Crystal structure and kinetic studies of a tetrameric type II β-carbonic anhydrase from the pathogenic bacterium Vibrio cholerae

Carbonic anhydrase (CA) is a zinc enzyme that catalyzes the reversible conversion of carbon dioxide to bicarbonate (hydrogen carbonate) and a proton. CAs have been extensively investigated owing to their involvement in numerous physiological and pathological processes. Currently, CA inhibitors are widely used as antiglaucoma, anticancer and anti-obesity drugs and for the treatment of neurological disorders. Recently, the potential use of CA inhibitors to fight infections caused by protozoa, fungi and bacteria has emerged as a new research direction. In this article, the cloning and kinetic characterization of the beta-CA from Vibrio cholerae (VchCA beta) are reported. The X-ray crystal structure of this new enzyme was solved at 1.9 angstrom resolution from a crystal that was perfectly merohedrally twinned, revealing a tetrameric type II beta-CA with a closed active site in which the zinc is tetrahedrally coordinated to Cys42, Asp44, His98 and Cys101. The substrate bicarbonate was found bound in a noncatalytic binding pocket close to the zinc ion, as reported for a few other beta-CAs, such as those from Escherichia coli and Haemophilus influenzae. At pH 8.3, the enzyme showed a significant catalytic activity for the physiological reaction of the hydration of CO2 to bicarbonate and protons, with the following kinetic parameters: a kcat of 3.34 x 10(5) s(-1) and a k(cat)/K-m of 4.1 x 10(7) M-1 s(-1). The new enzyme, on the other hand, was poorly inhibited by acetazolamide (K-i of 4.5 mu M). As this bacterial pathogen encodes at least three CAs, an alpha-CA, a beta-CA and a gamma-CA, these enzymes probably play an important role in the life cycle and pathogenicity of Vibrio, and it cannot be excluded that interference with their activity may be exploited therapeutically to obtain antibiotics with a different mechanism of action.