Mechanism
A zinc prosthetic group in the enzyme is coordinated in three positions by histidine side-chains. The fourth coordination position is occupied by water. This causes polarisation of the hydrogen-oxygen bond, making the oxygen slightly more negative, thereby weakening the bond.
A fourth histidine is placed close to the substrate of water and accepts a proton, in an example of general acid - general base catalysis (see the article "Acid catalysis"). This leaves a hydroxide attached to the zinc.
The active site also contains specificity pocket for carbon dioxide, bringing it close to the hydroxide group. This allows the electron-rich hydroxide to attack the carbon dioxide, forming bicarbonate.
Read more about this topic: Carbonic Anhydrase
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