Lipid boundary

Cardiolipin forms also tight complexes, with the adenine nucleotide translocator (ATN) affecting its translocator activity (Beyer and Nuscher, 1996). Six cardiolipin residues are tightly bound to lysines (Beyer and Klingenberg, 1985). Removal of these lipids renders the translocator inactive, but activity can be reconstituted by adding cardiolipin. It has also a pivotal role as a boundary lipid of various proteins such as NADH ubiquinone oxireductase (Hoch, 1992) or cytochrome c oxidase (Ushmorov el al., 1999 Vik el al., 1981). 

Das, T.K. and Mazumdar, S., 2000, Effect of Adriamydn on the boundary lipid structure of cytochrome c oxidase pico-second time-resolved fluorescence depolarization studies. Biophys. Chem. 86 15-28... 

Costa-Filho AJ, Crepeau R, Borbat P, Mingtao G, Freed JH. Lipid-gramicidin interactions dynamic structure of the boundary lipid by 2D-ELDOR. Biophys. J. 2003 84 3364-3378. 

Lipids do not form an ideal fluid, but exist as a mixture of diverse species that show preferences in associating with each other as a result of head group attractions or repulsions, and packing effects in the hydrocarbon core. Small transient microdomains are formed by specific and nonspecific protein—protein, protein-lipid, and lipid-lipid interactions. A variety of techniques have shown that membrane proteins are surrounded by a dynamic boundary layer of lipids with an average composition distinct from the bulk phase. Membrane proteins, through their preferential association with specific lipids, can induce microdomains consisting of these boundary lipids and the lipids that interact preferentially with the boundary lipids. In turn, these microdomains enhance the formation of protein clusters. 

Figure 8 Schematic cross-section of a chloroplast membrane showing an intrinsic protein spanning the membrane, with hydrophilic regions located at the membrane surfaces and a hydrophobic portion shaded) embedded within the non-polar interior of the lipid bilayer. Anderson 55 postulates that the chlorophyll molecules represented above with the hydrophobic portion of the chlorin ring shaded) are located as part of the boundary lipid of a chlorophyll-protein complex (Reproduced by permission from Nature, 1975, 253, 536)...

Lavialle et al. [74] have also obtained information on the lipid-protein boundary. These authors studied the interaction of melittin, a polypeptide consisting of 26 amino acid residues, with dimyristoyl phosphatidylcholine. The results illustrated in Fig. 20 show that for a lipid-melittin molar ratio of 14 1 two order-disorder transitions are observed, one above (at 29 °C) an one below (at 17 °C) the transition for the pure lipid (at 22.5 °C). The low temperature transition is associated with a depression of the main lipid phase transition while the 29°C transition is associated with the melting behavior of approximately seven inunobilized boundary lipids which surround the hydrophobic portion of the melittin. 

Quenching in memlManes can also be used to study boundary lipids, whidi are the lipid molecules surrounding a fluorophore or a membrane-bound votein. Suppose th a prc in is surrounded by a discrete number of lipid molecules and that the tiyiHophan fluorescence is accessible to quenchers in the membrane phase (Figure 9.3d). Then the number of boundary lipid molecules can be estimated from... 

On the basis of spin label evidence, Jost et al (12) have suggested that a layer of lipid molecules is immobilized in contact with integral protein molecules these "boundary lipids" exchange with surrounding lipids at much lower rate than lipids in the bulk bilayer. 

The demonstration of the existence of boundary lipids appears very important for the properties of mitochondrial proteins. A boundary layer would represent a dynamic outer surface of each protein, capable of affecting conformational plasticity and hence catalytic activity of the protein itself, and also its interactions with other proteins or different molecules (such as quinones ), besides affecting passive permeability in the areas surrounding the proteins. Evidence in favor and against boundary lipids is listed in Table V. 

Anesthetics appear to abolish the short-range immobilization induced by proteins on lipids (tentatively ascribed to boundary lipids) in other words, there is a labilization of lipid proteins interactions induced by the membrane perturbation. This appears to us as an indirect proof in favor of the existence of boundary lipids. The disordering effect in membranes is induced at low anesthetic concentrations, compatible with those known to cause general anesthesia. 

It is therefore justified to advance the hypothesis that anesthetics primarily alter lipid protein interactions at the level of the boundary lipids that exert their role on catalytic activity and conformation. 

The insertion of the hydrophobic core of cytochrome b into a lipid bilayer was found to result in an immobilization of 2-4 molecules of lipid per molecule of cytochrome b (Dehlinger et al., 1974). This so-called "boundary lipid" was also seen when cytochrome oxidase was added to lipid vesicles (Jost et al., 1973). 

Recent model studies with membrane proteins show that not all of the lipid of membranes is equally fluid. When cytochrome oxidase is titrated with increasing concentrations of phospolipid, the initial lipids bound (up to 0.2 mg per mg protein) show little movement of the fatty acid side chains. Lipids in excess of this concentration begin to exhibit the characteristics of a fluid bilayer. The immobilized portion of the phospholipids has been termed boundary lipids and is interpreted as representing a component which is directly bound to the hydrophobic proteins, forming an intermediary phase between the proteins and the lipid bilayer. 

On the question of boundary lipid, another point of view is that the absence in the high-resolution spectrum of a separate component attributable to immobilized lipid could be due to the difficulty of observing a very wide pattern of long T, (Yeagle, 1982). Systematic attempts to quantitate the amount of missing lipid have yielded reproducible results (Yeagle and Romans, 1981). This missing lipid could in principle be observed by spectrometers with faster data acquisition and the echo technique (to over-... 

To remove intrinsic proteins from a membrane it is necessary to solubilize the molecules using detergents which can break the hydrophobic interactions with lipids. The lipids adjacent to the proteins experience a different environment from the bulk lipid of the membrane and this has led to the concept of boundary lipids. ESR (electron spin-resonance) studies showed that lipids residing at the lipid-protein interface exhibited increased order parameters (restricted motion) in the acyl chain region. Recent experiments, however, paint a different picture. First, the lipid-protein... 

Although the general concept of long-lived boundary lipids surrounding intrinsic proteins does not appear to be viable at present, there are well documented cases where particular lipids are associated with particular proteins. For example, the purple membranes of Halobacterium halobium apart from having a rigidly defined protein structure also contain a unique lipid composition. The lipids appear to be interdigitated between the protein rods and have a composition quite distinct from the rest of the membrane. 

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