Interaction between lipids and proteins

The basis for the interaction between lipids and proteins is related to their amphiphilic nature and is due to their influence on the water structure, the so-called hydrophobic effect (Tanford, 1980). In general terms four alternative types of phases can occur in lipid-protein-water systems. An aqueous lipid solution can co-exist with a protein in the same solution alternatively a solution of molecular lipid-protein complexes can be formed. It is also possible that the lipid forms a liquid-crystalline phase (or an L2-phase) with water, such a phase can either solubilize a protein or co-exist with a protein solution. The first two interaction alternatives in water solution have been thoroughly discussed (Tanford, 1980). When the lipid concentration is below the CMC there is no interaction beside the eventual association of a few lipid molecules to the protein at certain high-affinity binding sites. At lipid concentrations above the CMC there is a so-called mass co-operative binding of numerous lipid molecules to each protein molecule and this leads to unfolding of the protein structure. 

Liquid-crystalline lipid-water phases can under certain conditions incorporate proteins. On the basis of studies of liquid-crystalline phases in various lipid-protein-water systems (Gulik-Krzywicki etal.y 1969 Rand and SenGupta, 1972), it was concluded that a prerequisite for the formation of composite liquid-crystalline phases is that the lipid is charged (Rand, 1976). Recently, however, a system with a neutral lipid was examined which appeared to form a lipid- 

A specific type of interaction between lipids and proteins is found in lipoproteins which transport triglycerides and cholesteryl esters in the plasma of mammalians. The largest lipoproteins, chylomicrons with a diameter between 800 A and 5000 A, and very-low-density lipoproteins (VLDL), with a diameter of 300-800 A, resemble emulsion droplets with a core of non-polar lipid and a surface coat of phospholipids and proteins (cf. Brown et ai, 1981). A physical characterization of chylomicrons has been reported (Parks et al.y 1981). Most of the plasma cholesterol occurs in low-density lipoprotein (LDL) which is a particle with a diameter of 200 A. The core consists of almost pure cholesteryl esters and a surface coat of a phospholipid monolayer and four tetrahedrally arranged apoproteins (Gulik-Krzywicki et aly 1979). The smallest particle, high-density lipoprotein (HDL), is a kind of molecular lipid-protein complex. 

The structures of egg yolk low-density lipoproteins have also been examined (Kamat et al.y 1972). These lipoproteins differ from plasma low-density lipoproteins mainly in the compositon of the non-polar lipids. Thus there is only about 0.8% of the particle weight of cholesteryl esters, whereas there are about 60% triglycerides. It follows that the particles with a diameter of about 300 A consist of a liquid core of neutral lipids covered by phospholipids and proteins. 

The most important condensed phase, formed by specific lipid-protein interaction, is the cell membrane discussed in the last paragraph. 

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A model that is consistent with these observations of the action of trypsin and phospholipase A and with the discontinuities in the All-composition curves (Figures 2 and 3) is one in which the lipid monolayer is not a continuous palisade of uniformly oriented lipid molecules but rather an assembly of surface micelles. In this model, proposed by Colacicco (4, 5), the protein first comes into contact with the lipid molecules at the periphery of the surface micelles and then inserts itself as a unit between them. This is the basis for the generalized nonspecific interaction between lipids and proteins which results in increase of surface pressure. One may thus explain the identical All values obtained with films of lecithin and 80 mole % lactoside by picturing the lecithin molecules outside and the lactoside molecules inside the surface micelles. In this model lecithin prevents the bound lactoside from interacting nonspecifically with globulin and produces the same increase in pressure as with a film of pure lecithin. In the mixed micelle the lactose moiety of the lactoside protrudes into the aqueous subphase. Contact of the protein with these or other nonperipheral regions of the surface micelle would not increase the surface pressure. 

Interactions Between Lipid and Protein Components in Supported Bilayers... 

The hydrophobic effect is a driving force in the formation of clathrate hydrates and the self-assembly of lipid bilayers. Hydrophobic interactions between lipids and proteins are the most important determinants of biological membrane structure. The three-dimensional folding pattern of proteins is also determined by hydrophobic interactions between nonpolar side chains of amino acid residues. 

The NMR and infrared data of Jenkinson et al (1969) on myelin and erythrocyte membrane suggests that there are differences in lipid-chain organization and interaction between lipid and protein in these two membrane systems. 

The enzyme is inactivated by detergent or phospholipase and reduction in activity correlates with the amount of lipids removed. Moreover, activity can be restored by adding lipids to the delipidized enzyme. The chemical composition and the molecular interaction between lipid and protein in vivo are unknown. 

Coulombic attractions are repressed at extreme pH. Because the weakening of other interactions is involved in dissociations, specific contributions of ionic attractions to membrane structure cannot be estimated from these data. Neither do these experiments give specific evidence from which the existence of important hydrophobic interactions between lipids and protein can be postulated. 

The structure of the lipid portion of membranes is potentially subject to regulation via variation of a number of independent parameters not related to interactions between lipid and protein phases. These parameters are the hydrocarbon chain-lengths of fatty acids and their degree of unsaturation, the nature of phospholipid headgroups, the cholesterol content of the membrane, and the presence or absence of certain ions. Further, it is becoming clear that subtle changes in the physical properties of membrane lipids correlate with changes in the behavior of membrane proteins. 

The interaction between lipids and partially folded LacY during refolding was found to be structurally specific. Both the chemical properties of the individual lipid molecules and the collective properties of phospholipid mixtures were determinants supporting proper protein folding (Bogdanov et al.,... 

Figure 3.4 Fluid mosaic model of a biological membrane. This model represents a biological membrane as a sea of lipids with a mosaic of associated proteins either floating on the surface or embedded within a fluid bilayer of lipids. This model is sufficient to describe many phenomena associated with membranes. It has been modified more recently to include the concept of membrane domains constrained over different timescales by interactions among lipids, between lipids and proteins, and between membrane proteins and the cytoskeletal network. (Modified from a public-domain image created by Mariana Ruiz Villarreal.)...

The rancid lipid odor profile is made up of a mixture of several volatile compounds. Among them, the trans, cw-alkadienals, and vinyl ketones have the lowest flavor threshold in oils, while the threshold of hydrocarbons (alkanes and alkenes) is the highest (Min, 1998). The sensory effects depend on the composition of the participating compounds and on the composition of the food matrix, while the rancid off-odors and off-flavors of foods emanate from the interactions between lipids and other components, especially proteins. 

Winter and co-workers elucidated the interaction between lipidated Ras protein and membrane and investigated the distributiOTi of Ras proteins in membrane microenvironments using two-photon fluorescence microscopy on giant unilamellar vesicles (GUVs) and tapping mode atomic force microscopy (AFM)... 

Lipids and phospholipids The study of phospholipid monolayers adsorbed on a mercury electrode and the interaction between phospholipids and proteins has been an active research topic for a number of years. The reason for this is obvious when one considers the currently accepted fluid mosaic model of the bilayer lipid membrane (BLM). In addition to its role as a structural element in cells, etc., the BLM is also important in some foods. Since there is enough phospholipid in milk to form a film on a greatly expanded oil-water interface, this lipid undoubtedly plays an important role in stabilizing dairy and other food products that utilize homogenized milk [123]. 

The presence of surfactants in drug formulations may produce unwanted side or toxic effects because of their interaction with proteins, lipids, membranes and enzymes. To fully understand these interactions, it is essential to have information on the metabolic fate of the ingested surfactant. Membrane disruption by surfactants involves binding of the surfactant monomers to the membrane components, followed by the formation of co-micelles of the surfactant with segments of the membrane. The interaction between surfactants and proteins can lead to solubilization of the insoluble-bound protein or to changes in the biological activity of enzyme... 

In the present study, the equilibrium surface tension reduction of adsorbed films from mixtures of proteins and lipids possessing different interactions in bulk solution are reported. The systems investigated are bovine serum albumin (BSA) and ovalbumin (OA) in sodium dodecylsulphate (SDS) and OA in 1-monocaproin. The interaction between SDS and proteins are well documented in literature and reviewed in reference [8]. SDS is known to bind to most proteins in a nonspecific, co-operative manner, thereby gen-... 

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