The coordination number of the metal ion in the active site of the enzyme carbonic anhydrase is 6. The metal ion in the active site is usually a zinc ion Zn2+ . The coordination number refers to the number of ligands atoms, ions, or molecules that are directly bonded to the central metal ion.In carbonic anhydrase, the zinc ion is coordinated by three histidine residues from the protein and a hydroxide ion OH- or water molecule H2O as the fourth ligand. The remaining two coordination sites are occupied by substrates or inhibitors during the catalytic process.The coordination number and geometry of the metal ion play a crucial role in the enzyme's catalytic activity. The zinc ion polarizes the hydroxide ion or water molecule, making it more nucleophilic and facilitating the conversion of carbon dioxide CO2 to bicarbonate ion HCO3- and a proton H+ . This process is essential for maintaining acid-base balance in the body and facilitating the transport of CO2 from tissues to the lungs.In summary, the coordination number of 6 for the metal ion in the active site of carbonic anhydrase is essential for its catalytic activity, as it allows the enzyme to bind and activate substrates, facilitating the conversion of CO2 to HCO3- and H+.