Reversible hydration of carbon dioxide. Essential to photosynthetic carbon dioxide fixation, supplies CO(2) to RuBisCO (ribulose bisphosphate carboxylase, rbcL-rbcS) in the carboxysome. H(+) + hydrogencarbonate = CO2 + H2O Binds 1 zinc ion per monomer, a water molecule forms the fourth ligand. Inhibited by ethoxyzolamide. kcat is 3340 sec(-1) for full length enzyme at pH 7.5. A hexamer formed by a trimer of dimers. Interacts with the first 260 residues of CcmM; both the N-terminal 206 residues and the C-terminal tail contribute to CcmM binding (PubMed:27729545). Interacts with full-length and the N-terminal 249 residues of CcmM. A probable CcmM-CcaA-CcmN complex as well as a CcaA-RuBisCO-CcmM complex can also be isolated (PubMed:17993516). This cyanobacterium makes beta-type carboxysomes. Associates with the shell portion of carboxysomes. Dimerization, activity and interactions with CcmM requires all but the last 60 amino acids of the protein (PubMed:17993516). The C-terminal tail (about 55 residues) inhibits CA activity about 10-fold; it might attenuate activity before encapsulation in the carboxysome (PubMed:27729545). Cells do not grow in normal air but do grow on 5% CO(2), called a high-CO(2) requiring phenotype, HCR; even at high CO(2) levels photosynthesis is less effective. Carboxyomes appear wild-type and contain RuBisCO but no longer have a HCO(3)- to CO(2) interconversion activity. Light-dependent CO(2) transport into the cell is unchanged. Belongs to the beta-class carbonic anhydrase family. Truncated N-terminus.