Structural insights into the inhibition of M. tuberculosis salicylate synthase (MbtI)

Tuberculosis represents the first cause of death from a single infectious agent in the world. This alarming trend is worsened by the growing emergence of resistant strains of Mycobacterium tuberculosis, which pose a serious threat to the public’s health. Therefore, the discovery of new antitubercular agents has still a critical importance. In this context, the mycobacterium-specific salicylate synthase MbtI, involved in the siderophore-mediated iron uptake pathway, has been recently validated as a pharmacological target for the development of novel antitubercular agents. A structure-based virtual screening allowed us to identify a competitive furan-based inhibitor of MbtI. With the aim of increasing its inhibitory effect, we explored the chemical space around this hit through a structure-activity relationship study, which led to the identification of more potent MbtI inhibitors. These compounds exhibited a promising antimycobacterial action, related to the reduced production of siderophores. The novel lead compounds were submitted to co-crystallization experiments to empirically define the binding mode of this class of compounds within the active site of MbtI. The structural data evidenced an unexpected binding mode, completely overturned with respect to the computational model. An investigation of the active site revealed that the absence of the magnesium ion in the crystal was likely responsible for the different pose. Therefore, we undertook a careful analysis of the role of the metal co-factor in the binding of the ligand through biochemical and structural experiments, which led to the disclosure of significant information regarding the catalytic activity of the enzyme. Our results allowed the rational modification of our scaffold, ultimately leading to the obtainment of improved compounds.