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

Abstract

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 β-CA fromVibrio cholerae(VchCAβ) are reported. The X-ray crystal structure of this new enzyme was solved at 1.9 Å resolution from a crystal that was perfectly merohedrally twinned, revealing a tetrameric type II β-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 β-CAs, such as those fromEscherichia coliandHaemophilus influenzae. At pH 8.3, the enzyme showed a significant catalytic activity for the physiological reaction of the hydration of CO₂to bicarbonate and protons, with the following kinetic parameters: ak_catof 3.34 × 10⁵ s⁻¹and ak_cat/K_mof 4.1 × 10⁷ M⁻¹ s⁻¹. The new enzyme, on the other hand, was poorly inhibited by acetazolamide (K_iof 4.5 µM). As this bacterial pathogen encodes at least three CAs, an α-CA, a β-CA and a γ-CA, these enzymes probably play an important role in the life cycle and pathogenicity ofVibrio, and it cannot be excluded that interference with their activity may be exploited therapeutically to obtain antibiotics with a different mechanism of action.