Dr. Mariano Andrea Scorciapino, Department of Chemical and Geological Sciences and Department of Physics, University of Cagliari, Italy
Title of the talk:
Hijacking active iron-transport systems in Gram-negative bacteria to deliver antibiotics. What can we learn by ligand-focused NMR investigations?
"The idea of the Trojan-Horse strategy to accumulate antibiotic molecules into the bacterial cell, instead of exploiting passive diffusion through porins, dates back to the 80’s. However, still few compounds have met with success, primarily because of the lack of knowledge about the many steps involved in the iron uptake, first of all the receptor recognition step. Iron is essential to life but it is mostly found in the form of Fe(III) highly insoluble minerals. In human, iron is transported as tightly bound to specific proteins and bacteria compete with the host for the low concentrated Fe(III). They either express receptors for host’s proteins and/or secrete low molecular weight scavengers, called siderophores. These are able to form a high stability complex with Fe(III), and the complex is then recognized by specific receptors in the outer membrane of Gram-negative bacteria, as part of a complex energy-coupled TonB-dependent transport system. The receptors typically have high-affinity binding sites for their cognate siderophore complex, which can be thus recognized and transported even at low concentrations. It is possible to synthesize siderophore-antibiotic conjugates able to mimic the properties of the former and to be recognized by proper receptors. In other words, targeted bacteria can be cheated because, in order to uptake the Fe(III), they will also uptake the antibiotic function of the molecular construct.
Siderophores are usually polydentate ligands that form six-coordinate complexes with Fe(III). This means that they are able to complete the octahedral coordination sphere of Fe(III) by having a sufficient number of donors, so that they form very stable 1:1 complexes. The most well characterized siderophores belong to this class, for which several 3D structure of the siderophore-receptor co-complex are also available. However, many siderophores with a number of donors lower than six do also exist. In such instances, either the solvent molecules occupy the missing positions of the metal’s octahedral configuration, or higher stoichiometry complexes have to be envisaged. No co-complex 3D structures have been obtained and published so far and we actually do not know which species is actually recognized by the receptor in these cases. Several promising siderophore-antibiotic conjugates belong to this group and a deeper understanding of the receptor recognition steps will be extremely useful to design and develop new conjugates with improved performance.
Our approach involves the combination of experiments and computer calculations to gain, first, a deeper characterization of the intrinsic behavior of the siderophore in solution. In particular, nuclear magnetic resonance and other spectroscopy techniques are used to investigate the complex formation equilibria, to collect geometrical parameters and to finally obtain a 3D structure model. Docking calculations are then used to compare the different complexes in terms of binding affinity for their receptor. Finally, ligand-detected NMR experiments can be used to obtain the clear evidence of what is the stoichiometry of the complex which is actually recognized by the receptor. Moreover, fundamental information like the epitope of binding and the dissociation constant can be determined."
All are kindly welcome!
Further informationa by Prof. Dr. Mathias Winterhalter, Professor of Biophysics - Email: m.winterhalter [at] jacobs-university.de - Link to Homepage: https://www.jacobs-university.de/ses/mwinterhalter