Membrane lipid structure/high temperature

In this context it is interesting to note that archaea, which possess S-layers as exclusive cell wall components outside the cytoplasmic membrane (Fig. 14), exist under extreme environmental conditions (e.g., high temperatures, hydrostatic pressure, and salt concentrations, low pH values). Thus, it is obvious one should study the effect of proteinaceous S-layer lattices on the fluidity, integrity, structure, and stability of lipid membranes. This section focuses on the generation and characterization of composite structures that mimic the supramolecular assembly of archaeal cell envelope structures composed of a cytoplasmic membrane and a closely associated S-layer. In this biomimetic structure, either a tetraether... 

The membrane is a dynamic assembly and things are diffusing rapidly in the plane of the bilayer. The middle of the bilayer has been likened to olive oil. As with oil, cooling the lipid bilayer will cause the hydrocarbons to become more ordered (structured). The side chains pack closer to each other, and the fluidity of the membrane is lower. Things that disrupt the ability of the side chains to pack in a regular fashion make the membrane more fluid (Fig. 3-4). These include high temperature, lipids with shorter chains (double bonds. The shorter lipids and the m-double bonds cause the occurrence of holes (packing defects). 

The lipid part of the membrane is essentially a two-dimensional liquid in which the other materials are immersed and to which the cytoskeleton is anchored. This last statement is not totally correct, as some membrane bound enzymes require the proximity of particular lipids to function properly and are thus closely bound to them. Simple bilayers formed from lipids in which both hydrocarbon chains are fully saturated can have a highly ordered structure, but for this reason tend to be rigid rather than fluid at physiological temperatures. Natural selection has produced membranes which consist of a mixture of different lipids together with other amphiphilic molecules such as cholesterol and some carboxylic acids. Furthermore, in many naturally occurring lipids, one hydrocarbon chain contains a double bond and is thus kinked. Membranes formed from a mixture of such materials can retain a fluid structure. The temperature at which such membranes operate determines a suitable mixture of lipids so that a fluid but stable structure results at this temperature. It will be seen that the lipid part of a membrane must, apart from its two-dimensional character, be disordered to do its job. However, the membrane bound proteins have a degree of order, as will be discussed below. 

Aseptic Lltration is necessary for parenteral formulations. Because both lipids and the structure of liposomes are unstable at high temperatures, conventional terminal steam sterilization is not suitable for liposome formulations. Thus, the membrane aseptic Lltration is the most reliable method for sterilizing liposome formulations. Since the possibility exists for the membrane being defective, it is advisable to test the integrity of the assembled unit by carrying out a bubble-point test. This test... 

Based on knowledge of the phase behaviour of membrane lipids it is possible to predict the events that accompany exposure of biological membranes to extremes of temperature. Because the destabilizing events are fundamentally different the effect of low temperature and high temperature on membrane structure and stability will be considered sepa rately. 

All of these events can be ascribed to the release of constraints imposed on non-bilayer lipids to maintain them in a bilayer phase required to achieve a stable membrane structure. Exposure to high temperatures serves to phase separate the non-bilayer lipids into discrete domains that appear to be devoid of membrane proteins. It is likely also that the permeability barrier properties are altered by this type of phase separation. In general, where gross non-bilayer phase separations have taken place it is difficult to imagine how the normal distribution of membrane components can be achieved and especially at a rate that would be consistent with the continued survival of the cell. 

Membrane lipids exhibit complex polymorphism as a function of temperature. A balance of lipid phase structure is believed to result from interaction of lipids with other membrane components and solutes in the aqueous phase. In general, this balance results in a formation of a fluid lipid bilayer matrix. Phase separations of lipid from other membrane constituents can be driven by exposure of membranes to temperatures outside the normal growth temperature. These can be the creation of gel phase domains at low temperature or the formation of nonbilayer structures at high temperature. Both types of lipid phase separation are associated with functional changes in the membrane including loss of selective permeability barrier properties. 

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