The metal ion in the active site of the enzyme carbonic anhydrase is zinc Zn . The coordination geometry of the zinc ion is tetrahedral, and its oxidation state is +2.The zinc ion is coordinated by three histidine residues from the protein and a hydroxide ion OH or water molecule HO as the fourth ligand. The tetrahedral coordination geometry and the +2 oxidation state of the zinc ion are crucial for the function of the enzyme.Carbonic anhydrase catalyzes the reversible hydration of carbon dioxide CO to form bicarbonate HCO and a proton H . The mechanism involves the nucleophilic attack of the zinc-bound hydroxide ion on the carbon dioxide molecule. The tetrahedral geometry of the zinc ion allows for proper orientation and stabilization of the hydroxide ion, making it a more effective nucleophile.The +2 oxidation state of the zinc ion also plays a role in stabilizing the transition state of the reaction and facilitating proton transfer. The positively charged zinc ion helps to polarize the bound water molecule, making it easier to lose a proton and form the hydroxide ion. Additionally, the zinc ion helps to stabilize the negatively charged bicarbonate intermediate, allowing for the release of the product and regeneration of the active site.In summary, the tetrahedral coordination geometry and +2 oxidation state of the zinc ion in the active site of carbonic anhydrase are essential for the enzyme's function, as they enable proper orientation and stabilization of the reacting species, facilitate proton transfer, and stabilize the transition state and intermediates.