Physical dimensions of a membrane component influence its phase preference upon hydration. A dimensionless packing parameter, S, given by S = V/al, where V is the hydrocarbon volume, a is the area of the head group, and I is the critical length of the hydrocarbon chain, is useful in determining the phase preference of a lipid, and the value of S usually lies between 0.5 and 1 for bilayers. Here, the value of S is calculated for phosphatidylcholine (PC) and lysophosphatidylcholine (lysoPC) as a function of chain length, and it is shown that diacylPC having an S value of <0.74 does not form bilayers. For example, diacylPC, up to a chain length of eight carbon atoms, forms only micelles, whereas higher homologs with S > 0.74 form bilayers. It is also shown that when lipid molecules having complementary shapes associate, the value of S becomes additive. Using the additivity of S, a number of experimental results for lipid mixtures can be explained. For example, lysoPC and cholesterol form lamellar structures between 45 and "40 mol% cholesterol, and the additive value of S for this region is between 0.74 and 1. Similarly, the additivity of S shows that the maximum amount of cholesterol that can be incorporated into PC bilayers is 50 mol%, in agreement with experimental studies.Molecular shape is an important consideration in membrane modeling. Based on the physical dimensions of a membrane component, its phase presence upon hydration and its location in the membrane can often be predicted. Taking into account interaction free energies, molecular geometry, and entropy, theoreticians have developed a dimensionless packing parameter, S, that is useful in determining the size and shape of lipid aggregates. S is given by S = V/al, where V is the hydrocarbon volume, a is the area of head group, and 1 is the critical length of the hydrocarbon chain (1-3). a, V, and I are all estimable or measurable (4), and the value of S can be calculated. The value of S determines the aggregate formed by lipids or any amphiphiles upon hydration. It has been shown that lipids aggregate to form spherical micelles (S
Gap junctions (GJ) are important regulators of cellular function. They provide channels for the direct movement of small molecules between cells and thus control cell-to-cell transfer of metabolites and the transmission of various stimuli. Gap junctions have been shown to be involved in a multitude of cellular processes ranging from cell synchronization and neuronal function to cell differentiation and carcinogenesis. Much knowledge has been gained in recent years concerning the structure and molecular organization of GJ proteins; yet, the mechanisms that control and modulate gap junction assembly and function are still not well understood. Although it is quite apparent that the GJ proteins assemble in the lipid milieu of the plasma membrane, and that the cluster of proteins assembled in the junction do function in a lipid environment, there is a general paucity of information on the role of lipids in the gap junction assembly process and in the function of gap junctions. The present review is a comprehensive account of current knowledge on gap junction lipids. We also discuss what is known to date on the involvement of lipids in gap junction formation. Special emphasis is being placed on the potential role of membrane cholesterol in gap junction assembly and function.
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