Carbonic anhydrases in plants and algae

Carbonic anhydrases catalyse the reversible hydration of CO2, increasing th e interconversion between CO2 and HCO3- + H+ in living organisms. The three evolutionarily unrelated families of carbonic anhydrases are designated al pha-, beta -and gamma -CA. Animals have only the alpha -carbonic anhydrase type of carbonic anhydrase, but they contain multiple isoforms of this carb onic anhydrase. In contrast, higher plants, algae and cyanobacteria may con tain members of all three CA families. Analysis of the Arabidopsis database reveals at least 14 genes potentially encoding carbonic anhydrases. The da tabase also contains expressed sequence tags (ESTs) with homology to most o f these genes. Clearly the number of carbonic anhydrases in plants is much greater than previously thought. Chlamydomonas, a unicellular green alga, i s not far behind with five carbonic anhydrases already identified and anoth er in the EST database. In algae, carbonic anhydrases have been found in th e mitochondria, the chloroplast thylakoid, the cytoplasm and the periplasmi c space. In C-3 dicots, only two carbonic anhydrases have been localized, o ne to the chloroplast stroma and one to the cytoplasm. A challenge for plan t scientists is to identify the number, location and physiological roles of the carbonic anhydrases.