The metal center in the enzyme carbonic anhydrase is a zinc ion Zn2+ . The coordination structure of this metal center is a distorted tetrahedral geometry. The zinc ion is coordinated to three histidine residues from the protein and a hydroxide ion OH- or water molecule H2O as the fourth ligand. The three histidine residues act as ligands, binding to the zinc ion through their imidazole nitrogen atoms.This coordination structure facilitates the enzyme's catalytic activity in the hydration of carbon dioxide CO2 through the following mechanism:1. The zinc-bound hydroxide ion OH- acts as a nucleophile, attacking the electrophilic carbon atom of CO2. This results in the formation of a bicarbonate ion HCO3- .2. A water molecule replaces the bicarbonate ion in the coordination sphere of the zinc ion, restoring the enzyme to its original state.3. The bicarbonate ion is released from the enzyme, allowing it to participate in various physiological processes such as pH regulation and respiration.The zinc ion plays a crucial role in the catalytic activity of carbonic anhydrase by polarizing the hydroxide ion or water molecule, making it a more effective nucleophile. Additionally, the zinc ion helps to stabilize the transition state of the reaction, lowering the activation energy and increasing the reaction rate. The coordination structure of the zinc ion in carbonic anhydrase allows for efficient and rapid hydration of carbon dioxide, making it one of the fastest enzymes known.