The coordination environment of zinc Zn in the active site of the enzyme carbonic anhydrase is a tetrahedral geometry. Zinc is a crucial metal ion in the active site, and it is coordinated to three histidine residues His from the protein and a water molecule or hydroxide ion H2O/OH- as the fourth ligand.The coordination of zinc in carbonic anhydrase contributes to the catalytic activity of the enzyme in the following ways:1. Polarization of water molecule: The presence of zinc ion in the active site polarizes the coordinated water molecule, making the oxygen atom more nucleophilic. This polarization facilitates the deprotonation of the water molecule to generate a hydroxide ion OH- .2. Facilitation of proton transfer: The zinc-bound hydroxide ion acts as a potent nucleophile, which attacks the carbon dioxide CO2 molecule, leading to the formation of bicarbonate ion HCO3- . The active site of carbonic anhydrase contains a proton shuttle residue, usually a histidine, which facilitates the transfer of the proton from the zinc-bound water molecule to the bulk solvent, thus completing the catalytic cycle.3. Stabilization of transition state: The tetrahedral coordination geometry of zinc in the active site helps stabilize the transition state during the nucleophilic attack of the hydroxide ion on the carbon dioxide molecule. This stabilization lowers the activation energy of the reaction, making the enzyme highly efficient in catalyzing the conversion of CO2 to HCO3-.In summary, the coordination environment of zinc in the active site of carbonic anhydrase plays a crucial role in the catalytic activity of the enzyme by polarizing the water molecule, facilitating proton transfer, and stabilizing the transition state during the conversion of CO2 to HCO3-.