minocycline and nitrocefin

By measuring quantitatively the active efflux of cephalosporins by the RND (resistance-nodulation-division) family efflux pump AcrB in intact cells of Escherichia coli, we found that the simultaneous presence of another substrate, such as chloramphenicol, benzene, cyclohexane, or Arg β-naphthilamide, significantly enhanced the extrusion of cephalosporins. The stimulation occurred also in a strain expressing the covalently linked trimer of AcrB, and thus cannot be ascribed to the enhanced assembly of the trimer from AcrB monomers. When Val139 of AcrB was changed into Phe, the stimulation by benzene was found to occur at much lower concentration of the solvent. A plausible explanation of these observations is that the AcrB pump is constructed to pump out very rapidly the solvent or chloramphenicol molecules, and thus the efflux of cephalosporins, which presumably bind to a different subsite within the large binding pocket of AcrB, can become facilitated. Computer simulations of ligand binding to AcrB, both by docking and by molecular dynamics simulations, produced results supporting and extending this hypothesis. Benzene and the cephalosporin nitrocefin can bind simultaneously to the distal binding pocket of AcrB, both in the wild type and in the V139F variant. Interestingly, while the binding position and strength of benzene are almost unaffected by the presence of nitrocefin, this latter substrate is significantly displaced toward the exit gate in both wild type and mutant transporter in the presence of benzene. Additionally, the cephalosporin efflux may be enhanced by the binding of solvents (sometimes to the cephalosporin-free protomer), which could accelerate AcrB conformational changes necessary for substrate extrusion.

Topics: Arginine; Benzene; Cefamandole; Cephalosporins; Chloramphenicol; Escherichia coli; Escherichia coli Proteins; Kinetics; Ligands; Minocycline; Molecular Docking Simulation; Molecular Dynamics Simulation; Multidrug Resistance-Associated Proteins; Protein Conformation; Protein Multimerization; Thermodynamics

The AcrB trimeric multidrug efflux transporter of Escherichia coli pumps out a very wide spectrum of compounds. Although minocycline and doxorubicin have been cocrystallized within the large binding pocket in the periplasmic domain of the binding protomer, nothing is known about the binding of many other ligands to this protein. We used computer docking to evaluate the interaction of about 30 compounds with the binding protomer and found that many of them are predicted to bind to a narrow groove at one end of the pocket whereas some others prefer to bind to a wide cave at the other end. Competition assays using nitrocefin efflux and covalent labeling of Phe615Cys mutant AcrB with fluorescein-5-maleimide showed that presumed groove-binders competed against each other, but cave-binders did not compete against groove-binders, although the number of compounds tested was limited. These results give us at least a hypothesis to be tested by more biochemical and genetic experiments in the future.

Topics: Binding, Competitive; Cephalosporins; Doxorubicin; Drug Resistance, Multiple; Escherichia coli; Escherichia coli Proteins; Fluoresceins; Ligands; Minocycline; Models, Chemical; Models, Molecular; Multidrug Resistance-Associated Proteins; Mutation; Protein Binding