The metal ion present in the active site of the enzyme carbonic anhydrase is zinc Zn . It plays a crucial role in the catalytic conversion of carbon dioxide CO to bicarbonate ion HCO .Carbonic anhydrase facilitates the reversible hydration of CO to form bicarbonate and a proton H as follows:CO + HO HCO + HThe zinc ion in the active site of carbonic anhydrase is coordinated to three histidine residues and a hydroxide ion OH . The hydroxide ion acts as a nucleophile and attacks the carbon atom of CO, which is a weak electrophile. The zinc ion helps to polarize and activate the hydroxide ion by lowering its pKa, making it a stronger nucleophile.During the reaction, the zinc-bound hydroxide ion attacks the CO molecule, forming a zinc-bound bicarbonate ion. The enzyme then releases the bicarbonate ion and binds a water molecule, which is deprotonated to regenerate the zinc-bound hydroxide ion. This process allows the enzyme to catalyze the conversion of CO to HCO rapidly and efficiently.In summary, the zinc ion in the active site of carbonic anhydrase plays a crucial role in the catalytic conversion of CO to HCO by activating the hydroxide ion and facilitating the nucleophilic attack on the CO molecule.