Rationalizing the Transport of Trojan Horse Compounds for Crossing the Outer Membrane of Gram- Bacteria

One of the challenges of modern medicine is to find efficient antibiotics to counteract infections. The main concern is represented by Gram negative species, where antibiotics have to cross the outer membrane for reaching their targets. Among the possible strategy for resolving the permeation of antibiotics is the use of TonB dependent transporters, such as those expressed to capture iron from the environment. Existing siderophore molecules enriched by anti-infective properties (Trojan-Horse candidates) can be transported efficiently inside the cell. The high-resolution structures obtained recently using X-ray crystallography have open the way to a more detailed knowledge of transport mechanism. Thanks to the new structures, we have applied molecular simulations in combination with NMR spectroscopy to investigate at molecular level the formation of the molecule-ion complex in solution, the binding of the complex to the transporter and finally its diffusion along the interior of the transporter. In general the ion-siderophore complex in solution is the one recognized by the transporter, in a precise recognition pocket. Upon the recognition, the binding can provide a unique allosteric signal appearing to activate other regions of the transporters in order to control the self-transport of the ligand. This effect upon ligand binding promotes a novel idea that the internal diffusion does not require a large conformational change of the transporter, as suggested earlier. We obtained detailed molecular data to understand how to rationalize siderophores able to use transporters to cross the outer membrane. The three fundamental steps are: formation of the complex in solution, binding and recognition on the transporter, and internal diffusion. Only the first two steps seem to play a key role in the transport and both depend on subtle interactions ion-siderophore-transporter, modulated by selecting precise chemical groups.