Insight into the putative specific interactions between cholesterol, sphingomyelin, and palmitoyl-oleoyl phosphatidylcholine

Abstract

The effects of cholesterol (Chol) on phospholipid bilayers include ordering of the fatty acyl chains, condensing of the lipids in the bilayer plane, and promotion of the liquid-ordered phase. These effects depend on the type of phospholipids in the bilayer and are determined by the nature of the underlying molecular interactions. As for Chol, it has been shown to interact more favorably with sphingomyelin than with most phosphatidylcholines, which in given circumstances leads to formation of lateral domains. However, the exact origin and nature of Chol-phospholipid interactions have recently been subjects of speculation. We examine interactions between Chol, palmitoylsphingomyelin (PSM) and palmitoyl-oleoyl-phosphatidylcholine (POPC) in hydrated lipid bilayers by extensive atom-scale molecular dynamics simulations. We employ a tailored lipid configuration: Individual PSM and Chol monomers, as well as PSM-Chol dimers, are embedded in a POPC lipid bilayer in the liquid crystalline phase. Such a setup allows direct comparison of dimeric and monomeric PSMs and Chol, which ultimately shows how the small differences in PSM and POPC structure can lead to profoundly different interactions with Chol. Our analysis shows that direct hydrogen bonding between PSM and Chol does not provide an adequate explanation for their putative specific interaction. Rather, a combination of charge-pairing, hydrophobic, and van der Waals interactions leads to a lower tilt in PSM neighboring Chol than in Chol with only POPC neighbors. This implies improved Chol-induced ordering of PSM's chains over POPC's chains. These findings are discussed in the context of the hydrophobic mismatch concept suggested recently.

FIGURE 1

Structures of palmitoyl-oleoyl-phosphatidylcholine (POPC, left), cholesterol (Chol, middle), and palmitoyl-sphingomyelin (PSM, right). Despite their broad structural similarities, POPC and PSM interact differently with Chol.

FIGURE 2

Snapshot of one monolayer of the simulated system. For clarity, PSM (red) and Chol (blue) are rendered with a space-filling model, while the POPC matrix is rendered with sticks. In both monolayers, the system features four PSM monomers, four Chol monomers, and four PSM-Chol dimers.

FIGURE 3

Area per lipid versus time for the simulated bilayer.

FIGURE 4

Electron densities of different lipids with and without specific nearest neighbors.

FIGURE 5

S_CD order parameters of the acyl chains of phospholipids without contact to a Chol molecule (upper graph) and of those that have a Chol as a nearest neighbor (lower graph). SPbase refers to the sphingosine chain of PSM.

FIGURE 6

Distribution of Chol tilt angles for those Chols with a PSM neighbor and those without.

FIGURE 7

Order parameter of Chol neighboring chains (mean S_CD averaged over all carbons in a chain) plotted against that Chol's tilt angle. The added lines indicate the average tilts of PSM neighboring Chol (25°) and monomeric Chol (33°). Neighborship to cholesterol was established using a 0.7 nm cutoff for the center-of-mass distances in the bilayer plane (the x,y plane).

FIGURE 8

Orientation distributions of choline headgroups, measured as the angle θ_PN of the P-N vector with the outward bilayer normal. Of all POPCs, only a small fraction is in contact with Chol. SM without chol refers to the monomeric non-Chol neighboring PSMs. POPC/PSM with chol nn contains only those lipids that actually have a Chol as nearest neighbor.

FIGURE 9

Simulation snapshots representing typical PSM orientations, for a PSM without Chol neighbor (left) and a PSM with Chol neighbor (right). The PSM and Chol in the right image form a charge pair between the headgroup positive charge and the Chol oxygen (see Charge-Pairing).

FIGURE 10

C₂ reorientational autocorrelation functions of vectors in the headgroup (P-N) and in the interfacial region for POPC/PSM molecules with (w) or without (no) Chol contact.

FIGURE 11

Hydrophobic interaction calculation. Densities of water and Chol are calculated inside small cylinders (of radius 0.7 nm), that are centered on Chol molecules (illustrated above on the purple-colored cholesterol molecule). Density overlap of water and Chol nonpolar carbons (highlighted by stripes) indicates unfavorable water contacts. The left profile is of non-PSM-paired Chol and the right one is of Chol that is paired to a PSM.