Lipid Flip-Flop and Pore Nucleation on Zwitterionic Bilayers are Asymmetric under Ionic Imbalance

Lipid flip-flop and its associated transient pore formation are key thermodynamic properties of living cell membranes. However, there is a lack of understanding of whether ionic imbalance that exists ubiquitously across cell membranes affects lipid flip-flop and its associated functions. Potential of mean force calculations show that the free-energy barrier of lipid flip-flop on the extracellular leaflet reduces with the presence of ionic imbalance, whereas the barrier on the intracellular leaflet is generally not affected. The linear decrease of the activation energy of lipid flip-flop on the extracellular leaflet is consistent with the experimentally measured conductance-voltage relationship of zwitterionic lipid bilayers. This suggests: 1) lipid flip-flop has a directionality under physiological conditions and phospholipids accumulate at a rate on the order of 105 µm^-2 h^-1 on the cytoplasmic side of cell membranes; 2) ion permeation across a lipid membrane is moderated by lipid flip-flop; 3) the energy barrier of pore formation is aligned with the weaker leaflet that has a lower energy of lipid flip-flop. The asymmetry of lipid flip-flop and pore nucleation may have substantial implications for protein translocation, signaling, enzymatic activities, vesicle fusion, and transportation of biomolecules on cell membranes.