In the case of carbonic anhydrase, a zinc Zn metal ion is present at the active site of the enzyme. The coordination environment around the metal ion is typically tetrahedral, with the zinc ion coordinated to three histidine residues His94, His96, and His119 in human carbonic anhydrase II and a hydroxide ion OH- or water molecule H2O as the fourth ligand. The hydroxide ion acts as a nucleophile in the catalytic mechanism of the enzyme.Carbonic anhydrase catalyzes the reversible hydration of carbon dioxide CO2 to bicarbonate HCO3- and a proton H+ . The zinc ion plays a crucial role in the enzyme's function by polarizing and activating the hydroxide ion or water molecule, which then attacks the CO2 molecule to form bicarbonate.Changes in the coordination geometry around the metal ion can significantly affect the function of the enzyme. For instance, if the coordination number or the ligands coordinating to the metal ion are altered, it can lead to a decrease in the enzyme's activity or even complete loss of function. This is because the geometry and ligands around the metal ion are crucial for stabilizing the transition state of the reaction and facilitating the catalytic mechanism.Additionally, the coordination environment can be affected by the presence of inhibitors, which can bind to the metal ion and disrupt its coordination with the native ligands. This can lead to a decrease in enzyme activity or complete inhibition of the enzyme. In the case of carbonic anhydrase, some inhibitors, such as sulfonamides, can bind to the zinc ion in the active site, replacing the hydroxide ion or water molecule and preventing the enzyme from carrying out its catalytic function.