Inhibition of the β-carbonic anhydrase from Streptococcus pneumoniae by inorganic anions and small molecules: Toward innovative drug design of antiinfectives?

The Gram-positive bacterium Streptococcus pneumoniae is a human respiratory tract pathogen that contributes significantly to global mortality and morbidity. It was recently shown that this bacterial pathogen depends on a conserved β-carbonic anhydrase (CA, EC 4.2.1.1) for in vitro growth in environmental ambient air and during intracellular survival in host cells. Hence, it is to be expected that this pneumococcal carbonic anhydrase (PCA) contributes to transmission and pathogenesis of the bacterium, making it a potential therapeutic target. In this study, purified recombinant PCA has been further characterized kinetically and for inhibition with a series of inorganic anions and small molecules useful as leads. PCA has appreciable activity as catalyst for the hydration of CO(2) to bicarbonate, with a k(cat) of 7.4×10(5)s(-1) and k(cat)/K(m) of 6.5×10(7) M(-1)s(-1) at an optimum pH of 8.4. Inorganic anions such as chloride, bromide, iodide, cyanate, selenocyanate, trithiocarbonate, and cyanide were effective inhibitors of PCA (K(I)s of 21-98μM). Sulfamide, sulfamic acid, phenylboronic, phenylarsonic acid, and diethyldithiocarbamate showed inhibition constants in the low micromolar/submicromolar range (K(I)s of 0.61-6.68μM), whereas that of the sulfonamide acetazolamide was in the nanomolar range (K(I)s 89nM). In conclusion, our results show that PCA can effectively be inhibited by a range of molecules that could be interesting leads for obtaining more potent PCA inhibitors. PCA might be a novel target for designing antimicrobial drugs with a new mechanism of action.