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 × 10(5)s(-1) and a k(cat)/K(M) of 3.3 × 10(7)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 (K(I)s in the range of 0.93-8.3 mM). The best TweCA inhibitors were hydrogen sulfide, sulfamate, sulfamide, phenylboronic acid and phenylarsonic acid, with K(I)s in the range of 9-90 μM, whereas acetazolamide inhibited the enzyme with a K(I) 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.