The specific metal center in the active site of the metalloenzyme carbonic anhydrase is a zinc ion Zn . The coordination geometry of the zinc ion in the active site is a distorted tetrahedral geometry. The zinc ion is coordinated to three histidine residues His94, His96, and His119 in human carbonic anhydrase II from the protein and a hydroxide ion OH or water molecule HO as the fourth ligand.The structure of the active site allows for the efficient catalysis of the hydration of carbon dioxide CO to bicarbonate HCO through a two-step mechanism:1. Nucleophilic attack: The zinc-bound hydroxide ion acts as a nucleophile and attacks the carbon atom of CO, which is a good electrophile. This leads to the formation of a zinc-bound bicarbonate ion Zn-HCO .2. Proton transfer: A nearby residue, typically a glutamate Glu106 in human carbonic anhydrase II , acts as a proton shuttle, facilitating the transfer of a proton from a water molecule to the zinc-bound bicarbonate ion. This results in the release of the bicarbonate ion HCO from the active site and the binding of a new water molecule to the zinc ion, regenerating the active form of the enzyme.The specific arrangement of the zinc ion and its ligands in the active site allows for the proper orientation of the reactants and stabilization of the transition states, thus enabling the efficient catalysis of the hydration of CO to HCO. The zinc ion plays a crucial role in activating the water molecule by polarizing the O-H bond, making the hydroxide ion a stronger nucleophile. Additionally, the zinc ion helps to stabilize the negative charge on the oxygen atom of the bicarbonate ion during the reaction.