Fig. 6: Proposed model for sAEL057 mode of action.

Mycobactin (MB) chelates ferric iron (Fe³⁺) from host storage proteins inside the phagosome. Iron-laden mycobactin is imported into the bacterial cytosol through the IrtA/B transporter. Ferric iron is reduced to ferrous iron (Fe²⁺) to facilitate release from mycobactin. Nitrogen atoms of sAEL057 molecules form coordinate covalent bonds with Fe²⁺ through the donation of electrons to Fe²⁺. The binding of sAEL057 to iron makes it inaccessible to Mtb, resulting in a reduced availability of intracellular iron. This leads to an iron deprivation transcriptional response, driven by the iron-sensing transcription factor IdeR, and is demonstrated by the upregulation of mycobactin and iron import genes and downregulation of iron storage genes. Additionally, we predict that catabolism of certain carbon substrates (such as cholesterol and glucose) have an increased reliance on iron-dependent proteins as compared to catabolism of other carbon substrates (such as long-chain fatty acids). Therefore, a secondary consequence of sAEL057’s iron chelation is altered cholesterol catabolism leading to reduced metabolic output and a compensatory upregulation of genes involved in the early steps of the enzymatic breakdown of cholesterol. Ultimately, this results in reduced growth and survival when Mtb is restricted to specific single carbon substrates. Created with BioRender.com, publishing license in Supplementary Material.