The metal center in the active site of the metalloenzyme carbonic anhydrase is a zinc ion Zn . The coordination geometry of the zinc ion is a distorted tetrahedral geometry. In this geometry, the zinc ion is coordinated to three histidine residues imidazole nitrogen atoms from the protein and a water molecule or hydroxide ion.The coordination geometry of the zinc ion contributes to the enzyme's function by facilitating the conversion of carbon dioxide CO to bicarbonate HCO and a proton H . The zinc ion activates the water molecule or hydroxide ion by polarizing the O-H bond, which increases the nucleophilicity of the oxygen atom. This allows the oxygen to attack the carbon dioxide molecule, leading to the formation of bicarbonate.Additionally, the tetrahedral geometry helps stabilize the transition state of the reaction, lowering the activation energy and increasing the reaction rate. The zinc ion also plays a role in proton shuttling, which is essential for the enzyme's catalytic function. The coordination geometry of the zinc ion in the active site of carbonic anhydrase is crucial for its catalytic activity and overall function in the enzyme.