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| [At physiological pH values, PC and PE carry a full negative charge on the phosphate and a full positive charge on the quaternary] ammonium – they are thus zwitterionic but electrically neutral. PS contain in addition to the negatively charged phosphate and the positively charged amino group, a negatively charged carboxyl group. At neutral pH, PS exhibits an overall negative charge. PG and PI carry a net negative charge, since the alcoholic moiety does not carry any positive charge to counterbalance the negative charge on the phosphate. The headgroup charges are held at the interface between the aqueous and hydrophobic phases by the organization of the membrane bilayers. Phospholipids play therefore an important role in the determination of the surface charge of the membrane. The division of phospholipids into classes according to the structure of their headgroup represents only one level of complexity. On a second level, each phospholipid class exhibits various fatty acid chain compositions. The acyl chain lengths vary usually between 12 and 26 carbon atoms and may be saturated or unsaturated. The number of carbon-carbon double bonds can reach as many as 6 per chain. The most abundant saturated chains contain 16 or 18 carbon atoms and the major unsaturated species are C18:1, C18:2 and C20:4. In this notation the first figure refers to the chain length, while the second one indicates the number of double bonds. Nearly all naturally occurring double bonds are cis isomers disrupt the ordered packing of the lipid chains, perturbing the membrane structure. Some branched fatty acids such as isomyristate or isopalmitate may also occur in the lipid chain composition. Another variation is for instance the inclusion of a cyclopropane ring in the fatty acid chain.
An overview of the lipid composition of mammalian plasma and intracellular organelle membranes, stemming from work of Jamieson and Robinson [1], shows that with a few exceptions, PC, PE and cholesterol appear to be the main components. PC are mainly composed of short chains and dipalmitoilphosphatidylcholine [sic] (DPPC), with saturated chains of 16 carbon atoms, is one of the principal component of this class of phospholipids. The principal unsaturated chains found in PC are 18:1 and 18:2. In PE, a relatively high proportion of polyunsaturated chains are found, especially 20:4 chains. The lipids found as membrane components are very diverse but have the same fundamental property in common – they are all amphipathic molecules, presenting separate polar and apolar regions and, for this reason, have natural propensity to form [bilayers structures in an aqueous environment.] 1. Jamieson, G. A.; Robinson, D. M. Mammalian Cell Membranes; Butterworth: London, 1977; Vol. 2. |
At physiological pH values, PC and PE carry a full negative charge on the phosphate and a full positive charge on the quaternary ammonium: they are thus zwitterionic but electrically neutral. PS contain, in addition to the negatively charged phosphate and the positively charged amino group, a negatively charged carboxyl group. At neutral pH, PS exhibit an overall negative charge. PG and PI carry a net negative charge, since the alcoholic moiety does not carry any positive charge to counterbalance the negative charge on the phosphate. The headgroup charges are held at the interface between the aqueous and hydropho-
[page 15:] bic phases by the organization of the membrane bilayer. Phospholipids play therefore an important role in the determination of the surface charge of the membrane. The division of phospholipids into classes according to the structure of their headgroup represents only one level of complexity. On a second level, each phospholipid class exhibits various fatty acid chain compositions. The acyl chain lengths vary usually between 12 and 26 carbon atoms and may be saturated or unsaturated. The number of carbon-carbon double bonds can reach as many as six per chain. The most abundant saturated chains contain 16 or 18 carbon atoms and the major unsaturated species are C18:1, C18:2, and C20:4. In this notation, the first figure refers to the chain length, while the second one indicates the number of double bonds. Nearly all naturally occurring double bonds are cis rather than trans. A cis double bond introduces a kink or bend in the molecule, so that cis isomers disrupt the ordered packing of the lipid chains, perturbing the membrane structure. Some branched fatty acids such as isomyristate or isopalmitate may also occur in the lipid chain composition. Another variation is for instance the inclusion of a cyclopropane ring in the fatty acid chain. [page 18:] Lipid composition of a few membranes An overview of the lipid composition of mammalian plasma and intracellular organelle membranes, stemming from the work of Jamieson and Robinson, is given in Table 1.1 [3]. Although these various membranes exhibit characteristic differences in their composition, some common features emerge. With a few exceptions, PC, PE, and cholesterol appear to be the main components. [...] PC are mainly composed of short chains and dipalmitoylphosphatidylcholine (DPPC), with saturated chains of 16 carbon atoms, is one of the principal component of this class of phospholipids. The principal unsaturated chains found in PC are 18:1 and 18:2. Sphingomyelin, in contrast, contains a great amount of long chains with 24 C-atoms with one double bond at the most. In PE, a relatively high proportion of polyunsaturated chains are found, especially 20:4 chains.
Concluding remarks The lipids found as membrane components are very diverse but have the same fundamental property in common: they are all amphipathic molecules, presenting separate polar and apolar regions and, for this reason, have a natural propensity to form bilayer structures in an aqueous environment. [3] G. A. Jamieson and D. M. Robinson. Mammalian Cell Membranes, volume 2. Butterworth, London, 1977. |
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