Most, if not all, biological membranes contain lipids that, in isolation, prefer to adopt a curved, hexagonal Hn phase rather than the normal, planar, bilayer phase (Cullis and de Kruijff 1979; Rietveld et al. 1993). Sometimes such lipids can be the major lipids in a membrane: for example, monogalactosyl diacylglycerol makes up about 50% of the total lipid in the chloroplast membrane (Shipley et al. 1973) and the E. coli inner membrane contains about 70% phosphatidylethanolamine (Har-wood and Russell 1984). It has been suggested that bacteria control the lipid compositions of their membranes to maintain a constant proportion of lipids favouring the hexagonal Hn phase (for a recent review, see Cronan 2003).
The tendency of phospholipids such as phosphatidylethanolamines to adopt a curved structure can be understood in terms of the forces present in a lipid bilayer, as shown in Fig. 6.5 (Gruner 1985; Seddon 1990; Lindahl and Edholm 2000). For a lipid monolayer to stay flat, the pressures illustrated in Fig. 6.5 must be in balance across the monolayer. If the lateral pressure in the chain region becomes greater than that between the headgroups, the monolayer will curl towards the aqueous region (Fig. 6.5). This is defined as a negative curvature. Conversely, if the lateral pressure between the headgroups becomes greater than that between the chains, the monolayer will curl towards the chain regions, a positive curvature (Fig. 6.5). A lip-
id such as a phosphatidylethanolamine has a tendency to adopt a negatively curved, hexagonal Hn phase because the small size of the phosphatidylethanolamine head-group relative to the cross-sectional area of the two fatty acyl chains gives the lipid an overall conical shape (Seddon 1990).
The tendency to curl becomes frustrated in a lipid bilayer. In a symmetrical bilayer (with identical conditions on each side) the two monolayers will both want to curve in the same way (either positive or negative) and so will counteract each other; the two monolayers cannot both curve in the same direction since this would create free volume in the interior of the bilayer. Thus the bilayer has to remain flat, in a state of physical frustration. Confining a monolayer with a non-zero spontaneous curvature to a planar form results in an elastic free energy stored in the bilayer. If the stresses in the bilayer become too great, the bilayer structure will become unstable and a non-bilayer phase will form.
Of course, in a real biological membrane it is essential that a lipid bilayer structure is maintained, and a lipid bilayer structure is maintained because the presence of intrinsic membrane proteins and bilayer-preferring lipids overcomes the tendency of lipids such as phosphatidylethanolamines to adopt a non-bilayer structure. Thus mixtures of di(C18:l)PC and di(C18:l)PE containing 15% or more di(C18:l)PC adopt a bilayer structure (Boni and Hui 1983), and a number of intrinsic membrane proteins (glycophorin A, cytochrome oxidase, the light-harvesting complex of photosystem II) have also been shown to stabilize a bilayer structure (de Kruijff et al. 1985; Simidjiev et al. 2000).
It is worth exploring further the idea that a lipid such as a phosphatidylethanolamine has an overall conical shape. If the "conical" shape of the phosphatidylethanolamine molecule were to be an accurate description of the overall shape of the phosphatidylethanolamine molecule in a lipid bilayer, incorporation of phosphatidylethanolamine into bilayers of phosphatidylcholine would have significant effects on the order parameter profile for the fatty acyl chains of the phosphatidylcholine molecule; inclusion of phosphatidylethanolamine would create a greater packing density toward the centre of the bilayer and a smaller packing density near the glycerol backbone region and thus increase order parameters for phosphatidylcholine chains at the terminal methyl ends of the chains and decrease order parameters at the carboxyl end. Such effects are not seen, addition of a phosphatidylethanolamine to a bilayer of a phosphatidylcholine increasing order parameters at all positions in the chains of the phosphatidylcholine (Fenske et al. 1990). Thus the phosphatidylethanolamine molecule increases packing density throughout the bilayer, consistent with the observation that order parameters for chains in phosphatidylethanolamines are greater than those in phosphatidylcholines at all positions of the chain (Perly et al. 1985; Lafleur et al. 1990). It has also been shown that, in mixtures of (C16:0,C18:1)PE and (C16:0,C18:1)PC, the order parameters for the palmitoyl chains in (C16:0,C18:1)PE and (C16:0,C18:1)PC are the same (Lafleur et al. 1990). Similarly, it has been shown in molecular dynamics simulations that order parameter profiles for the oleoyl chains in mixtures of di(C18:l)PC and di(C18:l)PE are identical for chains on the two classes of lipid (de Vries et al. 2004). Thus although the idea of a cone-shaped phosphatidylethanolamine molecule is helpful in explaining the preference of phosphatidylethanolamine for the hexagonal Hn phase, the acyl chains do not adopt a cone shape when mixed in a bilayer with phosphatidylcholine. The increase in order parameters seen on addition of a phosphatidylethanolamine to bilayers of a phosphatidylcholine has been attributed to the ability of the phosphatidylethanolamine headgroup to hydrogen bond with phosphatidylcholines or other phosphati-dylethanolamines, and to the smaller size of the phosphatidylethanolamine head-group (de Vries et al. 2004).
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