X-ray structure, thermodynamics, elastic properties and MD simulations of cardiolipin/dimyristoylphosphatidylcholine mixed membranes

Alexander L Boscia,Bradley W Treece,Dariush Mohammadyani,Judith Klein-Seetharaman,Anthony R Braun,Tsjerk A Wassenaar,Beate Klösgen,Stephanie Tristram-Nagle

doi:10.1016/j.chemphyslip.2013.12.010

Abstract

Cardiolipins (CLs) are important biologically for their unique role in biomembranes that couple phosphorylation and electron transport like bacterial plasma membranes, chromatophores, chloroplasts and mitochondria. CLs are often tightly coupled to proteins involved in oxidative phosphorylation. The first step in understanding the interaction of CL with proteins is to obtain the pure CL structure, and the structure of mixtures of CL with other lipids. In this work we use a variety of techniques to characterize the fluid phase structure, material properties and thermodynamics of mixtures of dimyristoylphosphatidylcholine (DMPC) with tetramyristoylcardiolipin (TMCL), both with 14-carbon chains, at several mole percentages. X-ray diffuse scattering was used to determine structure, including bilayer thickness and area/lipid, the bending modulus, KC, and SXray, a measure of chain orientational order. Our results reveal that TMCL thickens DMPC bilayers at all mole percentages, with a total increase of ∼6Å in pure TMCL, and increases AL from 64Å2 (DMPC at 35°C) to 109Å2 (TMCL at 50°C). KC increases by ∼50%, indicating that TMCL stiffens DMPC membranes. TMCL also orders DMPC chains by a factor of ∼2 for pure TMCL. Coarse grain molecular dynamics simulations confirm the experimental thickening of 2Å for 20mol% TMCL and locate the TMCL headgroups near the glycerol-carbonyl region of DMPC; i.e., they are sequestered below the DMPC phosphocholine headgroup. Our results suggest that TMCL plays a role similar to cholesterol in that it thickens and stiffens DMPC membranes, orders chains, and is positioned under the umbrella of the PC headgroup. CL may be necessary for hydrophobic matching to inner mitochondrial membrane proteins. Differential scanning calorimetry, SXray and CGMD simulations all suggest that TMCL does not form domains within the DMPC bilayers. We also determined the gel phase structure of TMCL, which surprisingly displays diffuse X-ray scattering, like a fluid phase lipid. AL=40.8Å2 for the ½TMCL gel phase, smaller than the DMPC gel phase with AL=47.2Å2, but similar to AL of DLPE=41Å2, consistent with untilted chains in gel phase TMCL.

Highlights

Cardiolipin (CL) refers to a group of unusual glycero-phospholipids which contain four instead of two acyl chains
At 20 mol% TMCL, wide-angle x-ray scattering (WAXS) revealed a gel/ fluid phase coexistence at 35°C, so the temperature was raised to 40°C in order for this sample to remain in the fluid phase
For pure TMCL, a gel/fluid phase coexistence prevailed at 45°C, so the temperature was raised to 50°C, where the sample was in the fluid phase

Summary

Introduction

Cardiolipin (CL) refers to a group of unusual glycero-phospholipids which contain four instead of two acyl chains. The acyl chain composition varies with species and cell state. Many investigations have been carried out to characterize CL’s overall structure (Lewis et al, 2007), headgroup structure (Haines, 2009; Tarahovsky et al, 2000), thermodynamic properties (Lewis and McElhaney, 2009; Nichols-Smith et al, 2004) , water permeability (Shibata et al, 1994), membrane fluidity (Yamauchi et al, 1981), ability to form the hexagonal II (HII) phase (Powell and Hui, 1996; Powell and Marsh, 1985; Rand and Sengupta, 1972; Seddon et al, 1983) and domains when mixed with other host phospholipids (Domenech et al, 2007; Frias et al, 2011; Lupi et al, 2008; Pinheiro et al, 1994; Sennato et al, 2005). In this work we present results from experiments and simulations; in both cases, CL contains the same chains as the host lipid, DMPC (dimyristoylphosphatidylcholine)

Methods

Results

Conclusion