio of the net rate to the forward rate of each elementary reaction, the rate-limiting step of the catalytic cycle was identified as the adsorption of CO₂, which was the same as that for zinc-cyclen, though the product-releasing step was regarded as more difficult in cobalt-cyclen, compared to zinc-cyclen.