The coordination geometry of the active site in the enzyme carbonic anhydrase is a distorted tetrahedral geometry. Carbonic anhydrase is a metalloenzyme that contains a zinc ion Zn2+ at its active site. The zinc ion is coordinated to three histidine residues His94, His96, and His119 in human carbonic anhydrase II and a water molecule or hydroxide ion H2O/OH- as the fourth ligand.This coordination geometry plays a crucial role in the catalytic activity of carbonic anhydrase. The enzyme catalyzes the reversible hydration of carbon dioxide CO2 to bicarbonate HCO3- and a proton H+ , which is an essential process in various physiological functions such as respiration, pH regulation, and ion transport.The mechanism of carbonic anhydrase involves two main steps:1. Nucleophilic attack: The zinc-bound hydroxide ion OH- acts as a nucleophile and attacks the carbon atom of CO2, leading to the formation of bicarbonate HCO3- . The tetrahedral geometry of the active site allows the hydroxide ion to be properly positioned for nucleophilic attack.2. Proton transfer: The active site must regenerate the zinc-bound hydroxide ion for the next catalytic cycle. This is achieved through a proton transfer from a zinc-bound water molecule to a nearby residue usually a histidine , which then transfers the proton to the bulk solvent. The tetrahedral geometry facilitates the proton transfer by orienting the water molecule and the nearby residue in an optimal arrangement.In summary, the distorted tetrahedral coordination geometry of the active site in carbonic anhydrase is crucial for its catalytic activity. It allows for the proper positioning of the zinc-bound hydroxide ion for nucleophilic attack on CO2 and facilitates the proton transfer necessary for the regeneration of the active site.