In aqueous solution, carbon dioxide forms carbonic acid
:CO + HO  HCO
This reaction is very slow in the absence of a catalyst, but quite fast in the presence of the hydroxide ion
:CO + OH  hydrogencarbonate|
A reaction similar to this is almost instantaneous with carbonic anhydrase. The structure of the active site in carbonic anhydrases is well known from a number of crystal structures. It consists of a zinc ion coordinated by three imidazole nitrogen atoms from three histidine units. The fourth coordination site is occupied by a water molecule. The coordination sphere of the zinc ion is approximately tetrahedral. The positively-charged zinc ion polarizes the coordinated water molecule, and nucleophilic attack by the negatively-charged hydroxide portion on carbon dioxide proceeds rapidly. The catalytic cycle produces the bicarbonate ion and the hydrogen ion as the equilibrium
:HCO   + H
favours dissociation of carbonic acid at biological pH values.