Lipids function

The length of the acyl chains (and hence membrane thickness) was also important - not surprisingly because the sugar transporter is known to contain 12 membrane-spanning hydrophobic domains. In effect the thickness of the membrane has to be right for the correct arrangement of the hydrophobic domains. 

2 MODIFICATIONS OF MEMBRANE LIPIDS DURING TEMPERATURE ADAPTATION 

We have already seen that membrane lipids can exist in the gel- or liquid-crystalline phases, depending on the prevailing temperature. Whereas isolated lipid samples show a sharp gel-liquid phase transition, naturally occurring lipids show broad non-cooperative transitions due to the heterogeneity of their acyl chains. For a series of molecular species of, say, phosphatidylcholine, the temperature Tc) for the gel-liquid phase transition depends on the fatty acid composition (with cis unsaturated fatty acids lowering Tc) and even on the position of acyl esterification (Table 8.2). 

It used to be thought that the membranes of natural organisms could not function when their lipids were in the gel-crystalline phase and that changes in growth or other physiological parameters seen in prokaryotes on lowering temperature could be explained simply by proposing that phase transitions had taken place. However, it is now clear that while a correlation can 

The process of temperature adaptation has been studied in some detail in cyanobacteria by Murata s group and in the protozoan Tetrahymena and the alga Dunaliella by Thompson in Texas. A number of lipid changes have been observed by these workers and others (Table 8.3) but they occur on a different time scale. A sudden change in growth temperature could 

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Our studies on the surface pressure-area isotherms of MGDG and the mixture of PS II core complex and MGDG indicate the presence of both PS II core complex and MGDG in the monolayer. MGDG molecules diluted the PS II core complex concentration in the monolayer. MGDG lipid functions as a support for the protein complex and the resulting mixture forms higher-quality films than PS II core complex alone. 

Covalent attachment of proteins to the surface of liposomal bilayers is done through reactive sites created on the head groups of phospholipids with the intermediary use of a crosslinker or other activating agent. The lipid functional groups described in Section 1 of this chapter are modified according to the methods discussed in Section 2 to be reactive toward specific target... 

Mutant or knockout mice defective in specific enzymes involved in lipid synthesis have provided powerful tools for genetic analysis of lipid function in the nervous system. For example, disruption of the genes for ceramide galactosyl transferase or galactosyl ceramide sulfotransferase,... 

By analogy to the solution-phase approaches, the introduction of lipid functionalities on peptides on the solid support can also follow two general approaches, either using prelipidated building blocks in the standard solid-phase peptide synthesis or via selective lipidation on resin. " " ... 

The following sections present suggested protocols for creating protein-bearing liposomes. Each method utilizes specific lipid functional groups and targets amines, sulfhydryls, aldehydes, or carboxylates on the protein molecules. 

Ringsdorf and coworkers used monolayer techniques for the creation of lipid functionalized streptavidin layers.252,253 In their pioneering work, a monolayer of biotinylated phospholipids initially created a two-dimensional docking matrix for the immobilization of streptavidin. In an effort to further expand this system while maintaining control over its structure, bis-biotinylated bifunctional linkers were employed. Addition of these linkers to concanavalin A created a new protein-based bidentate linker. Upon addition to the monolayer of the lipid-functionalized streptavidin a second biotilynated protein layer was created. Upon the further addition of streptavidin, they achieved in a step-by-step construction process well-defined alternating protein triple layers of streptavidin and concanavalin A (Figure 7.37b). A similar approach has more recently been used to crosslink... 

Close modulation of the physical states of lipids is important in a variety of other contexts. Lipids play major roles as buoyancy-regulating devices in which temperature-induced alterations in lipid density play important functions in setting the overall buoyancy of the organism (e.g., the spermaceti organ of sperm whales see Clarke, 1979). Cuticular lipids function as important barriers to water movement in terrestrial arthropods (Gibbs, 1998 Hadley, 1981), and the chemical compositions and physical states of these lipids manifest temperature responses comparable in many ways to those observed for membrane lipids. 

Fig. 5.—Proposed Structure of the Lipid Functional Group of the Group C Meningococcal Polysaccharide. 

Munro JC, Frank CW (2004) Adsorption of lipid-functionalized poly(ethylene glycol) to gold surfaces as a cushion for polymer-supported lipid bilayers. Langmuir 20 3339-3349... 

Introduction of lipid functionalities into peptides can follow two general strategies, independent of solid-phase or solution-phase chemistry. Either the lipid groups are introduced via coupling of prelipidated amino acids to the peptide (Fig. 3a) or they are introduced via selective lipidation of a peptide (Fig. 3b-e). Both the use of prelipidated building blocks (14-16) and of resin lipidation (17-25) have been investigated for the synthesis of lipidated peptides on solid support. 

Figure 3 Different approaches for the introduction of lipid functionalities, here exemplified via the farnesyl group, into peptides. (A) Lipidated amino acid building blocks. (B) Substitution of bromoalanine with a nucleophile. (C) Alkylation or acylation of a free thiol functionality of a cysteine. (D) Conjugate addition of a nucleophile (e.g., farnesylthiolate) to a dehydroalanine. (E) Conjugate addition of a nucleophile to aziridine-2-carboxylic acid containing...

The only piece of experimental evidence for the orientation of apoA-I helices on DMPC discoidal complexes has come from the use of polarized attenuated total reflection infrared (ATR-IR) spectroscopy (Brasseur et al. (1990 Wald et al., 1990). Although internal reflection spectroscopy is not new (Harrick, 1967), its combination with polarized IR measurements of oriented biological membranes is a more recent application, which allows the spectrum to be taken in the presence of water. The technique has been used most frequently to study the orientation of lipid functional groups relative to the membrane plane in pure lipid bilayers (Fringeli and Gunthard, 1981 Holmgre et al., 1987 Okamura et al., 1990 Hubner and Mantsch, 1991), but has also been applied to studies of... 

Larsson, K., Tailoring Lipid Functionality in Foods, Trends Food Sci. Technol. 5 311-315... 

Lipids are molecules that are insoluble in water and soluble in nonpolar solvents. Lipids function to store energy and form cell membranes. 

The pleiotropic function of even a single lipid and its widespread distribution within a given cell poses difficult challenges to defining roles for a lipid in cellular processes. Unlike proteins lipids display no inherent catalytic activity and are not encoded by genes so that many of the initial clues to lipid function... 

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