Exploring Amphotericin B-Membrane Interactions: Free Energy Simulations

Abstract

Amphotericin B (AmB) is a membrane-active polyene antibiotic used to treat serious fungal infections. Biological action of AmB is due to the formation of transmembrane channels. Membrane sterols are known to be crucial for the AmB antifungal activity, i.e. AmB is more active against fungal cell membranes containing ergosterol than against the mammalian membranes with cholesterol.We explored molecular determinants of AmB selectivity for ergosterol-containing membranes using computational methods. By means of molecular dynamics simulations we studied various aspects of the interactions between AmB and lipid bilayers of different composition (containing or not 30% of ergosterol or cholesterol). More precisely, we examined (1) AmB insertion into a membrane, (2) changing tilt angle between AmB and the bilayer plane (for AmB embedded in a membrane) as well as (3) AmB dimerization in a membrane. To provide a thermodynamic description of these processes, we calculated the free energy profiles describing each of them in the three different membrane systems. The results indicate that at low, chemotherapeutically relevant concentrations of AmB at which the antibiotic expresses its channel-forming activity, AmB is mostly monomeric in ergosterol-containing membranes and it exists predominantly as a dimer in cholesterol-containing (and sterol-free) ones. We also show that compared to the other two studied membranes, it is the most favorable for AmB to insert the ergosterol-containing bilayer. The differences in the behavior patterns of AmB in bilayers of different composition are mainly of energetic origin. From the free energy profiles for (2) and (3) we also determine the most preferred location and orientation of AmB within the studied bilayers.