Inhibition of the R1 fragment of the cadmium-containing zeta-class carbonic anhydrase from the diatom Thalassiosira weissflogii with anions.

We investigated the catalytic activity and inhibition of both the zinc and cadmium-containing R1 fragment of the zeta-class carbonic anhydrase (CA, EC 4.2.1.1) from the marine diatom Thalassiosira weissflogii. Our data prove that these enzymes are not only very efficient catalysts for the physiological reaction, but also sensitive to sulfonamide and anion inhibitors, with inhibition constants from the nanomolar to millimolar range. Acetazolamide inhibited the two enzymes with K(I)s in the range of 58-92 nM. The best anion inhibitors of Cd-R1 were thiocyanate, sulfamate and sulfamide, with K(I)s of 10-89 microM, whereas the best Zn-R1 anion inhibitors were sulfamate and sulfamide with K(I)s of 60-72 microM. These enzymes were only weakly inhibited by chloride, bromide or sulfate, main anion components of sea water, with inhibition constants in the range of 0.24-0.85 mM. Thus, similarly to CAs belonging to other classes, the zeta-class CA (with either cadmium or zinc ions at the active site) was inhibited by both anions and sulfonamides.