Fig. 8. Proposed catalytic mechanism of Ni-CA II.

In Ni-CA II, octahedral coordination is maintained throughout the whole catalytic cycle. The significant consequence is that one of the three bound water molecules experiences steric hindrance with the CO₂ binding. In addition, the nucleophilic attack geometry is distorted (Fig. 5b), suggesting less efficient conversion into HCO₃⁻. The formed HCO₃⁻ is strongly bound to Ni²⁺ ion in a bidentate mode as in the Co-CA II but is directly displaced by two inflowing water molecules. Finally, proton transfer occurs via an altered network (possibly, W_Ni′ → W_Ni → W2 → His64) to restore the catalytic cycle.