Lipoproteins surface

The general structure of lipoproteins is shown schematically in Figure 3. The core of the lipoprotein contains the more hydrophobic lipids namely cholesterol ester (CE) and triglyceride (TG) and is surrounded by a surface monolayer consisting of the more polar phospholipid (PL) and free cholesterol (FC). Apoproteins are associated with the lipoprotein surface. The proportional composition of human plasma lipoproteins is given in Table 7. 

Barter et al. (B9) have calculated the apparent Km for the esterification of HDL3 and LDL cholesterol. The for free cholesterol associated with HDL3 is seven times less than for free cholesterol associated with LDL. However, if these calculations are expressed in terms of lipoprotein surface area, instead of free cholesterol concentration, the Km for LDL and HDL as LCAT substrates may well be comparable. 

D3g-dodecylphosphocholine (DPC) micelles were used to provide a lipid environment. This molecule, which possesses a phosphocholine head group and a single Ci2 hydrocarbon chain, mimics the phospholipid component of lipoprotein surface monolayers. About 40 DPC molecules form a micelle which has molecular weight of about 16 kDa. Thus, the apoLp-IH/DPC (1 1 protein/micelle) complex... 

For simplicity of calculation, the core was assumed to contain all of the triglyceride and cholesteryl ester, although it is known that small amounts of the core lipids are dissolved in the surface monolayer, where they represent about 3 mol% of the surface lipids, and a larger fraction, about one ninth of the cholesterol, is dissolved in the core (Miller and Small, 1987). The presence of core lipids in the lipoprotein surface is very important metabolically, for the lipases and transfer proteins have access to these core lipids without having to penetrate the surface monolayer. For the calculation of composition, density, and size, however, the effects of component transfer between surface and core affect these quantities about one part in the fourth significant figure, and have been neglected in Table II. 

IV. Structural Studies of Apolipoprotein B on Low-Density Lipoprotein Surfaces... 

Electron microscopy of negative-stained native LDL samples does not resolve the distribution of apoB 100 on the lipoprotein surface. However, in electron micrographs of LDL-antibody complexes, antibodies, recognized as small, Y-shaped objects, have been observed protruding from the LDL particles (Chatterton et ai, 1991). Therefore, monoclonal antibodies directed against apoB can be used to locate specific regions of apoB on the LDL surface. Because each LDL contains only a single copy... 

Another independent approach to the mapping of apoB on the lipoprotein surface arose unexpectedly during studies on the mechanism of lipoprotein assembly in hepatocyte cell lines, and this will be described next. 

In humans, apoA-IV is found primarily in the free protein (nonlipoprotein) portion of plasma. Although the reason is not clear, it is possible that the lack of class A motif in the amphipathic helical domains of human apoA-IV causes it to associate poorly with the lipoprotein surface. In rats, however, apoA-IV is seen on HDLs. Examination of individual amphipathic helical domains of rat apoA-IV does show the presence of the class A motif in its structure, thus supporting our hypothesis that the class A motif is essential for binding of apolipoproteins to lipoproteins. 

Fig. 4. A model showing the proposed unfolding of apoLp-IIl on a lipoprotein surface, showing only the a-carbon backbone of the protein. It is proposed that the protein binds to the surface via one end and that helices 3 and 4, and helices 1,2, and 5, then move relative to each other (filled arrows) around hinges in the loops connecting helices 4 and 5, and 2 and 3 (unfilled arrows). The hydrophobic side chains, which are in the interior of the protein in the folded state, face the lipoprotein surface in the unfolded state.

Studies of the physical properties of UC, reviewed in Chapter 6 of this volume, have contributed much to our understanding of the role of this Upid in membranes and lipoprotein surfaces. The shape and polarity of UC promote its association with the phosphoUpids of membranes and Upoproteins, and this association has important effects on membrane fluidity and permeability. The physical properties of long-chain fatty acid esters of cholesterol, on the other hand, differ strikingly from those of UC, and cause these esters to be largely excluded from phospholipid bilayers and monolayers and to aggregate instead in oil droplets. 

The cholesterol esterification reaction is thought to occur primarily on the surface of high-density lipoproteins (HDL) where both of the lipid substrates and one or more apohpoprotein activators are located (Fig. 2A). The enzyme presumably binds transiently to the same surface, and catalyzes the transfer of a fatty acyl group mainly from the C-2 position of PC to the S S-hydroxyl group of UC. Because linoleic acid, arachidonic acid, and oleic acid usually predominate in the C-2 position, the CE formed are rich in these fatty acids. Once formed, the CE for the most part leave the lipoprotein surface. They partition into the core of the HDL or are transferred to other lipoproteins by the plasma CETP (see later). Meanwhile, the lysoPC formed by the LCAT reaction equilibrates with other lipoproteins and particularly with albumin. These changes in distribution of CE and lysoPC presumably account for the fact that the reaction is essentially irreversible. 

The formation of plasma lipoprotein CE begins in cells of the intestinal mucosa and liver (Fig. 2B). ACAT activity in these cells leads to the formation of cholesteryl palmitate and cholesteryl oleate, which can be included in nascent chylomicrons, VLDL, and apparently also small spherical HDL [15,80-82], These esters presumably accumulate when more than enough UC is present in the cells to provide for the requirements of cell membranes and lipoprotein surfaces. (For a more detailed discussion of ACAT activity in intestinal epithelial cells, see Chapter 5.)... 

The situation changes rapidly once lipoprotein lipase begins to attack the TG in the lipoprotein core. Enzyme-mediated hydrolysis of the TG leads to a marked decrease in particle volume, whereupon much of the lipoprotein surface becomes superfluous [85]. As a result, UC, PC, and apo C dissociate from the particle and become available for interaction with HDL [86]. Action of LCAT on the HDL then leads to the formation of CE. Some of this CE remains with the HDL, but most of it is transferred by CETP to apo B-containing lipoproteins and perhaps to cells (Fig. 2A). 

Trace amounts of many other proteins are recovered in purified fractions of plasma lipoproteins. Some, like albumin, have plausible functions both at the lipoprotein surface [binding unesterified fatty acids newly generated from lipoprotein triacylglycerol (TG)] and in free solution to ensure osmotic balance between cells and their surroundings. Blood-clotting proteins are present in low amounts in TG-rich lipoproteins, but it is not clear that they play a unique biological role in lipoproteins. Many lipoprotein-associated proteins have no known function in lipid transport. 

Apo Al, like apo B, is secreted mainly from liver and intestinal cells. Unlike apo B, however, apo Al is secreted in lipid-free or lipid-poor from. The cholesterol and phospholipids that are transferred to HDLs move down concentration gradients driven by the plasma enzyme lecithinxholesterol acyltransferase (LCAT). LCAT, which is bound to HDL, converts cholesterol and phosphatidylcholine to insoluble CE and lysophos-phatidylcholine (Section 3.4), which is soluble and is transferred to albumin in the plasma. Cholesterol has a small but significant solubility and, as a result, can be transferred spontaneously from cell and lipoprotein surfaces to apo Al. Cholesterol may also be transferred as a result of molecular collision between lipoprotein particles. The LCAT reaction consumes equal amounts of cholesterol and phospholipids, but the rate at which phospholipids are transferred spontaneously between cells and lipoproteins is much lower... 

Concanavalin A Heparin sepharose Hydroxylapatite of the lipoprotein surface... 

It has been difficult to evaluate both the quantitative importance in for example net protein transfer and the mechanism of the very low density-to-low density conversion process. This conversion is more complex than the simple removal of triglyceride by lipolysis. The low density lipoprotein class contains less triglyceride, more protein and cholesterol ester, and about the same relative amount of free cholesterol and phospholipid as the very low density lipoprotein class (Table 3). The free cholesterol/phospholipid ratio is the same for both lipoprotein classes. Indeed, the free cholesterol/lecithin mole ratios are 1.2 and 1.1 for very low density and low density lipoproteins respectively. Stable 1 1 complexes between cholesterol and phospholipid are possible (Vandenheuvel 1963). A cholesterol-phospholipid complex could form a portion of the lipoprotein surface... 

Peripheral tissues take up more cholesterol from VLDL than they require, and export the surplus onto HDL for return to the liver for catabolism. HDL are secreted from the liver as a lipid-poor protein, and take up cholesterol from tissues by the action of lecithin—cholesterol acyltransferase at the lipoprotein surface. 

Fig. 3. Topology of surface components of HDL. The cross-hatched areas represent the phospholipid polar head group, while the solid eliptical areas represent cholesterol. This scheme depicts two adjacent molecules of apoA-1 spread as a monomolecular film on the lipoprotein surface with phospholipid molecules interspersed between the structural domains of the apolipoprotein. (Edelstein et al., 1979, used with permission).

The surface area is an important determinant of lipoprotein function. When compared on the basis of the number of lipoprotein particles ml, about 90% of the total lipoprotein particles in the plasma are HDL (Table 3). By contrast, in males, HDL have only about half of the total lipoprotein surface area. In females, HDL contributes about 75% of the total lipoprotein surface area because of the relative abundance of HDL. In terms of the core volume of lipoproteins, LDL contains about half the lipoprotein core volume in both males and females. 

Dynamics of lipoprotein structure. The individual rate constants for absorption and desorption of the lipoprotein components from the lipoprotein surface are designated d, ... 

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). 

Two possible mechanisms of lipid transfer between lipoproteins that do not involve proteins are the following. In the first mecha-nism, lipid molecules escape slowly from the lipoprotein surface in the rate-limiting step and are taken up rapidly from the aqueous medium. In the second mechanism, transfer requires the formation of a collision complex of the donor and acceptor lipoproteins that... 

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