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submitted on 18.05.2020 and posted on 19.05.2020by Jiajun Wang, Jigneshkumar Dahyabhai Prajapati, Ulrich Kleinekathöfer, Mathias Winterhalter
The effect of divalent ions
on the permeability of norfloxacin across the major outer membrane channels from
E. coli (OmpF, OmpC) and E. aerogenes (Omp35, Omp36) has been
investigated at the single channel level. To understand the rate limiting steps
in permeation, we reconstituted single porin into planar lipid bilayers and analyzed
the ion current fluctuations caused in the presence of norfloxacin. To obtain
an atomistic view, we complemented the experiments with millisecond-long free
energy calculations based on temperature-accelerated Brownian dynamics
simulations to identify the most probable permeation pathways of the
antibiotics through the respective pore. Both, experimental analysis
and computational modelling, suggest that norfloxacin is able to permeate through
the larger porins, i.e., OmpF, OmpC, and Omp35, whereas it only binds to
the slightly narrower porin Omp36. Moreover,
divalent ions can bind to negatively charged residues inside the porin, reversing the ion selectivity of the pore. In addition, the divalent ions can chelate with the fluoroquinolones and alter their physicochemical properties. The results
suggest that the conjugation must break with either one of them when the antibiotics molecules bypass the lumen of the porin, with the conjugation
to the antibiotic being more stable than that to the pore. In general, the permeation or binding process of
fluoroquinolone in porins occurs irrespective of the presence of divalent ions, but the presences of divalent ions can
vary the kinetics significantly.