Biological Chemistry - Berezov T. T., Korovkin B. F. 1998

Chemistry of Conjugated Proteins
Lipoproteins

In recent years, significant progress has been made in elucidating the chemical nature and Structure of Lipoproteins (LPs). This Class of complex Proteins consists of a protein moiety and a prosthetic group represented by a lipid. Specifically, neutral fats, free Fatty acids, Phospholipids, and cholesteryl esters have been identified within lipoproteins. Lipoproteins are widespread in nature—found in plants, animal Tissues, and microorganisms—and perform diverse biological Functions. They are integral components of Cell membranes and intracellular Biomembranes of The Nucleus, Mitochondria, and microsomes (structured lipoproteins), and also exist in a free state (primarily in Blood Plasma). Furthermore, lipoproteins include the thromboplastic protein of lung tissue, lipovitellin of chicken egg yolk, and certain milk phospholipids, among others. It has been established that lipoproteins are involved in the structural and complex Organization of myelin sheaths, Nerve Tissue, METABOLISM/14.html">Chloroplasts, photoreceptor and electron transport systems, retinal rods and cones, and more.

Most LPs are synthesized in the Liver or the intestinal mucosa. They contain a hydrophobic lipid core surrounded by Polar Lipids and a shell of proteins known as apolipoproteins (apoproteins). There are 8 distinct types of apoproteins: apo-AI, AII, B, CI, CII, CIII, D, and E. Typically, LPs contain up to 5% CARBOHYDRATES (glucose, galactose, hexosamines, fucose, sialic acid), which means some of them are also classified as Glycoproteins.

Serum lipoproteins are subdivided into distinct classes based on their electrophoretic mobility (along with blood proteins) and density upon ultracentrifugation. These include low-density lipoproteins (LDLs), very-low-density lipoproteins (VLDLs), high-density lipoproteins (HDLs), very-high-density lipoproteins (VHDLs), and intermediate-density lipoproteins (IDLs) (Table 2.3).

The functions and significance of individual LP classes in The Development of arteriosclerosis and atherosclerosis are discussed in detail in Chapter 17.

Mechanism of binding between the protein component and lipids. Evidence suggests that The formation of lipoproteins involves non-covalent forces of various types, determined by the presence or absence of ionized atomic groups in the lipid component. When phospholipids participate in lipoprotein formation, an ionic bond is established between them and the protein molecule (Fig. 2.4).

The existence of hydrophobic interactions between the non-polar groups of the lipid component (e.g., fatty acid radicals) and the protein molecule has also been proven. Most often, a combination of different non-covalent forces acts within lipoproteins, facilitating the Formation of the highly ordered double protein-lipid structure characteristic of biomembranes.

Table 2.3. Classification and Main Properties of human serum LPs