Effects of Internal Bilayer Pressures and Curvature Stress

Although, as described above, lateral pressures in the headgroup and chain regions of a lipid bilayer must balance, the distribution of lateral pressures within the chain region are not uniform (Lindahl and Edholm 2000; Gullingsrud and Schulten 2004). It has been suggested that the lateral pressure profile in a lipid bilayer could be an important factor in determining the conformational equilibrium of a membrane protein (Cantor 1997). For example, if changes in the cross-sectional area of a membrane protein in going from one conformation to another are different at different depths in the membrane, then it has been suggested that the equilibrium constant describing the conformational change will depend on lateral pressure profile through a series of PA V work terms, where P and A Prefer, respectively, to pressures and volume changes at particular depths in the bilayer (Cantor 1997). The largest tension in a lipid bilayer is located close to the glycerol backbone region, where close packing is required to prevent exposure of the fatty acyl chains to water (Lindahl and Edholm 2000; Gullingsrud and Schulten 2004). However, it could be argued that expansion of a membrane protein close to the o

Fig. 6.12.Possible effects of stored curvature elastic energy on the function of membrane proteins. In (a) the binding of an extrinsic membrane protein Is increased In the presence of lipids favouring the hexagonal Hn phase, as the fatty acyl chains of neighbouring lipid molecules can distort to fill the free volume created by Insertion of the extrinsic membrane protein Into the lipid headgroup region. In (b) the presence of lipids favouring the hexagonal HN phase shifts the conformation equilibrium of an intrinsic membrane protein towards the conformation with the greatest hydrophobic thickness glycerol backbone region need not result in any increased exposure of lipid fatty acyl chains to water, so that expansion of the protein in this region does not require any work against a lipid pressure. Further, it has been argued above, from a comparison of the properties of simple hydrocarbons with the properties of the hydrocarbon core of a lipid bilayer, that the hydrocarbon core of a lipid bilayer is not in any way special as far as its bulk properties are concerned.

It has been suggested that lateral pressure profiles in lipid bilayers will be very dependent on the detailed structure of the phospholipid (Cantor 1997). However, molecular dynamics simulations suggest that the pressure profiles in bilayers of di(C18:l)PC and di(C18:l)PE are rather similar, and differences in pressure profile between di(C18:l)PC and di(C18:l)PE are likely to be too small to be functionally significant (Gullingsrud and Schulten 2004). It has also been shown experimentally that the activity of the Ca2+-ATPase of sarcoplasmic reticulum, although very sensitive to fatty acyl chain length, is not sensitive to features of the fatty acyl chains such as unsaturation (East et al. 1984).

Curvature stress has also been suggested to be a significant factor in the function of membrane proteins. It has been suggested that the stored curvature elastic energy in a bilayer containing a non-bilayer preferring lipid such as phosphati-dylethanolamine can be used to increase the binding of a peripheral membrane protein to the membrane (Attard et al. 2000); as shown in Fig. 6.12 insertion of an intrinsic membrane protein into the headgroup region of the bilayer will release some of the stored curvature elastic energy. It has also been suggested that stored curvature elastic energy could alter the conformational equilibrium of an intrinsic membrane protein (Botelho et al. 2002). If the hydrophobic thickness of one conformation of a protein is greater than that of an alternative conformation, then the presence of lipids favouring the hexagonal Hn phase could shift the equilibrium in favour of the conformation with the greatest hydrophobic thickness (Fig. 6.12). However, as described in Sect. 6.3.1, it is also possible that any observed effects could be explained in terms of direct lipid-protein interactions.

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