Periplasmic α-carbonic anhydrase of Helicobacter pylori (HpαCA), an oncogenic bacterium in the human stomach, is essential for its acclimation to low pH. It catalyses the conversion of carbon dioxide to bicarbonate using zinc as the cofactor. In H. pylori, Neisseria spp., Brucella suis and Streptococcus pneumoniae this enzyme is the target for sulfonamide antibacterial agents. In 1970s, before the discovery of H. pylori, these compounds were used as antiglaucoma drugs. The observation that they also heal peptic ulcers with a very low recurrence rate has eventually led to the hypothesis that they kill H. pylori by inhibiting HpαCA. The talk will focus on our structural and functional studies of HpαCA to understand the mechanism of action of sulfonamide inhibitors and enable assessment of HpαCA as a target for drug design. Structural analyses correlated with inhibition data will be presented for the complexes of HpαCA with sulfonamides acetazolamide and methazolamide. The structures reveal that two sulfonamide oxygen atoms of the inhibitors are positioned proximal to the putative location of the oxygens of the substrate in the Michaelis complex, whilst the zinc-coordinating sulfonamide nitrogen occupies the position of the catalytic water. The structures are consistent with acetazolamide acting as site-directed, nanomolar inhibitors of the enzyme by mimicking its reaction transition state. Additionally, inhibitor binding provides insights into the channel for substrate entry and product exit. The presentation will also incorporate data on the full-genome sequencing of H. pylori mutants with spontaneous resistance to these compounds and a discussion of the possible biochemical mechanisms by which the resistance may occur. These data inform future efforts to optimise the antimicrobial activity of sulfonamides.