The coordination sphere of the zinc ion Zn in the active site of carbonic anhydrase enzyme consists of four coordination sites. Three of these sites are occupied by histidine amino acid residues from the enzyme, and the fourth site is occupied by a water molecule or a hydroxide ion OH .The coordination sphere plays a crucial role in the enzyme's catalytic activity in converting carbon dioxide CO to bicarbonate HCO . The zinc ion in the active site acts as a Lewis acid, polarizing the bound water molecule and facilitating its deprotonation to form a hydroxide ion OH . This hydroxide ion is a strong nucleophile, which can attack the carbon atom of the carbon dioxide molecule, leading to the formation of bicarbonate.During the catalytic cycle, the bicarbonate ion is released from the active site, and a new water molecule binds to the zinc ion, restoring the enzyme to its original state and allowing it to participate in another round of catalysis. The coordination sphere of the zinc ion in carbonic anhydrase is essential for the enzyme's function, as it enables the formation of the reactive hydroxide ion and stabilizes the transition state during the reaction, thereby increasing the rate of CO hydration to bicarbonate.