Mechanisms of Carbonic Anhydrase Catalysis and Inhibition

The catalytic mechanism of carbonic anhydrase (CA, EC 4.2.1.1) at least for the physiological CO₂ hydration is well understood, with the CO₂-bicarbonate interconversion due to the nucleophilic attack of a zinc hydroxide species on the substrate molecule bound in a hydrophobic pocket, followed by a proton transfer reaction for the generation of the nucleophilic species of the enzyme, which is also the rate-determining one. There are five CA inhibition mechanisms described in detail to date, including (i) zinc-binding: (ii) anchoring to the zinc-bound water/hydroxide ion; (iii) occluding the active-site entrance; (iv) binding outside the active site; and (v) halogen-bond interaction with the proton shuttle of the enzyme. More than 50 different chemotypes were identified, which act in this way, whereas there are few compounds for which the inhibition mechanism is not clear yet. The many new classes of CA inhibitors also allowed the identification of isoform-selective agents for the many CA isoforms described in humans and thus, also the possibility of new pharmacological applications.