In the active site of carbonic anhydrase, the coordination geometry of the zinc ion is tetrahedral. The metal ion oxidation state of zinc is +2 Zn2+ . The ligand binding sites consist of three histidine residues His94, His96, and His119 in human carbonic anhydrase II and a water molecule or hydroxide ion H2O/OH- as the fourth ligand.These structural features contribute to the enzyme's catalytic activity in the following ways:1. The tetrahedral coordination geometry of the zinc ion allows for optimal interaction with the substrate CO2 and the nucleophilic hydroxide ion. The geometry helps to stabilize the transition state and lower the activation energy of the reaction.2. The Zn2+ ion has a strong affinity for electron-rich ligands, such as water or hydroxide ions. This affinity helps to polarize the water molecule, making it more nucleophilic and facilitating the attack on the carbon atom of CO2.3. The three histidine residues provide a stable coordination environment for the zinc ion, ensuring that it remains in the active site during the catalytic cycle. Additionally, these residues help to position the zinc ion and the substrate in the optimal orientation for catalysis.4. The water molecule or hydroxide ion bound to the zinc ion plays a crucial role in the enzyme's catalytic activity. In the first step of the reaction, the zinc-bound water molecule loses a proton to a nearby residue such as Glu106 in human carbonic anhydrase II , generating a nucleophilic hydroxide ion. This hydroxide ion then attacks the carbon atom of CO2, forming bicarbonate HCO3- . In the final step, the bicarbonate ion is released from the active site, and a new water molecule binds to the zinc ion, resetting the enzyme for another round of catalysis.Overall, the coordination geometry, metal ion oxidation state, and ligand binding sites of zinc in the active site of carbonic anhydrase are essential for the enzyme's ability to efficiently catalyze the conversion of CO2 to bicarbonate and protons.