The coordination geometry of the metal ion in the active site of the metalloenzyme carbonic anhydrase is a distorted tetrahedral geometry. The metal ion is usually a zinc ion Zn2+ , which is coordinated to three histidine residues from the protein and a hydroxide ion or water molecule as the fourth ligand.The active site of carbonic anhydrase facilitates the enzymatic reaction of converting carbon dioxide CO2 to bicarbonate ion HCO3- through a two-step process:1. Activation of water molecule: The zinc ion polarizes the water molecule or hydroxide ion in the active site, making it more nucleophilic. This is achieved by the positively charged Zn2+ ion attracting the electron density of the water molecule's oxygen atom, which weakens the O-H bond and increases the nucleophilicity of the oxygen.2. Nucleophilic attack on CO2: The activated water molecule or hydroxide ion performs a nucleophilic attack on the carbon atom of CO2, which is electrophilic due to its partial positive charge. This results in the formation of a bicarbonate ion HCO3- and a proton H+ .The distorted tetrahedral geometry of the zinc ion in the active site allows for optimal orientation and interaction between the metal ion, the water molecule or hydroxide ion , and the CO2 molecule. This arrangement ensures efficient catalysis of the reaction, making carbonic anhydrase one of the fastest enzymes known, with a turnover rate of up to 10^6 reactions per second.