Non-bilayer-forming lipids

Holland JW, Cullis PR, Madden TD. Poly(ethyleneglycol)-lipid conjugates promote bilayer formation in mixtures of non-bilayer-forming lipids. Biochemistry... 

Non-bilayer-forming lipids are also required for protein translocation across the membrane of E. coli. The only non-bilayer-forming lipid in E. coli mutants lacking PE is CL. Protein translocation into inverted membrane vesicles prepared from PE-lacking cells (now enriched in CL) is reduced with divalent cation-depletion but can be enhanced by inclusion of Mg or Ca [ 1 ]. Protein translocation in the absence of divalent cations was restored by incorporation of non-bilayer PE (18 1 acyl chains) but not by bilayer-prone PE (14 0 acyl chains). These results indicate that lipids with a tendency to form non-bilayer stmctures provide a necessary environment for translocation of proteins across the membrane. 

On the other hand, the neutral non-bilayer-forming lipid MGlcDG can substitute for PE in cell division (M. Wikstrbm, 2004), suggesting that common properties of these two lipids, such as the ability to form non-bilayer structures and dilution of negative charge, are important for cell division [1]. 

Phase separation of non-bilayer forming lipids could be responsible for initiating the modification in the thylakoid membrane structure... 

The phase structure of total polar lipid extracts of chloroplast thylakoid membranes when dispersed in physiological salt solutions is characterised by non-lamellar arrangements of lipid C4D. This contrasts markedly with the general belief that the dominant form of the lipids in the photosynthetic membrane is in a bilayer configuration. The conclusion that must be drawn from this is that the interation of the different polar lipids with the other membrane components imposes a bilayer structure and prevents phase separation of non-bilayer forming lipids into separate domains within the membrane. 

The above discussion indicates that a knowledge of the phase behaviour of individual molecular species of lipid comprising the matrix of the thylakoid membrane of higher plant chloroplasts can be informative of the factors governing the stability of the membrane. Perhaps the most important conclusion is that the presence of non-bilayer forming lipids is not simply required to facilitate the dynamic functions such as membrane fusion etc., but also to play a role in the creation of oligomeric functional complexes of the different membrane proteins. 

DGDG + SQDG) as well as phospholipids (PG + PC) are equally distributed in both monolayers but MGDG, a non-bilayer forming lipid, is by far the most predominant class in the outer leaflet. Its role remains... 

The release profiles suggest that the lysoPPC and PA hydrolysis products, which are formed in a 1 1 molar ratio by SPLA2, are incorporated into the target membranes [51], leading to the increase in the permeability of the target liposomal membranes. These hydrolysis products, due to their non-bilayer forming molecular shapes, induce a curvature stress [52, 53] and/or form small-scale lipid domains [33, 36], which lead to membrane defects and consequently increased membrane... 

Important factors when considering the enhanced hydrolysis at interfaces are the substrate environment in the monolayer and the need to transfer a substrate molecule from this monolayer to the active site. Interfacial disorder may provide an important parameter that facilitates such transfer of substrate to the active site. Phospholipase activity is enhanced under conditions that affect phospholipid fluidity, packing density of the phospholipids, and polymorphism of the aggregate. A highly ordered structure seen with phosphatidylcholine either above or below the transition temperature tends to give low rates of hydrolysis. Discontinuities in such ordered structures occur at temperatures close to the transition temperatures and the presence of other lipids such as anionic lipids or non-bilayer-forming phospholipids promote catalysis by perturbing the interface. 

Dmg transport during the pharmacokinetic phase represents a compromise between the increased solubility of the ionized form of a drug and the increased ability of the non-ionized form to penetrate the lipid bilayer of cell membranes. A drug must cross many lipid barriers as it travels to the receptor that is its site of action. Yet cell membranes... 

Phospholipids are the major lipid building blocks most membranes and their molecules comprise of a hydrophobic (acyl chain) and a hydrophilic (polar) head group. The relative size of the hydrophobic tails and hydrophilic head of the molecule characterizes the molecular shape and determines the structure of the molecular assemblies in contact with w ater. Molecules with polar and non-polar regions (PC, PS, PI, Sphm) of equal size have a cylindrical shape and form lipid bilayers. Molecules that have a larger non-polar region are cone-shaped (PE, PA, Choi, Car), and form reversed micelles, in contact to water. When the polar region is larger (lysophospholipids) the molecule assembles an inverted cone and form micelles. Fig. (8). 

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