Cloning, characterization and anion inhibition study of the δ-class carbonic anhydrase (TweCA) from the marine diatom Thalassiosira weissflogii

We investigated the catalytic activity and inhibition of the δ-class carbonic anhydrase (CA, EC 4.2.1.1) from the marine diatom Thalassiosira weissflogii, TweCA. The enzyme, obtained by cloning the synthetic gene, was an efficient catalyst for the CO2 hydration, its physiological reaction, with a kcat of 1.3 × 105 s−1 and a kcat/KM of 3.3 × 107 M−1 s−1. A range of inorganic anions and small molecules were investigated as inhibitors of TweCA. Chloride and sulfate did not inhibit the enzyme (KIs >200 mM) whereas other halides and pseudohalides were submillimolar-millimolar inhibitors (KIs in the range of 0.93-8.3 mM). The best TweCA inhibitors were hydrogen sulfide, sulfamate, sulfamide, phenylboronic acid and phenylarsonic acid, with KIs in the range of 9-90 μM, whereas acetazolamide inhibited the enzyme with a KI of 83 nM. This is the first kinetic and inhibition study of a δ-class CA. However, these enzymes are widespread in the marine phytoplankton, being present in haptophytes, dinoflagellates, diatoms, and chlorophytic prasinophytes, contributing to the CO2 fixation by sea organisms. A phylogenetic analysis with all five genetic families of CAs showed that α- and δ-CAs are evolutionarily more related to each other with respect to the γ-CAs, although these three families clustered all together. On the contrary, the β- and ζ-CAs are also related to each other but phylogenetically much more distant from the α-, γ and δ-CA cluster. Thus, the study of δ-CAs is essential for better understanding this superfamily of metalloenzymes and their potential biotechnological applications in biomimetic CO2 capture processes, as these enzymes are part of the carbon concentrating mechanism used by many photosynthetic organisms.