Non bilayer phases

Under appropriafe condihons some aqueous phospholipids can exisf in non-bilayer phases, a facf fhaf may be of considerable biological imporfance. In the presence of Ca + some pure phospholipids can be converfed to fhe inverted hexagonal or Hjj phase (Fig. phase fhe phospholipid... 

All of these events can be ascribed to the release of constraints imposed on non-bilayer lipids to maintain them in a bilayer phase required to achieve a stable membrane structure. Exposure to high temperatures serves to phase separate the non-bilayer lipids into discrete domains that appear to be devoid of membrane proteins. It is likely also that the permeability barrier properties are altered by this type of phase separation. In general, where gross non-bilayer phase separations have taken place it is difficult to imagine how the normal distribution of membrane components can be achieved and especially at a rate that would be consistent with the continued survival of the cell. 

The central role of the phospholipid bilayer in biological membrane organization was outlined in the previous chapter. Bilayers composed of single defined phospholipids provide an excellent model system for in depth study of lipid membrane properties. In the present chapter the structure and thermodynamics of phospholipid bilayers, as determined by a variety of different physical techniques, will be reviewed. Particular attention will be paid to the or-dered-fluid lipid phase transition and its use in studying bilayer properties. The effects of hydration, surface electrostatics and ion interactions will be considered. Discussion is also given of the transformation to non-bilayer phases. 

Phospholipid(s) 379, 380,382 - 387, 392. See also Specific substances bilayer diagram 391 head groups, functions of 396 inverted hexagonal phase 397 31P NMR 397 non-bilayer structures 397 Phosphomannomutase 654 Phosphomutases 526 Phosphonamidate 626s... 

As stated, biological membranes are normally arranged as bilayers. It has, however, been observed that some lipid components of biological membranes spontaneously form non-lamellar phases, including the inverted hexagonal form (Figure 1.9) and cubic phases [101]. The tendency to form such non-lamellar phases is influenced by the type of phospholipid as well as by inserted proteins and peptides. An example of this is the formation of non-lamellar inverted phases by the polypeptide antibiotic Nisin in unsaturated phosphatidylethanolamines [102]. Non-lamellar inverted phase formation can affect the stability of membranes, pore formation, and fusion processes. So-called lipid polymorphism and protein-lipid interactions have been discussed in detail by Epand [103]. 

The second part of the model assumes that an influence on the functioning of the membrane-integrated proteins is possible even without a specific drag-protein interaction. This is understandable if one considers another parameter that characterizes cells or vesicles, namely their curvature. This is a measure of internal stress and depends on the tendency of bilayers to assume a non-bilayer conformation, for instance a hexagonal or cubic phase (see Section 1.1.3.1 and Figure 1.9). This transformation can, for example, be detected and measured by X-ray or neutron diffraction techniques. 

According to electron micrographic evidence (principally comparison with bicontinuous cubic phases), phases are an accurate representation of so-called "non-bilayer" conformations of membranes. It must be stressed, however, that this conformation is a true bilayer. 

Phospholipase C activity is not directly influenced by the formation of non-bilayer structures. However, the presence of lipids (e.g., PE) with a tendency to form such structures stimulates the enzyme even under conditions at which purely bilayer phases exist. Conversely, sphingomyelin, a stabilizer of the bilayer phase, inhibits the enzyme. Thus, phospholipase C appears to be regulated by the overall geometry and composition of the bilayer (M.B. Ruiz-Arguello, 1998) supporting the hypothesis that the collective physical properties of the lipid bilayer can modulate the activities of membrane-associated proteins. 

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