Fig. 7: Proposed catalytic mechanism of Co-CA II.

In Co-CA II, CO₂ binding and the catalytic conversion of CO₂ to HCO₃⁻ occur in the same way as in the Zn-CA II with tetrahedral geometry. However, the HCO₃⁻ displacement and proton transfer process are significantly altered due to the coordination expansion to octahedral geometry during catalysis. This octahedral coordination allows bidentate binding mode of HCO₃⁻ and reorganization of negative charge of HCO₃⁻ toward Co²⁺ ion, allowing stronger HCO₃⁻ binding to metal ion. To dissociate the product, proton transfer first occurs via an altered pathway (possibly, W_Co,octa → W2 → His64) and W_Co,octa is converted into the hydroxyl ion. This negatively charged hydroxyl ion then pushes away the bound product, and the tetrahedral coordination is restored for the next catalytic cycle.