Phospholipids lipid insertion into bilayers

The multilamellar bilayer structures that form spontaneously on adding water to solid- or liquid-phase phospholipids can be dispersed to form vesicular structures called liposomes. These are often employed in studies of bilayer properties and may be combined with membrane proteins to reconstitute functional membrane systems. A valuable technique for studying the properties of proteins inserted into bilayers employs a single bilayer lamella, also termed a black lipid membrane, formed across a small aperture in a thin partition between two aqueous compartments. Because pristine lipid bilayers have very low ion conductivities, the modifications of ion-conducting... 

Because membranes components participate in nearly every cell activity their structures are also dynamic and far from the equilibrium states that are most readily understood in biophysical terms. Newly synthesized bilayer lipids are initially associated with endoplasmic reticulum (Ch.3) whereas phospholipids initially insert into the cytoplasmic leaflet while cholesterol and sphingolipids insert into the luminal endoplasmic reticulum (ER) leaflet. Glycosylation of ceramides occurs as they transit the Golgi compartments, forming cerebrosides and gangliosides in the luminal leaflet. Thus, unlike model systems, the leaflets of ER membranes are asymmetric by virtue of their mode of biosynthesis. 

The precise mechanism how proapoptotic Bcl-2 members regulate the exit of cytochrome c and how phospholipids participate in this process is still controversial. There are two prevailing models (Figure 3) In model 1 Bcl-2 members insert into the lipid bilayer of the mitochondrial outer membrane and form channels that facilitate exit of cytochrome c and other apoptogenic... 

Lipids also show asymmetrical distributions between the inner and outer leaflets of the bilayer. In the erythrocyte plasma membrane, most of the phosphatidylethanolamine and phosphatidylserine are in the inner leaflet, whereas the phosphatidylcholine and sphingomyelin are located mainly in the outer leaflet. A similar asymmetry is seen even in artificial liposomes prepared from mixtures of phospholipids. In liposomes containing a mixture of phosphatidylethanolamine and phosphatidylcholine, phosphatidylethanolamine localizes preferentially in the inner leaflet, and phosphatidylcholine in the outer. For the most part, the asymmetrical distributions of lipids probably reflect packing forces determined by the different curvatures of the inner and outer surfaces of the bilayer. By contrast, the disposition of membrane proteins reflects the mechanism of protein synthesis and insertion into the membrane. We return to this topic in chapter 29. 

However, lipid bilayers are impermeable to ions and most polar molecules, with the exception of water, so they cannot, on their own, confer the multiple dynamic processes which we see in the function of biological membranes. All of this comes from proteins, inserted into the essentially inert backbone of the phospholipid bilayer (Figure 3.27), which mediate the multiple functions which we associate with biological membranes, such as molecular recognition by receptors, transport via pumps and channels, energy transduction, enzymes, and many more. Biomembranes are noncovalent assemblies of proteins and hpids, which can best be described as a fluid matrix, in which lipid (and protein molecules) can diffuse rapidly in the plane of the membrane, but not across it. 

A central and widely used tool for membrane anchoring is the post-translational attachment of hydrophobic residues, such as fatty adds, isoprenoids (see Fig. 3.12) or complex glycolipids (see Fig. 31.15) to spedfic amino add side chains of target proteins. These lipid moieties of lipidated proteins favor membrane assodation by increasing the affinity of the protein to the membrane. Because of their hydrophobic nature, the membrane anchors insert into the phospholipid bilayer and thus mediate... 

The mechanism by which phospholipid inserts into the outer membrane is unclear. Pulse-chase experiments indicate that newly synthesised PE is first located in the inner leaflet of the outer membrane and later rotates ( flip-flops ) through the lipid bilayer to become part of the external lipid leaflet. Attempts to visualise discrete sites of PE insertion into the outer membrane have failed. This is not surprising since the lateral diffusion time... 

Since both the free and the bound forms of the lipoprotein are located exclusively in the outer membrane, there are two possible ways in which a superhelical assembly could interact with the outer membrane (1) The interaction could occur through the three fatty acids attached to the amino-terminal amino acid of the lipoprotein, as suggested by Braun. In this case, the hydrocarbon chains of the fatty acids stick out of the assembly and penetrate into the phospholipid bilayer of the outer membrane. Therefore, the protein part of the assembly protrudes from the inside surface of the outer membrane. This model would predict that the peptidoglycan layer should be at least 76 A apart from the outer membrane, which is not likely. (2) Alternatively, the whole assembled structure, with a height of 76 A, penetrates through the 75-A-thick outer membrane with hydrophobic interaction between the surface of the assembly and the lipid bilayer of the outer membrane. This arrangement is further stabilized by the three hydrocarbon chains at the amino-terminal end of the individual molecules, which could be flipped back over the helix and inserted into the bilayer (Fig. 14). In order to arrange the hydrocarbon chains as shown in Fig. 14, the side chains of two serine residues at the amino terminus are made to face upward, which makes the uppermost part of the assembly hydrophilic, as a part of the surface of the outer membrane. 

Fig. 14. Schematic illustration of the outer membrane structure. A superhelix made of six a-helices is shown to be inserted into the outer membrane and to span the full 75-A-thick membrane. The three hydrocarbon chains attached at the top of each molecule are flipped over, hanging down from the top, and are anchored in the lipid bilayer of the outer membrane. At the bottom (carboxyl-terminal ends of the lipoproteins) of the assembly, two molecules are linked to the peptidoglycan layer, as shown by small bars. The peptidoglycan layer is illustrated by rectangular blocks (for the glycan chains) and small bars (for the peptide portions) which are cross-linking the glycan chains. Phospholipids forming the lipid bilayer are shown by hydrophilic, open, circular heads and hydrophobic, hatched, long tails. Channel opening of 7- and 8-membered assemblies are also illustrated on the surface of the outer membrane.

Insertion of modified receptors into liposomal bilayers. Bilayers of liposomes consist of phospholipid assemblies that hold individual Upid molecules by weak van der Waals forces. Ligands or receptors with surface-active groups can be inserted into liposomal bilayers (Fig. 5c). Proteinaceous receptors are usually nonamphiphilic, and hence they can be modified with alkyl chains or lipids that can be positioned along the hydrophobic part of bilayers. Then the relatively hydrophilic receptor part is exposed to the liposomal surface and can interact with ligands of cell surfaces [37,38]. 

Insertion of PEG-lipid into conventional liposome phospholipid bilayer had substantially increased their circulation half-life. Pharmacokinetic of PEGylated liposome is clearly modified by the presence of the PEG-lipid extended circulation time was reported as reviewed (4). 

Membrane conformational changes are observed on exposure to anesthetics, further supporting the importance of physical interactions that lead to perturbation of membrane macromolecules. For example, exposure of membranes to clinically relevant concentrations of anesthetics causes membranes to expand beyond a critical volume (critical volume hypothesis) associated with normal cellular function. Additionally, membrane structure becomes disorganized, so that the insertion of anesthetic molecules into the lipid membrane causes an increase in the mobility of the fatty acid chains in the phospholipid bilayer (membrane fluidization theory) or prevent the interconversion of membrane lipids from a gel to a liquid form, a process that is assumed necessary for normal neuronal function (lateral phase separation hypothesis). 

In 1985 Tyminski etal. [55, 56] reported that two-component lipid vesicles of a neutral phospholipid, e.g. DOPC, and a neutral polymerizable PC, bis-DenPC (15), formed stable homogeneous bilayer vesicles prior to photopolymerization. After photopolymerization of a homogeneous 1 1 molar lipid mixture, the lipid vesicles were titrated with bovine rhodopsin-octyl glucoside micelles in a manner that maintained the octyl glucoside concentration below the surfactant critical micelle concentration. Consequently there was insufficient surfactant to keep the membrane protein, rhodopsin, soluble in the aqueous buffer. These conditions favor the insertion of transmembrane proteins into lipid bilayers. After addition and incubation, the bilayer vesicles were purified on a... 

By inserting hydrophobic antiobesity compounds into the liposomal bilayers, marine phospholipids would boost the effect of the inserted hydrophobic antiobesity compounds. When marine phospholipids are served as liposomal drinks, they would be more effective than adding into solid foods or feeds. These facts were borne out by Okada et al. (2011). They carried out the following experiment. Brown seaweed (Undaria pinnatifida) lipid containing fucoxanthin (UL) encapsulated into scallop midgut gland phospholipids (PL) liposomes were prepared to see the promotional effect of marine phospholipid liposome on antiobesity. Animal model used in their study was 3-week-old male diabetic-obese... 

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