Fluidity of membrane lipids

T he use of spin-label probes to investigate cell membrane structure and function clearly demonstrates the fluidity of membrane lipid structures (1,2,3,4) however, a spin-label probe sees only its immediate environment. Predictions (5, 6, 7, 8) and data (9, 10, 11, 12) show that the introduction, for example, of a substituted oxazolidine ring as part of a typical amphiphatic lipid molecule can also significantly perturb a normal lipid environment. Consequently, some quantitative observations that used spin-label techniques need revision while others may be reduced to the level of qualitative predictions. 

SFAs (Cronan and Gelmann, 1973). As a result, the fluidity of membrane lipids returns to their original state, or close to it, with restoration of normal cell activity at the lower temperature. 

Like general anesthetics, ethanol appears to act by changing the fluidity of membrane lipids, leading to a perturbed function of the membrane proteins. During development of tolerance to ethanol, the membrane phospholipids acquire more saturated fatty acids, which seems to counteract the effects of ethanol on membrane function. 

The size, structure, and fluidity of membrane lipids are also important because those aspects of the molecules make it possible for them to pack efficiently into a variety of convoluted bilayer membrane structures with various degrees of curvature and flexibihty. Tliat flexibility also makes possible the inclusion of the various other important components of the cell wall, including proteins, carbohydrates, and cholesterol. In terms of the geometric concepts discussed previously (see Fig. 15.15), one can visualize where one class of lipid will have a critical packing factor. Pc (= v/adc) < 1, which will produce a truncated cone shape, while another will have F > i for an inverted truncated cone. Combinations of the two can then accommodate the inclusion of, for example, proteins and cholesterol, while maintaining an overall planar structure (or a given degree of curvature), or increase curvature to produce a smaller associated unit. 

In Section II.A, it was pointed out that the Arrhenius plots of sugar transport rates have discontinuities at the same temperature at which spin-label probes of the bulk lipid phase undergo discontinuous changes. This implies a sensitivity on the part of the protein to changes in the mobility (fluidity) of membrane lipids. In this section, we will examine this point in greater detail. 

Size, structure, and fluidity of membrane lipids are also important characteristics because those aspects of the amphiphilic molecules make it possible for them to efficiently pack into a variety of bilayer membrane structures with various degrees of curvature and flexibility. That flexibility makes possible the inclusion of other important components of the cell wall, including proteins, glycoproteins, and cholesterol. 

Huang, L. and A. Haug. 1974. Regulation of membrane lipid fluidity in Acholeplasma laidlawii Effect of carotenoid pigment content. Biochim. Biophys. Acta 352 361-370. 

Mansilla, M.C., Cyhulski, L.E., Albanesi, D. and de Mendoza, D. (2004) Control of membrane lipid fluidity by molecular fhermosensors. Journal of Bacteriology,... 

The rs is often equated with the term membrane fluidity, which itself is a vague term relating to the motional condition of membrane lipids. Nevertheless, membrane fluidity continues to be a useful concept in studies with natural cell membranes. This subject has been rigorously reviewed elsewhere 2 34) and will therefore not be dealt with in detail here. In spite of the problem that rs contains both rate and orientational contributions (see... 

C. D. Stubbs, Membrane fluidity Structure and Dynamics of membrane lipids, Essays in Biochemistry 19, 1-39 (1983). 

The fluidity of membranes primarily depends on their lipid composition and on temperature. At a specific transition temperature, membranes pass from a semicrystalline state to a more fluid state. The double bonds in the alkyl chains of unsaturated acyl residues in the membrane lipids disturb the semicrystalline state. The higher the proportion of unsaturated lipids present, therefore, the lower the transition temperature. The cholesterol content also influences membrane fluidity. While cholesterol increases the fluidity of semicrystalline, closely-packed membranes, it stabilizes fluid membranes that contain a high proportion of unsaturated lipids. 

Unsaturated fatty acid chains do not pack together in the bUayer as tightly as saturated fatty acid chains these properties contribute to different degrees of fluidity of membranes of different lipid composition. 

This cascade however may be propagated throughout the cell unless terminated by a protective mechanism (see below) or a chemical reaction such as disproportionation, which gives rise to a non-radical product. Polyunsaturated fatty acids, found particularly in membranes, are especially susceptible to free radical attack. The effects of lipid peroxidation are many and various. Clearly, the structural integrity of membrane lipids will be adversely affected. In the lipid radical produced, the sites of unsaturation may change, thereby altering the fluidity of the membrane (see chap. 3). Lipid radicals may interact with other lipids and... 

Cholesterol - an essential component of mammalian cells - is important for the fluidity of membranes. With a single hydroxy group, cholesterol is only weakly am-phipathic. This can lead to its specific orientation within the phospholipid structure. Its influence on membrane fluidity has been studied most extensively in erythrocytes. It was found that increasing the cholesterol content restricts molecular motion in the hydrophobic portion of the membrane lipid bilayer. As the cholesterol content of membranes changes with age, this may affect drug transport and hence drug treatment. In lipid bilayers, there is an upper limit to the amount of cholesterol that can be taken up. The solubility limit has been determined by X-ray diffraction and is... 

The earliest general model of adaptation to temperature in membrane lipids focused on the physical state ( static order or viscosity [= 1 / fluidity ]) of the bilayer. The finding that the physical state of membrane lipids from Escherichia coli cultured at different temperatures was similar at the different growth temperatures led to the homeoviscous adaptation hypothesis, which states that lipid composition is modified during thermal acclimation to facilitate retention of a relatively stable membrane physical state (Sinensky, 1974). At the outset of any discussion of homeoviscous adaptation, it is important to examine carefully what is meant by physical state (or the related terms static order, viscosity, and fluidity ). In such an analysis, one must also consider the physical methods that are used to make such measurements—and the limitations of these techniques. 

The potential consequences of the peroxidation of membrane lipids include loss of polyunsaturated fatty acids, decreased lipid fluidity, altered membrane permeability, effects on membrane-associated enzymes, altered ion transport, release of material from subcellular compartments, and the generation of cytotoxic metabolites of lipid hydroperoxides. The physiological significance of lipid peroxidation products is shown in Table 1. 

Membranes, Fluidity of Membrane Proteins, Properties of Membrane Fusion, Mechanisms of Lipid BUayers, Properties of Lipid Rafts Biosensors... 

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