The coordination geometry of the active site in the metalloenzyme carbonic anhydrase is a distorted octahedral geometry. The metal ion in the active site is typically a zinc ion Zn , which is coordinated by three histidine residues and a water molecule or hydroxide ion. The remaining two positions in the octahedral geometry are occupied by the substrate carbon dioxide and an additional water molecule during the catalytic process.The coordination geometry of the metal ion in the active site plays a crucial role in the enzyme's catalytic activity. The distorted octahedral geometry allows for the proper orientation and stabilization of the substrate and the nucleophilic attack by the hydroxide ion. The zinc ion polarizes the coordinated water molecule, facilitating the deprotonation of water to generate a nucleophilic hydroxide ion. This hydroxide ion then attacks the carbon dioxide molecule, converting it to bicarbonate.The geometry also ensures that the active site can accommodate the structural changes that occur during the catalytic cycle, such as the binding of the substrate, the nucleophilic attack, and the release of the product. The flexibility of the coordination geometry allows for rapid and efficient catalysis, making carbonic anhydrase one of the fastest enzymes known.