The coordination geometry of the metal ion in the active site of carbonic anhydrase enzyme is a distorted tetrahedral geometry. The metal ion, which is usually zinc Zn2+ , is coordinated to three histidine residues and a water molecule or hydroxide ion.This coordination geometry plays a crucial role in the function of the enzyme. Carbonic anhydrase catalyzes the reversible hydration of carbon dioxide CO2 to bicarbonate HCO3- and a proton H+ . The zinc ion in the active site serves as a Lewis acid, polarizing the bound water molecule or hydroxide ion, making it more nucleophilic. This allows the nucleophilic attack on the carbon dioxide molecule, facilitating the formation of bicarbonate.The tetrahedral geometry also helps stabilize the transition state of the reaction, lowering the activation energy and increasing the reaction rate. Additionally, the coordination of the zinc ion to the histidine residues helps maintain the proper orientation and positioning of the active site residues, ensuring efficient catalysis.In summary, the distorted tetrahedral coordination geometry of the metal ion in the active site of carbonic anhydrase is essential for the enzyme's function, as it facilitates the catalysis of the CO2 hydration reaction and stabilizes the transition state.