Carbonic anhydrase is a metalloenzyme that contains a zinc Zn ion in its active site. The coordination number of the zinc ion in the active site is 4, which means that it is coordinated to four different ligands.The geometry of the metal ion in the active site is tetrahedral. The zinc ion is coordinated to three histidine residues His94, His96, and His119 from the protein and a hydroxide ion OH or a water molecule HO as the fourth ligand. The three histidine residues are part of the enzyme's polypeptide chain, while the hydroxide ion or water molecule is a substrate or a product of the reaction.The coordination environment of the zinc ion facilitates the enzymatic function of carbonic anhydrase by stabilizing the transition state and lowering the activation energy of the reaction. The zinc ion polarizes the water molecule, making it more nucleophilic and facilitating the attack on the carbon dioxide CO molecule. The hydroxide ion acts as a nucleophile, attacking the carbon atom of the CO molecule, leading to the formation of bicarbonate ion HCO .Additionally, the zinc ion helps to stabilize the negatively charged transition state by providing electrostatic interactions. The tetrahedral geometry of the active site also ensures proper orientation of the reacting molecules, which is crucial for the reaction to proceed efficiently.In summary, the coordination number of the metal ion Zn in the active site of carbonic anhydrase is 4, and its geometry is tetrahedral. The coordination environment of the metal ion plays a crucial role in facilitating the enzymatic function of carbonic anhydrase by stabilizing the transition state, lowering the activation energy, and ensuring proper orientation of the reacting molecules.