Apolipoproteins complexes with lipids

The crystal structure of L. migratoria apoLp-III was obtained for the protein in its lipid-free state. The lipid-bound structure of apoLp-III, however, is more interesting since it represents the active form of the protein. To date, no detailed structural reports for exchangeable apolipoproteins in complex with lipid have been reported. The crystal structure of lipid-free apoLp-III demonstrated that the five amphipathic helices orient in such a way that their hydrophobic faces are directed toward each other to form a hydrophobic core while the hydrophilic faces of the helices are exposed to solvent. It has been hypothesized that, upon binding to a... 

Fig. 4. Density gradient ultracentrifugation of apolipoprotein A-IV/phospholipid/cholesterol complexes. Apo A-IV-1 (wild type) and Apo A-IV-2 (variant) /lipid complexes were prepared by the dialysis detergent procedure and subjected to density-gradient ultracentrifugation as outlined in Fig. 2. Note that the variant Apo A-IV-2 peptide forms stable complexes with lipids as the wild type, Apo A-IV-1...

Apolipoproteins ( apo designates the protein in its lipid-free form) combine with lipids to form several classes of lipoprotein particles, spherical complexes with hydrophobic lipids in the core and hydrophilic amino acid side chains at the surface (Fig. 21-39a). Different combinations of lipids and proteins produce particles of different densities, ranging from chylomicrons to high-density lipoproteins. These particles can be separated by ultracentrifugation (Table 21-2) and visualized by electron microscopy (Fig. 21-39b). 

Free fatty acids are transported as complexes with serum albumin. Cholesterol, triacylglycerols, and phospholipids are transported as protein-lipid complexes called lipoproteins. Lipoproteins are spherical, with varying amounts and kinds of proteins at their surfaces. The protein components, of which at least ten exist, are called apolipoproteins. Lipoproteins are classified in terms of their density. 

These proteins are of great interest because of their relationship to coronary heart disease (CHD). Lipoproteins, a group of macromicellar complexes of lipids and proteins, are closely associated with the risk of developing CHD. Structurally, lipoprotein particles contain a nonpolar lipid core of triglycerides and cholesterol esters and a polar surface that is comprised of apolipoproteins and unesterified cholesterol and phospholipids. There are three principle classes of lipoproteins ... 

Cholesterol, which is largely insoluble in aqueous m a, travels through the blood circulation in the form of Upoprotein complexes. The plasma lipoproteins are a family of globular particles that share common structural features. A core of hydrophobic lipid, principally triacylglycerols (triglycerides) and cholesterol esters, is surrounded by a hydrophilic monolayer of phospholipid and protein (the apolipoproteins) [1-3]. Lipid-apolipoprotein interactions, facihtated byi amphi-pathic protein helices that segregate polar from nonpolar surfaces [2,3], provide the mechanism by which cholesterol can circulate in a soluble form. In addition, the apolipoproteins modulate the activities of certain enzymes involved in Upoprotein metabolism and interact with specific cell surface receptors which take up Upopro-teins by receptor-mediated endocytosis. Differences in the Upid and apoUpoprotein compositions of plasma Upoproteins determine their target sites and classification based on buoyant density. 

Although PL liposomes are favored systems for the study of apolipoprotein binding to PL surfaces, vesicle-apolipoprotein complexes are not the ideal models for lipoproteins. Vesicles have an interior water compartment not present in lipoproteins, are incapable of solubilizing large amounts of neutral lipids within the PL bilayer, and are too large to mimic the surface curvature of HDL. Thus, methods have been developed to prepare small, micellar complexes of exchangeable apolipoproteins (in particular apo Al) with lipids that mimic discoidal and spherical HDL in shape, composition, and functional properties. For LDL and VLDL, microemulsions and emulsions of lipids of selected diameter and composition, with added apo B 100, make good models of the native lipoproteins. 

Intermediate-density lipoproteins and LDLs are generated in the circulation by lipolysis of TGs within CM and VLDLs (Chapter 19). Apo B is an essential component of CM, VLDLs, and LDLs. Unlike the exchangeable plasma apolipoproteins (apo E, apo Al, apo A2, and apo C), apo B does not exchange among lipoproteins and is present in plasma only in association with lipid. In addition to apo B, VLDLs and CM contain apo E and apo C CM contain small amounts of apo Al and apo A2. In contrast, apo B is the only apolipoprotein of LDLs. HDLs are particles of diverse composition that are generated in the circulation by complex lipid transport processes (Chapter 19). 

The lipoproteins are macromolecules with varying complexes of lipids where the hydrophobic lipid portions—cholesterol esters and triglycerides—are localized at the core of the molecules. The amphipathic surface layers surrounding the core contain the apolipoproteins and phospholipids. The lipoproteins vary in size, density, lipid composition, and apolipoprotein constituents, and they ean be classihed by size, the flotation rate determined by ultracentrifugation, or their electrophoretic mobilities. Put simply, the density of a lipoprotein particle is determined by the relative amounts of lipid and protein contained in the particle. Chylomicrons and very low density lipoproteins have the highest lipid content and the lowest protein content thus, very excessive amounts of chylomicrons float on the surface of plasma. In descending order of size, the broad lipoprotein fractions (with their electrophoretic mobility) are... 

These results show two distinct features of lipophorin biosynthesis during the larval stage. First, the nascent lipophorin produced in the fat body by de novo synthesis is an apolipoprotein-phospholipid complex that derives its transported lipids from the midgut. Second, lipophorin biosynthesis is not coupled to fat intake, as is the case with vertebrates. These processes are illustrated in Fig. 6 and fit observations made on lipid storage in larvae. Thus, it has been shown that more than 70% of the fatty acids in the diet are stored as TG in the larval fat body (Tsuchida and Wells, 1988). Although the fat body can convert carbohydrates to fatty... 

Lipoproteins are soluble complexes of proteins (apolipoproteins) and lipids that transport lipids in the circulation of all vertebrates and even insects. Lipoproteins are synthesized in the liver and the intestines, arise from metabolic changes of precursor lipoproteins, or are assembled at the cell membranes from cellular lipids and exogenous lipoproteins or apolipoproteins. In the circulation, lipoproteins are highly dynamic. They undergo enzymatic reactions of their lipid components, facilitated and spontaneous lipid transfers, transfers of soluble apolipoproteins, and conformational changes of the apolipoproteins in response to the compositional changes. Finally, lipoproteins are taken up and catabolized in the liver, kidney, and peripheral tissues via receptor-mediated endocytosis and other mechanisms. This chapter deals with the composition and structure of human lipoproteins. 

Several methods are known for the reconstitution of HDL-like complexes from pure components (i) spontaneous formation of HDL discs from dimyristoylphosphatidyl-choline liposomes (ii) detergent-mediated reconstitution of HDL discs with various PL and (iii) co-sonication of apolipoproteins and lipids to form either discoidal or spherical HDL analogs [19]. 

Some portions of the apolipoprotein molecules are nonpolar (hydro-phobic), and these are usually oriented toward the inside (near the lipid portion) of the complex. Polar amino acid side chains in the protein portions are oriented toward the outside of the complex, where they associate with the aqueous environment, rendering the complex soluble in blood plasma. This type of structure resembles that of micelles. 

A contribution to the understanding of lipoprotein metabolism would be the definition of the metabolic role of the lysolecithin-rich d 1.21 lipoprotein. This protein-phospholipid complex may simply represent a degradation product of the high-density lipoprotein after removal of the other lipid moieties. On the other hand, it could be considered as a newly synthesized product from the liver on its way to further lipidation to form a more complete class of lipoproteins. Its possible identity with the apolipoprotein described by Roheim and Eder (1961 Roheim et al., 1964) remains to be established. 

Much study has been devoted to the problem of lipoprotein analysis in blood plasma using H NMR spectroscopy. This has been comprehensively reviewed recently by Ala-Korpela. Lipoproteins are complex particles that transport molecules normally insoluble in water. They are spherical with a core region of triglyceride and cholesterol ester lipids surrounded by phospholipids in which are embedded various proteins known as apolipoproteins. In addition, free cholesterol is found in both the core and surface regions. The lipoproteins are in a dynamic... 

Because of their relatively large hydrophobic surface area, apolipoproteins, in the absence of lipids, readily self-associate in aqueous solution (Stone and Reynolds, 1975 Vitello and Scanu, 1976). The rate of desorption of apolipoproteins from lipoprotein surfaces has not been studied systematically. Extensive studies of the reverse process, which is the assembly of lipid apoprotein complexes, have been conducted in considerable detail. The dynamic of lipid-protein interactions have been studied primarily with in vitro model systems. Analysis of the association of apolipoproteins with various phospholipid aggregates have provided important clues about the nature of the kinetically important steps in the transfer of apolipoproteins between lipoproteins (Pownall et al., 1977 1978a Massey et al., 1981a Mantulin et al., 1981). 

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