Reversible hydration of carbon dioxide (PubMed:14729686, PubMed:17012396, PubMed:18258595). Essential for chemolithotrophic carbon dioxide fixation, supplies CO(2) to RuBisCO (ribulose bisphosphate carboxylase, cbbL-cbbS) in the carboxysome (Probable) (PubMed:18258595). There are estimated to be 40 CsoSCA dimers per carboxysome (Ref.5). Unlike beta-carboxysomes, alpha-carboxysomes (Cb) can form without cargo protein. CsoS2 is essential for Cb formation and is also capable of targeting foreign proteins to the Cb. The Cb shell assembles with the aid of CsoS2; CsoS1A, CsoS1B and CsoS1C form the majority of the shell while CsoS4A and CsoS4B form vertices. CsoS1D forms pseudohexamers that probably control metabolite flux into and out of the shell. H(+) + hydrogencarbonate = CO2 + H2O The fourth ligand is a water molecule. Binds 1 Zn(2+) per monomer, in PDB:2FGY a second Zn(2+) is seen, but one of its ligands is an accidentally introduced mutation. Carbonic anhydrase activity is inhibited by ethoxyzolamide, dithiothreitol, cyanide, and divalent metal chelators dipicolinic acid and nitrilotriacetic acid. kcat is 8.9 x 10(4) sec(-1) for CO(2) hydration at pH 8.0, and 4.6 x 10(4) sec(-1) for hydrogencarbonate dehydration at pH 7.0. Optimum pH is 8.0-8.5 for CO(2) hydration, and pH 6.0 for hydrogencarbonate dehydration. Homodimer, may form filaments. Coomassie staining of gels and immunogold staining shows this is a minor shell protein (PubMed:10816046, PubMed:14729686). The protein is probably found inside the carboxysome, as its addition to mutant organelles does not complement the deletion (PubMed:18258595). This cyanobacterium makes alpha-type carboxysomes (Ref.5, PubMed:18258595). The N-terminal domain is physically isolated from the rest of the protein and may serve to anchor the protein to the carboxysome shell or to RuBisCO; in PDB:2FGY the N-terminal domain binds Zn(2+) which is thought to be non-physiological. The central domain bears the catalytic Zn(2+), while the C-terminal domain is structurally similar to the central catalytic domain but cannot bind Zn(2+). Cells do not grow in normal air but do grow on 5% CO(2), called a high-CO(2) requiring phenotype, hcr. Loss of CA activity, requires 3.5 X more inorganic carbon for half-maximal CO(2) fixation. Carboxysomes are approximately normal except that ratio of CsoS2A:CsoS2B shifts from 1:1 to 2:1. Required for growth in ambient air (PubMed:31406332). Expression of 10 genes for alpha-carboxysome (Cb) proteins (cbbL-cbbS-csoS2-csoS3-csoS4A-csoS4B-csoS1C-csoS1A-csoS1B-csoS1D) in E.coli generates compartments that resemble Cb, contain RuBisCO and have its catalytic activity, showing it is possible to make artificial, functional Cb using these 10 genes. Cargo proteins can be targeted to these organelles (PubMed:22184212). Artificial Cb assembly in E.coli requires csoS2-csoS4A-csoS4B-csoS1C-csoS1A-csoS1B-csoS1D (but not the gene for carbonic anhydrase, csoS3). Targeting proteins to the organelle requires at least one of the CsoS2 C-repeats; 3 repeats gives the best localization. A nanoreactor of the Cb shell proteins has been engineered which generates H(2) using a ferredoxin-hydrogenase fusion (AC P07839-Q9FYU1) and a flavodoxin/ferredoxin--NADP reductase (AC A0A0K3QZA5) targeted separately to the Cb; the hydrogenase has first to be matured and activated by HydGXEF (AC Q8EAH9, Q8EAH8, Q8EAH7 and Q8EAH6 respectively). Encapsulation increases H(2) production about 20% during anaerobic growth, and over 4-fold more during aerobic growth (PubMed:33116131). Belongs to the beta-class carbonic anhydrase family. CsoSCA subfamily.