δ-Carbonic Anhydrases: Structure, Distribution, and Potential Roles

Abstract Carbonic anhydrases (CAs) are found in five different classes, all of which catalyze the reversible hydration of carbon dioxide. One of these five classes, δ-CA, is restricted to marine photosynthetic eukaryotes such as diatoms, dinoflagellates, and coccolithophores, and changes in the expression of δ-CAs in response to variations in carbon dioxide concentration suggest that they play an important role in the carbon-concentrating mechanism of these organisms. Intriguingly, primary structure alignments suggest the presence of a single large insert in the δ-CAs compared with the α-CAs. In addition, the aligned regions between the two classes share a common predicted secondary structure. Lastly, tertiary structure predictions suggest a common structure with a zinc atom in the active site coordinated by three histidine residues in both classes. These bioinformatic analyses suggest that δ-CAs may share a common ancestry with the α-CAs.