Carbonic Anhydrase: New Insights for an Ancient Enzyme*□S

CO2 H2O^ HCO3 H (Eq. 1) The first carbonic anhydrase was purified from erythrocytes in 1933 (1) followed by the characterization of several mammalian isozymes that dominated research on carbonic anhydrase until recently. Although it has been known since the 1940s that carbonic anhydrase is ubiquitous in plants (2), where it is essential for CO2 fixation, relatively few studies had been reported. Until 1994, only five carbonic anhydrases had been purified from prokaryotes; however, a recent survey has established that the enzyme is widely distributed among phylogenetically and physiologically diverse prokaryotes, indicating a far greater role for this enzyme in nature than previously recognized (3). The comparison of sequences and crystal structures of the mammalian and plant enzymes demonstrates that they evolved independently and have been designated the and -class, respectively. An additional independently evolved -class was reported in 1994 (4) for which phylogenetic analyses predict an ancient origin (3). This review discusses dramatic advances over the past 3 years regarding the structure and biological chemistry of carbonic anhydrases.