Investigating the inhibition mechanism and catalytic cycle of MbtI, the salicylate synthase from Mycobacterium tuberculosis

According to the latest annual report published by the World Health Organisation, tuberculosis is the first cause of death from a single infectious agent worldwide. Moreover, the growing emergence of resistant strains of Mycobacterium tuberculosis (Mtb) poses a serious threat to the public’s health. Therefore, the discovery of new antitubercular agents has still a critical importance.[1] In this context, the mycobacterium-specific salicylate synthase MbtI has been recently validated as a promising pharmacological target. This Mg2+-dependent enzyme is involved in the siderophore-mediated iron uptake, a key pathway for the survival and pathogenicity of Mtb in the host.[2] A structure-based virtual screening allowed us to identify a competitive furan-based inhibitor of MbtI, which was taken as a starting point for a thorough structure-activity investigation. Our studies led to the development of potent enzymatic inhibitors, also exhibiting a promising antimycobacterial action.[3,4] In an attempt to deeply understand the inhibition mechanism of this class of compounds, we performed biochemical investigations: these studies suggested the possibility of a Mg2+-independent binding, despite the interaction with the catalytic metal had been a cornerstone of MbtI inhibition for years. Further computational analyses and experimental data seemed to support our hypothesis, but it was only with crystallisation studies that we obtained a definitive characterisation of the binding mode. Our structural investigations also provided new insights into the conformational shifts of the active site, in relation to the catalytic state of the enzyme.[5] Overall, these results pave the way for the rational modification of our scaffold, which will hopefully lead to the obtainment of improved antitubercular candidates.