A alpha-carbonic anhydrase (CA, EC 4.2.1.1) has been purified and characterized biochemically from the mollusk Mytilus galloprovincialis. As in most mollusks, this alpha-CA is involved in the biomineralization processes leading to the precipitation of calcium carbonate in the mussel shell. The new enzyme had a molecular weight of 50 kDa, which is roughly two times higher than that of a monomeric alpha-class enzyme. Thus, Mytilus galloprovincialis alpha-CA is either a dimer, or similar to the Tridacna gigas CA described earlier, may have two different CA domains in its polypeptide chain. The Mytilus galloprovincialis alpha-CA sequence contained the three His residues acting as zinc ligands and the gate-keeper residues present in all alpha-CAs (Glu106-Thr199), but had a Lys in position 64 and not a His as proton shuttling residue, being thus similar to the human isoform hCA III. This probably explains the relatively low catalytic activity of Mytilus galloprovincialis alpha-CA, with the following kinetic parameters for the CO2 hydration reaction: k(cat) = 4.1 x 105 s(-1) and k(cat)/K-m of 3.6 x 10(7) M-1 x s(-1). The enzyme activity was poorly inhibited by the sulfonamide acetazolamide, with a K-i of 380 nM. This study is one of the few describing in detail the biochemical characterization of a molluskan CA and may be useful for understanding in detail the phylogeny of these enzymes, their role in biocalcification processes and their potential use in the biomimetic capture of the CO2.
