Lipid bilayers interaction with proteins

How do proteins and the lipid bilayer interact with each other in membranes ... 

For reasons discussed in detail by Singer (1971) the current concept of membrane structure is that of a lipid bilayer intercalated with proteins, referred to as the fluid mosaic model (Fig. 11). It is based primarily on thermodynamic arguments (Tanford, 1973) concerned with the fact that it is energetically favorable for the ionic portion of an amphipathic substance to be in direct contact with water while its hydrophobic tail is sequestered from water and interacts with other nonpolar molecules. The role of hydrophobic interactions in protein conformation was first emphasized by Kauzman (1959). It is not our purpose here to review the area of membrane structure (see articles by Singer, 1971 Tanford, 1973), but certain points are important to the discussion of effector functions. 

The artificial lipid bilayer is often prepared via the vesicle-fusion method [8]. In the vesicle fusion process, immersing a solid substrate in a vesicle dispersion solution induces adsorption and rupture of the vesicles on the substrate, which yields a planar and continuous lipid bilayer structure (Figure 13.1) [9]. The Langmuir-Blodgett transfer process is also a useful method [10]. These artificial lipid bilayers can support various biomolecules [11-16]. However, we have to take care because some transmembrane proteins incorporated in these artificial lipid bilayers interact directly with the substrate surface due to a lack of sufficient space between the bilayer and the substrate. This alters the native properties of the proteins and prohibits free diffusion in the lipid bilayer [17[. To avoid this undesirable situation, polymer-supported bilayers [7, 18, 19] or tethered bilayers [20, 21] are used. 

The method utilizing ID NMR is simple and eonvenient. Henee the NMR method diseussed here ean be applied to the systematie investigation of the membrane irug inter-aetions, elosely related to the vital function in biomembranes. It is expected that the application can be extended to the lipid-peptide interaction and protein uptake. We are now applying the method to apolipoprotein binding with lipid bilayers and emulsions. Preferential protein binding sites in membranes can be specified by NMR on the molecular level. 

As mentioned earlier, fullerene molecules can destroy the virions, but do not affect living cells. It is possible to suppose that the differences of the structures of virion envelope and cell membrane are the main reason for this phenomenon the outer side of virion envelope is enriched with protein molecules, whereas the outer side of cell membranes is more lipophylic. On the one hand, fullerene molecules can interact with proteins (Belgorodsky et al., 2006), and on the other hand, their penetration into a lipid bilayer does not destroy them (Ikeda et al., 2005 Piotrovsky, 2006). So it is not unlikely that the difference in the structure of outer side is the main driving force of the observed differences in the response of virions and cells in the presence of C60. 

Some workers have suggested that the lauryl chain is of intrinsic biological importance in relation to its ability to disrupt lipid bilayers, having the optimal physical properties of lipophilicity and size, but as C12 compounds are also maximally irritant to the skin (28) where simple lipoidal barrier membranes are probably not involved, other factors are no doubt implicated. Dominguez al. (29) have considered Schott s (26) approach to the biological uniqueness of the dodecyl chain, but have postulated that its properties of skin penetration are related to the conformation of the chain, especially when adsorbed to or interacting with protein. Dominguez e postulate that... 

Line shape analysis of the static 31P NMR spectra and its corresponding CSA values have been successfully used to study the perturbation effect induced by proteins. 31P data for PLs bilayers interacting with antimicrobial peptide (AMP) magainin-2, aurein-3,3, incorporated into structures of supramolecular lipid assemblies such as toroidal pores and thinned bilayers have been reported.90 Various types of PL systems (l-palmitoyl-d3i-2-oleoyl-s -glycero-3-phosphotidylcholine... 

Figure 12.17. Integral and Peripheral Membrane Proteins. Integral membrane proteins (a, b, and c) interact extensively with the hydrocarbon region of the bilayer. Nearly all known integral membrane proteins traverse the lipid bilayer. Peripheral membrane proteins d and e) bind to the surfaces of integral proteins. Some peripheral membrane proteins interact with the polar head groups of the lipids (not shown).

The main stabilizing feature of biological membranes is hydrophobic interactions among the molecules in the lipid bilayer. The phospholipids in the lipid bilayer orient themselves so that their polar head groups interact with water. Proteins in the lipid bilayer interact favorably in their hydrophobic milieu because they typically have hydrophobic amino acid residues on their outer surfaces. 

Lipids are not covalently bound in membranes but rather interact dynamically to form transient arrangements with asymmetry both perpendicular and parallel to the plane of the lipid bilayer. The fluidity, supermolecular-phase propensity, lateral pressure and surface charge of the bilayer matrix is largely determined by the collective properties of the complex mixture of individual lipid species, some of which are shown in Fig. 8.1. Lipids also interact with and bind to proteins in stiochiometric amounts affecting protein structure and function. The broad range of lipid properties coupled with the dynamic organization of lipids in membranes multiplies their functional diversity in modulating the environment and therefore the function of membrane proteins. 

Micelles have been used to build lipid bilayers around membrane proteins captured on sensor surface to mimic their natural environment. A good example of this type of application involves the study of G-protein coupled receptors (GPRCs). Receptor was captured on an LI chip surface using an immobilized antibody that recognizes an additional tag presented on the receptor followed by injection of micelles to form the hpid bilayer [19] (Fig. 12). Others have used this approach to monitor interactions of hg-ands and G-proteins with GPRCs [20]. This approach may provide a general method for studying a variety of membrane-associated systems and even ion channels. 

The mechanism for this action may involve interaction with protein receptors in free nerve endings or induction of changes in membrane permeability through a disruption of the lipid bilayer. 

In parallel, another important (although less direct) technique for measuring forces between macromolecules or lipid bilayers was developed, namely, the osmotic stress method [39-41]. A dispersion of vesicles or macromolecules is equilibrated with a reservoir solution containing water and other small solutes, which can freely exchange with the dispersion phase. The reservoir also contains a polymer that cannot diffuse into the dispersion. The polymer concentration determines the osmotic stress acting on the dispersion. The spacing between the macromolecules or vesicles is measured by X-ray diffraction (XRD). In this way, one obtains pressure-versus-distance curves. The osmotic stress method is used to measure interactions between lipid bilayers, DNA, polysaccharides, proteins, and other macromolecules [36]. It was particularly successful in studying the hydration... 

Interactions of the accumulated lipophilic substances with the cytoplasmatic membranes and especially hydrophobic parts of the cell or cell membranes. Lipid-lipid interactions and interactions between proteins and lipids of the membrane structure (lipid bilayers, membrane-embedded proteins) are discussed. 

Protems can be physisorbed or covalently attached to mica. Another method is to innnobilise and orient them by specific binding to receptor-fiinctionalized planar lipid bilayers supported on the mica sheets [15]. These surfaces are then brought into contact in an aqueous electrolyte solution, while the pH and the ionic strength are varied. Corresponding variations in the force-versus-distance curve allow conclusions about protein confomiation and interaction to be drawn [99]. The local electrostatic potential of protein-covered surfaces can hence be detemiined with an accuracy of 5 mV. 

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