Food Chemistry

Module 3. Food lipids

Lesson 10
LIPOPROTEIN – DEFINITION, CLASSIFICATION AND INVOLVEMENT IN THE FORMATION OF BIOLOGICAL MEMBRANES

10.1 Introduction

Lipoproteins are aggregates, consisting of proteins, polar lipids and triacylglycerols; which are water soluble and can be separated into protein and lipid moieties by an extraction procedure using suitable solvents. This indicates that only non-covalent types of bonds are involved in the formation of lipoproteins. The aggregates are primarily stabilized by hydrophobic interactions between the apolar side chains of hydrophobic regions of the protein and the acyl residues of the lipid. In addition, there is a contribution to stability by ionic forces between charged amino acid residues and charges carried by the phosphatides. Hydrogen bonds play a small role in binding lipids molecule as there are only few sites available for such linkages. In wheat flour, the lipoprotein complex consists of prolamine and glutelin attached to glycolipids by hydrogen bonds and hydrophobic forces. Thus, lipoproteins are held together only by non-covalent bonds.

10.2 Classification

Lipoproteins exist as globular particles in an aqueous medium. They are solubilized from biological sources by buffers with high ionic strength, by a change of pH or by detergents in the isolating medium. The latter, a more drastic approach, is usually used in the recovery of lipoproteins from membranes. Lipoproteins are characterized by ultracentrifugation. Since lipids have a lower density (0.88–0.9 g/ml) than proteins (1.3–1.35 g/ml), the separation is possible because of differences in the ratios of lipid to protein within a lipoprotein complex.

The lipoproteins of blood plasma have been thoroughly studied. They are separated by a stepwise centrifugation in solutions of NaCl into three fractions with different densities.

A. The “very low density lipoproteins” (VLDL): The density of these types of lipoproteins is <1.006 g/ml): The VLDL fraction can be separated further by electrophoresis into chylomicrons (the lightest lipoprotein, density <1.000 g/ml) and pre-β-lipoprotein.
B. The “low density lipoproteins” (LDL): The density of LDL is 1.063 g/ml. Lipoproteins in the LDL fraction from an electrophoretic run have mobility close to that of blood plasma β-globulin. Therefore, the LDL fraction is denoted as β-lipoprotein.
C. The “high density lipoproteins” (HDL): The density of HDL is 1.21 g/ml. An analogous designation of α-lipoprotein is assigned to the HDL fraction.

Chylomicrons, the diameters of which range from 1000–10,000Å, are small droplets of triacylglycerol stabilized in the aqueous medium by a membrane-like structure composed of protein, phosphatides and cholesterol. The role of chylomicrons in blood is to transport triacylglycerols to various organs, but preferentially from the intestines to adipose tissue and the liver. The milk fat globules have a structure similar to that of chylomicrons. Certain diseases related to fat metabolism (hyperlipidemias) can be clinically diagnosed by the content and composition of the plasma lipoprotein fractions. Electron microscopy studies have revealed that the fat globules in milk have small particles attached to their membranes; these are detached by detergents and have been identified as LDL.

10.3 Involvement in the Formation of Biological Membranes

Membranes that compartmentalize the cells and many subcellular particles are formed from two main building blocks: proteins and lipids (phospholipids and cholesterol). The glycerophospholipids are the main structural component of biological membranes, such as the cellular plasma membrane and the intracellular membranes of organelles; in animal cells the plasma membrane physically separates the intracellular components from the extracellular environment. The glycerophospholipids are amphipathic molecules (containing both hydrophobic and hydrophilic groups) that contain a glycerol core linked to two fatty acid-derived "tails" by ester linkages and to one "head" group by a phosphate ester linkage. While glycerophospholipids are the major component of biological membranes, other non-glyceride lipid components such as sphingomyelin and sterols (mainly cholesterol in animal cell membranes) are also found in biological membranes. In plants and algae, the galactosyl diacylglycerols, and sulfoquinovosyl diacylglycerols, which lack a phosphate group, are important components of membranes of chloroplasts and related organelles and are the most abundant lipids in photosynthetic tissues, including those of higher plants, algae and certain bacteria. Differences in membrane structure and function are reflected by the compositional differences of membrane proteins and lipids. Studies of membrane structure are difficult since the methods for isolation and purification profoundly change the organization and functionality of the membrane.

Model membranes are readily formed. The major forces in such events are the hydrophobic interactions between the acyl tails of phospholipids, providing a bilayer arrangement. In addition, the amphipathic character of the lipid molecules makes membrane formation a spontaneous process. The acyl residues are sequestered and oriented in the non-polar interior of the bilayer, whereas the polar hydrophilic head groups are oriented toward the outer aqueous phase.

Another arrangement in water that satisfies both the hydrophobic acyl tails and the hydrophilic polar groups is a globular micelle. Here, the hydrocarbon tails are sequestered inside, while the polar groups are on the surface of the sphere. There is no bilayer in this arrangement hydrophobic lipid tails.

Fig. 10.1 Arrangements of polar acyl lipids in aqueous medium



Fig. 10.2 Fluid mosaic model of a biological membrane

The favored structure for most phospho- and glycolipids in water is a bimolecular arrangement, rather than a micelle. Globular proteins, often including enzymes, are found in animal cell membranes and are well embedded or inserted into the bimolecular layer. Some of these so-called integral membrane proteins protrude through both sides of the membrane (fluid mosaic model). Although integral proteins interact extensively with the hydrophobic acyl tails of membrane lipids they are mobile within the lipid membrane.