Carbonic anhydrase is a metalloenzyme that contains a zinc Zn ion in its active site. The coordination number of the zinc ion in the active site is 4. The geometry of the metal center is tetrahedral. The zinc ion is coordinated to three histidine residues from the protein and a water molecule or hydroxide ion.The role of the metal center in the catalytic mechanism of carbonic anhydrase is to facilitate the interconversion of carbon dioxide CO2 and water H2O into bicarbonate HCO3- and a proton H+ . The zinc ion in the active site acts as a Lewis acid, polarizing the water molecule or hydroxide ion bound to it. This increases the nucleophilicity of the oxygen atom, allowing it to attack the carbon atom of CO2 more effectively.The mechanism of carbonic anhydrase can be summarized in the following steps:1. The zinc-bound water molecule loses a proton to a nearby residue usually a histidine , generating a zinc-bound hydroxide ion.2. The hydroxide ion attacks the carbon atom of CO2, forming a zinc-bound bicarbonate ion.3. The bicarbonate ion is released from the active site, and a new water molecule binds to the zinc ion, resetting the enzyme for another catalytic cycle.In summary, the coordination number of the metal center in carbonic anhydrase is 4, with a tetrahedral geometry. The metal center plays a crucial role in the enzyme's catalytic mechanism by polarizing the water molecule or hydroxide ion, facilitating the interconversion of CO2 and H2O into HCO3- and H+.