The active site of carbonic anhydrase contains a zinc ion Zn2+ as the metal center. The coordination geometry of the zinc ion is tetrahedral. The ligands involved in the coordination are three histidine residues imidazole nitrogen atoms from the protein and a hydroxide ion OH- or water molecule H2O as the fourth ligand.The coordination environment plays a crucial role in the enzyme's catalytic activity and selectivity towards carbon dioxide hydration. The zinc ion activates the hydroxide ion or water molecule by polarizing the O-H bond, making the oxygen more nucleophilic. This activated hydroxide ion can then attack the carbon dioxide molecule, leading to the formation of bicarbonate ion HCO3- .The tetrahedral geometry of the zinc coordination site provides the necessary steric constraints and electronic environment for the selective binding of carbon dioxide. The active site also contains a hydrophobic pocket that helps to orient the CO2 molecule correctly for the nucleophilic attack by the hydroxide ion.Additionally, the enzyme's active site has several amino acid residues that participate in proton transfer steps during the catalytic cycle, facilitating the rapid interconversion between carbon dioxide and bicarbonate. These residues include a histidine residue that acts as a proton shuttle and a network of water molecules that form hydrogen bonds with the enzyme's amino acid residues.In summary, the coordination geometry and ligands in the active site of carbonic anhydrase contribute to the enzyme's catalytic activity and selectivity towards carbon dioxide hydration by providing the necessary steric and electronic environment for the reaction to occur efficiently and selectively.