LIPID MOLECULAR SHAPES AND MEMBRANE ARCHITECTURE

A simple biomembrane like erythrocyte contains well over hundred lipid species with diverse molecular shapes differing in the number of acyl chains, chain length, unsaturation and head group composition. A deli cate balance between these molecular shapes is necessary in order to h ave a functional membrane. It is well established that the activities of a number of membrane-bound enzymes and other properties such as agg regation, spontaneous vesiculation, pathophysiological properties and lipid-protein interactions of lipids depend on the acyl chain length, unsaturation and head group composition. In fact, the molecular shape of a phospholipid molecule, as modulated by changes in chain length, u nsaturation and head group composition, is probably what is affecting the above mentioned properties. The molecular shape of a lipid depends on a dimensionless packing parameter, S, the value of which influence s the size and shape of aggregate formed upon hydration. In fact, the additivity of S values of lipid mixtures explains a number of experime ntal observations. The molecular shape concept, although very simple, explains many membrane phenomena like complementarity of molecular sha pes of non-bilayer lipids to form stable bilayers. Membrane permeabili ty is controlled to a large extent by lipid packing which depends upon molecular shapes. In fact, membranes maintain their lamellar structur e by delicately balancing the composition of bilayer-forming and non-b ilayer-forming lipids indicating that complementarity of molecular sha pes is essential to maintain the permeability barrier. Based on this, the complementary molecular shape model of cell membrane is proposed.