Requirement for carbonic anhydrase activity in processes other than photosynthetic Inorganic carbon assimilation

A number of non-green plant tissues have high rates of HCO₃⁻-consuming reactions in the cytosol, i.e. C₄ dicarboxylic acid production preceding organic acid anion transport into dicarboxylate consuming compartments in N₂-fixing root nodules, in lipogenic tissues, and in thermogenic aroid spadices and, in the case of lipogenic tissues, in acetyl CoA incorporation into lipid in plastid stroma. Since inorganic C supply to the cytosol or stroma by decarboxylation reactions, and by transmembrane fluxes, involves only CO₂, the HCO₃⁻ consumed in the rapid metabolic processes must originate from hydration (hydroxylation) of CO₂. Computations based on the first-order rate constant for uncatalysed conversion of CO₂ to HCO₃⁻ and the most likely in vivo CO₂ concentration show that the uncatalysed reaction is possibly adequate to supply the observed HCO₃⁻ requirement in the HCO₃⁻-consuming compartments. However, carbonic anhydrase activity is well established in legume root nodules, and also appears to occur in aroid spadices. In addition to coping with any heterogeneities in HCO₃⁻, consumption in the cytosol, the root nodule activity may be involved in optimizing haemoglobin function. Further work is needed on carbonic anhydrase expression is tissues with rapid HCO₃⁻ consumption, especially in view of reports of negligible carbonic anhydrase activity in some non-green plant tissues. Other possible roles of carbonic anhydrase in non-green plant tissues are briefly discussed.