The coordination geometry of the active site metal ion in the metalloenzyme carbonic anhydrase is a distorted octahedral geometry. The metal ion, which is usually zinc Zn2+ , is coordinated by three histidine residues and a hydroxide ion OH- or water molecule H2O in the active site. The remaining two positions in the octahedral geometry are occupied by substrates or inhibitors during the enzymatic reaction.This coordination geometry plays a crucial role in the mechanism of enzymatic activity of carbonic anhydrase. The enzyme catalyzes the reversible hydration of carbon dioxide CO2 to form bicarbonate HCO3- and a proton H+ . The mechanism involves two steps:1. Nucleophilic attack: The hydroxide ion OH- coordinated to the zinc ion acts as a nucleophile and attacks the carbon atom of CO2, forming bicarbonate HCO3- . The zinc ion helps to polarize and activate the hydroxide ion for the nucleophilic attack.2. Proton transfer: A water molecule replaces the bicarbonate in the coordination sphere of the zinc ion. The zinc-bound water molecule is then deprotonated by a nearby residue usually a histidine to regenerate the hydroxide ion OH- , which is ready for the next round of catalysis.The distorted octahedral coordination geometry of the active site metal ion in carbonic anhydrase allows for the proper orientation and activation of the hydroxide ion and substrates, facilitating the efficient catalysis of the reaction.