Lipid gymnastics: evidence of complete acyl chain reversal in oxidized phospholipids from molecular simulations

Abstract

In oxidative environments, biomembranes contain oxidized lipids with short, polar acyl chains. Two stable lipid oxidation products are PoxnoPC and PazePC. PoxnoPC has a carbonyl group, and PazePC has an anionic carboxyl group pendant at the end of the short, oxidized acyl chain. We have used MD simulations to explore the possibility of complete chain reversal in OXPLs in POPC-OXPL mixtures. The polar AZ chain of PazePC undergoes chain reversal without compromising the lipid bilayer integrity at concentrations up to 25% OXPL, and the carboxyl group points into the aqueous phase. Counterintuitively, the perturbation of overall membrane structural and dynamic properties is stronger for PoxnoPC than for PazePC. This is because of the overall condensing and ordering effect of sodium ions bound strongly to the lipids in the PazePC simulations. The reorientation of AZ chain is similar for two different lipid force fields. This work provides the first molecular evidence of the "extended lipid conformation" in phospholipid membranes. The chain reversal of PazePC lipids decorates the membrane interface with reactive, negatively charged functional groups. Such chain reversal is likely to exert a profound influence on the structure and dynamics of biological membranes, and on membrane-associated biological processes.

Figure 1

POPC and its two oxidation products investigated in this work. The partial charges for the carbonyl and carboxyl carbon, as well as oxygen atoms are shown.

Figure 2

Simulation snapshots of a single PazePC and PoxnoPC molecule from the 25% OXPL simulations. The images were taken at uniform intervals during the last 50 ns and superimposed. The oxidized sn-2 chains are shown in yellow, except for the terminal functional group. The rest of the atoms are shown in blue. The horizontal black line represents the approximate average location of the phosphate groups in one bilayer leaflet.

Figure 3

The concentration-dependent tilt angle distribution of the oxidized sn-2 chains of PazePC (a) and PoxnoPC (b). The numbers in the legend refer to the number of OXPLs present in each simulation. The tilt angle was defined as the angle between the bilayer normal and the vector from the last to the first carbon atom of the oxidized sn-2 acyl chain. A tilt angle of 0° indicates an sn-2 chain in the hydrocarbon interior of the bilayer and antiparallel to the bilayer normal. A tilt angle of 180° indicates an sn-2 chain pointing into the aqueous phase and parallel to the bilayer normal. The distribution for a POPC oleoyl chain (thick gray line) is from the pure POPC simulation.

Figure 4

Density distribution of carboxyl (COO-) group of PazePC and carbonyl (CHO) group of PoxnoPC for the 25% OXPL simulations. The density of the phosphate is from the pure POPC simulation and was scaled by a factor of ∼0.067 to facilitate comparison.

Figure 5

The overall electron density of the pure POPC bilayer, and for POPC bilayer with 25% OXPL.

Figure 6

Average area per lipid (a) and thickness (b) of the lipid bilayers at various concentrations of OXPL. The errors were estimated using a block-averaging approach and cannot be seen in some cases because they are smaller than the size of the symbols. The highest block size was 14 ns. The thickness was calculated as the distance between the average positions of the phosphate groups in the two leaflets.

Figure 7

Tilt angle distribution for the AZ chain the 25% PazePC simulations, with and without salt. The definition of tilt angle is the same as in Fig. 3.

Figure 8

Partial density of the carboxyl (COO⁻) and phosphate (PO₄) groups in the 12.5% OXPL simulations with the CHARMM (CHM) and the GROMACS (GMX) force fields. The density of phosphate was scaled by a factor of 0.05 to facilitate comparison.

Figure 9

The order parameters for the palmitoyl chain of POPC lipids as a function of the amount of PoxnoPC (a) and PazePC (b) present. The numbers in the legend refer to the number of OXPL present in each simulation.

Figure 10

The electrostatic potential profile across a single monolayer for the pure POPC and the 25% OXPL simulations. The potential was calculated from double integration of Poisson's equation along the bilayer normal.