Meta-Biol

• Pathways

Carbonic anhydrase IV (CA4) anchored to extracellular face of the plasma membrane (Wistrand et al. 1999) hydrates carbon dioxide (CO2) to yield bicarbonate (HCO3-) and a proton (H+) (Zhu & Sly 1990, Okayuma et al. 1992, Baird et al. 1997, Innocenti et al. 2004). During the reaction a hydroxyl group bound by the zinc ion (Zn2+) of CA4 attacks the CO2 molecule to directly form HCO3- (reviewed in Lindskog 1997). The HCO3- is displaced by water, which is then deprotonated by a histidine residue to recreate the Zn2+:hydroxyl group. Depending on the concentrations of reactants the reaction is reversible.