Lipid-Protein Complexes

Studies related to the interaction of hydroperoxides with proteins have shown that, in the absence of oxygen, linoleic acid 13-hydroperoxide reacts with N-acetylcysteine, yielding an adduct of which one isomer is shown  

However, in the presence of oxygen, covalently bound amino acid-fatty acid adduct formation is significantly suppressed instead, the oxidized fatty acids listed in Table 3.35 are formed. 

The difference in reaction products is explained in the reaction scheme shown in Fig. 3.33 which gives an insight into the different reaction pathways. The thiyl radical, derived from cysteine by abstraction of an H-atom, is added to the epoxyallyllic radical only in the absence of oxygen (pathway 2 in Fig. 3.33). In the presence 

Compound Threshold value for bitter taste (mmol/1) 

13-Hydroperoxy-cis-9,trans-11 -octa-decadienoic aeid not bitter  

---

Langmuir-Blodgett (LB) films of proteins (Tiede 1985, Hwang et al. 1977, Furuno et al. 1988, Lvov et al. 1991) and lipid-protein complexes (Fromherz 1971, Phillips et al. 1975, Mecke et al. 1987, Kozarac et al. 1988) were intensively studied and characterized by different techniques. 

Uittenbogaard, A, Everson, WV, Matveev, SV, and Smart, EJ, 2002. Cholesteryl ester is transported from caveolae to internal membranes as part of a caveolin-annexin II lipid-protein complex. J Biol Chem 277,4925—4-931. 

A. Dietary fats are packaged by the enterocytes into chylomicrons, a very large type of lipid-protein complex or lipoprotein, for export to other organs. 

Prior to phospholipid analysis, it is imperative to extract the lipids from their matrix and free them of any nonlipid contaminants. Phospholipids are generally contained within the lipid fraction, which may be recovered by the traditional Bligh and Dyer or Folch extraction procedure (9,22). In any phospholipid extraction method it is recommended to include a rather polar solvent in addition to a solvent with high solubility for lipids. The former is needed to break down lipid-protein complexes that prevent the extraction of the lipids in the organic phase. Traditionally, mixtures of chloroform and methanol (especially 2 1, v/v) have been recommended. These are washed with water or aqueous saline to remove nonlipid contaminants. Comparing the recovery of phospholipids, Shaikh found that the neutral phospholipids PC, PE, SPH as well as DPG were nearly quantitatively extracted by all solvent systems studied (Table 1), although Bligh and Dyer, in which the lower phase was removed only once, was somewhat worse (23). 

During the 1960s, various alternatives to the Davson-Danielli model were proposed. Some investigators abandoned the idea of a phospholipid bilayer and suggested instead that membranes consist of aggregates of lipid-protein complexes. However, in 1972, Jon Singer and Garth Nicol-... 

Observations were made of lipid-protein phases in which the structure is determined mainly by the protein. Raman spectroscopy is a useful method for structure analysis of such phases. The structures described above were analyzed successfully by an x-ray diffraction technique. Lipid-protein complexes, however, are often amorphous, and alternative methods to study their structures are therefore needed. It was demonstrated that Raman spectroscopy can be used to obtain structural information about lipid-protein interaction (16, 17). It is thus possible to determine the conformation as well as the type of environment of the lipid molecules. With the protein, interpretation is more complicated. It is usually possible to determine whether the complex has the same protein conformation as the component used in the preparation, or, if a change occurs, it may be possible to correlate it with denaturation of the pure protein. For complexes formed by long-chain alkyl phosphates and insu-... 

Biological Implications of Structural and Electrical Properties of Lipids. It is rather obvious that the structure of lipids is very important in connection with the function of living cells since most physiological processes occur in lipid environment. There is, for example, evidence that lipid-protein complexes are necessary for the proper functioning of mitochondria (56). Although lipids are most important in providing a suitable material for functional complexes (ionic channels, electron transport systems, receptor units, etc.), their own physical properties are certainly... 

Krause in the 1930 s proposed that the chromophore of rhodopsin was similar chemically to the polymethine compounds known as cyanine dyes which are also used to sensitize photographic emulsions to the longer wavelengths of the visible spectrum. He proposed that the chromophore was bound to a very large lipid/protein complex with a molecular weight near 800,000. 

Lipids are a complex group of substances, which include the long-chain fatty acids and their derivatives, sterols and steroids, carotenoids, and other related isoprenoids. It is evident that the term lipid denotes a wide range of compounds that appear to have little obvious interrelation. However, although these compounds possess widely different structures, they are derived in part from similar biological precursors and exhibit similar physical and chemical characteristics. Furthermore, most lipids occur naturally in close association with protein, either in membranes as insoluble lipid-protein complexes or as soluble lipoproteins of the plasma. 

While there is no doubt that the major attractive forces maintaining lipids in membranes are noncovalent in nature, there is excellent evidence in the literature showing that a small percentage of the membrane lipids is covalently linked to membrane proteins. These lipids are highly specific in nature and will be discussed briefly below. Normally these lipid protein complexes are not found in the organic solvent phase of a typical (lipid) extraction procedure. Rather they would be found in the water-rich phase of such an extraction approach. Basically there are four specific classes of lipid covalent binding to protein ... 

Blood plasma contains a number of soluble lipoproteins, which are classified, according to their densities, into four major types. These lipid-protein complexes function as a lipid transport system. Isolated lipids are insoluble in blood, but they are rendered soluble, and therefore transportable, by combination with specific proteins, the so-called lipoproteins. There are four basic types in human blood (1) chylomicrons, (2) very low density lipoproteins (VLDL), (3) low-density lipoproteins (LDL). and (4) high-density lipoproteins (HDL). Their properties are summarized in Table 6.2. 

While a temperature-dependent IR spectrum allows one to examine specific elements of a transition, a DSC thermogram enables the visualization of transitions in their entirety and the calculation of associated thermodynamic parameters. The IR and DSC thermal profiles for identically treated samples of hydrated porcine SC are shown in Fig. 3. The results of a series of thermograms for intact, delipidized, fractionated, and reheated SC as well as extracted lipids suggest that these three major transitions near 60,70, and 95°C in intact SC are due to intercellular lipid, a lipid-protein complex associated with the comeocyte membrane, and intracellular keratin, respectively. Evidence supporting these deductions is elegantly presented by Golden et al. [33]. More recently, the presence of a subzero lipid transition at -9°C has also been reported [34]. 

Lau et al. [188] have shown that calcium ions are linked to phospholipids. The binding of calcium with hpids inhibits the formation of lipid/protein complexes. The decrease in mineral salts, particularly in magnesium and calcium ions, during processes 2 and 3 promotes the lipid/protein complexes formation. This phenomenon was confirmed by higher precipitation levels for lipids and proteins in process 3 in comparison with process 1 (Table 21.11). Equation 21.1 also means that an increase of components should improve the lipid/protein complexes formation. This increase of may be obtained by a concentration step of whey solutions by ultrafiltration. 

One important experimental result was available, the quantitative measurement of the fraction of each secondary structural element by circular dichroism (CD) on purified lipid-protein complexes. This provided a constraint that allowed a careful evaluation of the secondary structure predictions derived from the various approaches, some of which were developed for water-soluble proteins and therefore of uncertain reliability for proteins in a lipid environment. The data from these analyses were combined using an integrated prediction method to arrive at a consensus secondary structure model for each protein. The integrated method involved 36 steps, with independent predictions at each step. The final model was based on an evaluation of the various predictions, with judicious intervention by the authors. As an aid to developing the appropriate weighting of all the data, they carried out the analysis for apoE-3 without reference to the available crystal structure (Wilson et al., 1991), then used the known structure of the HDL-binding amino-terminal domain of apoE-3 as feedback to reevaluate the weighting. 

Match each description below with the appropriate lipid-protein complex. 

Schmitz G, Muller G (1991) Structure and function of lamellar bodies, lipid-protein complexes involved in storage and secretion of cellular lipids. J Lipid Res 32 1539-1570... 

Receptor-mediated endocytosis plays a key role in cholesterol metabolism (p. 745). Some cholesterol in the blood is in the form of a lipid—protein complex called low-density lipoprotein (LDL). Low-density lipoprotein... 

Lipid oxidation in muscle foods is one of the major deteriorative reactions causing losses in quality during processing and storage. The oxidation of unsaturated fatty acids leads to formation of free radicals and hydroperoxide. These intermediary compounds are unstable and cause the oxidation of pigments, flavors, and vitamins. Oxidized unsaturated lipids bind to protein and form insoluble lipid-protein complexes. This accounts for toughened texture and poor flavor of frozen seafoods (Khayat and Schwell, 1983). 

Hoppe, G. et al. Accumulation of oxidized lipid-protein complexes alters phagosome maturation in retinal pigment epithelium. Cell Mol. Life Sci. 61 (2004b) 1664-74. 

A variety of lipid-protein complexes are used in the body to transport relatively water-insoluble lipids, such as triglycerides and cholesterol, in circulating blood. These complexes are commonly called lipoproteins they contain both proteins and lipids in varying concentrations. The density of these lipoproteins depends on the relative amounts of protein, because lipids are less dense than protein. Low density lipoproteins, or LDLs, have a relatively higher ratio of lipid to protein. LDLs are used to transport cholesterol and triglycerides from the liver to the tissues. In contrast, high density... 

Lipids are nonpolar molecules and are relatively insoluble in aqueous solutions. At low concentrations, cholesterol and cholesterol esters, as well as other lipids, may form microscopic droplets called chylomicrons (lipid-protein complexes) that are somewhat stable in solution. At high concentrations, the lipids would form larger droplets and clog blood vessels, so they must be transported as complexes of lipid and protein called lipoproteins. Lipoproteins are complexes of lipid and precursor protein molecules called apolipoproteins. 

---