Multiple functions of carbonic anhydrase in the crustacean gill

The enzyme‐catalysed hydration of CO2 gas by carbonic anhydrase produces the ionic species H+ and HCO3−. The chemical species involved in the reaction scheme are further used in a variety of different physiological and biochemical processes, each of which at least potentially relies on carbonic anhydrase to keep those chemical species from being limiting. Two such processes, CO2 excretion and ion uptake, occur in the crustacean gill. On the surface it would appear that these processes are mutually exclusive in a single organ, as the former requires the net dehydration of HCO3−, and the latter requires the net hydration of CO2. That carbonic anhydrase can meet the requirements of both processes is due to its compartmentalization within different subcellular fractions of the gill tissue. Carbonic anhydrase associated with the basal‐lateral branchial membrane mobilizes hemolymph HCO3− to CO2 for excretion, while intracellular (cytoplasmic) carbonic anhydrase, through the catalysed hydration of CO2, provides H+ and HCO3− for use in Na+ and Cl− uptake.