The β-Carbonic Anhydrases from Mycobacterium tuberculosis as Drug Targets

Three β-carbonic anhydrases (CAs, EC 4.2.1.1), encoded by the gene Rv1284 (mtCA 1) Rv3588c (mtCA 2) and Rv3273 (mtCA 3) are present in the human pathogen Mycobacterium tuberculosis. These enzymes were cloned and they showed appreciable catalytic activity for CO2 hydration, with kcat of 3.9 x 105 s-1, and kcat/Km of 3.7 x 107 M-1.s-1 for mtCA 1, of 9.8 x 105 s-1, and kcat/Km of 9.3 x 107 M-1.s-1 for mtCA 2 and kcat of 4.3 x 105 s-1, and a kcat/Km of 4.0 x 107 M-1.s-1 for mtCA 3, respectively. The Rv3273 gene product is predicted to be a 764 amino acid residues polypeptide, consisting of a sulfate transporter domain (amino acids 121-414) in addition to the β-CA mentioned above (which is encoded by residues 571-741). All these enzymes were inhibited appreciably by many sulfonamides and sulfamates, in the nanomolar - micromolar range, whereas some subnanomolar inhibitors were also reported for two of them (mtCA 1 and mtCA 3). As sulfonamides also efficiently inhibit dehydropteroate synthetase (DHPS), the contribution of mtCAs and DHPS inhibition to a possible antimycobacterial action of these drugs must be better understood. It has been however proven that mtCAs are druggable targets, with a real potential for developing antimycobacterial agents with a diverse mechanism of action compared to the clinically used drugs for which many strains exhibit multi-drug resistance and extensive multi-drug resistance, although for the moment no in vivo inhibition of the bacteria could be evidenced with the presently avilable drugs due to lack of penetrability through the mycolic acid cell wall of M. tuberculosis.