The coordination number of the metal ion in the active site of the metalloenzyme carbonic anhydrase is 6. The metal ion in carbonic anhydrase is usually zinc Zn , which is coordinated by three histidine residues, one water molecule or hydroxide ion , and two additional ligands that can be either water molecules or other amino acid residues.The coordination number affects the catalytic activity of the enzyme by influencing the geometry and electronic properties of the active site. In carbonic anhydrase, the coordination number of 6 allows the zinc ion to have an octahedral geometry, which is crucial for its catalytic activity.The zinc ion in the active site plays a critical role in the catalytic mechanism of carbonic anhydrase. It activates a water molecule or hydroxide ion by polarizing it, making it more nucleophilic. This activated water molecule then attacks the carbon dioxide substrate, leading to the formation of bicarbonate ion HCO and a proton H . The coordination number of 6 ensures that the zinc ion can maintain its optimal geometry and electronic properties for efficient catalysis.In summary, the coordination number of the metal ion in the active site of carbonic anhydrase is 6, and it plays a crucial role in the enzyme's catalytic activity by influencing the geometry and electronic properties of the active site.