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

Chemistry of Conjugated Proteins
Glycoproteins

Glycoproteins are complex Proteins containing heterooligosaccharide groups In addition to a simple protein or peptide moiety. Nowadays, they are commonly referred to as Glycoconjugates. A glycoconjugate consists of a carbohydrate component (glycan fraction) covalently linked to a non-carbohydrate moiety (aglycan fraction) represented by a protein, peptide, amino acid, or lipid.

The surging interest in the science of CARBOHYDRATES—glycobiology—is currently driven by the discovery that alterations in glycoconjugate Structure play a critical role in the Pathogenesis of diseases such as Cancer, HUMAN IMMUNODEFICIENCY VIRUS (HIV), rheumatoid Arthritis, asthma, and others. It has been shown that impaired glycosylation (see Chapter 14) of the two Major Classes of glycoconjugates (glycoproteins and gangliosides) leads either to the accumulation of metabolic precursors or to the synthesis of truncated carbohydrate chains. Furthermore, carbohydrate components have been found to play a pivotal role in the interaction between certain Viruses and target Cells. Specifically, the gp120 glycoprotein of the human immunodeficiency virus (which has a high carbohydrate content) exhibits a high affinity for the CD₄ glycoprotein on T-lymphocytes. In this highly specific recognition process, glycosylated fragments most likely play a crucial pathogenetic role. It is also known that rheumatoid arthritis is often characterized by the synthesis of aberrant Antibodies (abnormal IMMUNOGLOBULINS, all of which are glycoproteins) featuring unusually short sugar chains, thereby triggering an autoimmune response. These Examples clearly demonstrate that, alongside glycobiology, The Emergence of such fields as glycopathology and glycotherapy is now widely recognized.

In addition to glycoproteins, Proteoglycans are another distinct group consisting of a protein core and glycosaminoglycans (formerly known as mucopolysaccharides). The latter are composed of complex carbohydrate chains containing amino sugars, uronic acids, sulfuric acid, and individual Monosaccharides. Typical glycosaminoglycans include hyaluronic acid, chondroitin sulfate, and heparin, The chemical composition, structure, and functions of which are discussed in detail in Chapter 21.

Typical glycoproteins include the majority of protein Hormones, substances secreted into Body Fluids, membrane-bound complex proteins, all antibodies (immunoglobulins), Plasma Proteins, milk proteins, Ovalbumin, interferons, Complement factors, Blood group determinants, and receptor proteins, among others. This far-from-exhaustive list demonstrates that all glycoproteins perform specialized functions: they mediate Cell Adhesion, MOLECULAR AND CELLULAR recognition, and the antigenic activity of tumor cells, while also exerting protective, hormonal, and antiviral effects.

While the overall Chemical composition of glycoproteins is well established, their precise structure has been mapped for only a subset of these molecules. Heterooligosaccharide chains are attached to the polypeptide backbone, containing from 2 to 10 (less frequently 15) monomeric residues of hexoses (galactose and mannose, rarely glucose), pentoses (xylose, arabinose), and a terminal carbohydrate moiety most commonly represented by N-acetylgalactosamine, L-fucose, or sialic acid. Unlike proteoglycans, glycoproteins lack uronic acid and sulfuric acid residues.

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N-acetylgalactosamine

L-fucose (6-deoxygalactose)

Sialic acid (N-acetylneuraminic acid)

The types of linkages between carbohydrate components and proteins have been elucidated only for a select group of glycoproteins with known amino acid Composition and Structure (such as immunoglobulins and hormones). These include O-glycosidic bonds (involving the OH groups of Serine, Threonine, and hydroxylysine), N-glycosidic bonds (involving the amide groups of asparagine, less frequently glutamine, or the ω-NH₂ groups of Lysine and Arginine), and ester glycosidic bonds with the free COOH groups of glutamic and aspartic acids.

The synthesis of glycoproteins occurs in the Ribosomes of The Endoplasmic reticulum (within the cisternae), followed by the attachment of sugar chains (post-synthetic glycosylation). The protein is then transported to The Cell's Biomembranes, where it is incorporated into Membrane Proteins or secreted.

The carbohydrate components are covalently linked to the nitrogen of an asparagine residue within the protein molecule. However, the oligosaccharide moiety is first attached to a lipid carrier, dolichol phosphate (a lipid containing 15 to 20 isoprene units), and then transferred to the polypeptide chain in the endoplasmic reticulum, releasing the carrier:

Dolichol phosphate (n = 15-30)

The synthesized glycoproteins are subsequently transported to the Golgi apparatus, where final glycosylation and sorting take place.

The structure of one of several heterooligosaccharide residues in a glycoprotein molecule, specifically immunoglobulins, can be represented by the following scheme (using Abbreviations: Glc — glucose, GlcNAc — N-acetylglucosamine, Gal — galactose, Man — mannose, NeuAc — N-acetylneuraminic acid):

Let us examine the currently available data on the synthesis, structure, and properties of several glycoproteins.

Interferons. Interferons are inhibitors of viral Replication for many types of viruses. Several types of interferons have been discovered (a, ß, and y), some of which are produced using Introduction/32.html">Genetic Engineering techniques. These are relatively small, complex proteins with a molecular weight ranging from 25,000 to 38,000–40,000 in various animal species and humans. They are produced by cells in response to the introduction of viral nucleic acid, thereby limiting viral aggression (infection). It is also known that a group of species-specific a-interferons is synthesized by macrophages, whereas y-interferon is produced by T cells and stimulated by interleukin-2 (see "Lymphokines"). Furthermore, y-interferon has been shown to enhance the cytotoxic activity of macrophages, T cells, and natural killer cells. Interferons possess antiproliferative activity and are considered key protective proteins not only against viral infections but also in tumor pathology.

It should be noted, however, that the molecular mechanisms by which interferons inhibit viral replication remain unclear. It is only known that interferons inhibit The Biosynthesis of all proteins (both host and viral), most likely at the translational level. It is possible that interferon induces the synthesis of a specific inhibitory protein, which then binds to ribosomes and blocks Translation, or that interferon converts one of the active eukaryotic protein initiation factors into an inactive factor via phosphorylation.

Immunoglobulins. Immunoglobulins, or antibodies, also belong to the class of glycoproteins and perform a protective function by neutralizing foreign substances entering the Organism — Antigens of any chemical nature. Immunoglobulins are synthesized by plasma cells, which originate from lymphocytes. The Study of Immunity has developed into an independent science — immunology, which examines the Structure and function of antibodies in general and immunoglobulins in particular. Below we present modern data on some physicochemical properties and the structure of human immunoglobulins (Table 2.4). There are 5 classes of immunoglobulins: IgG, IgM, IgA, IgD, and IgE. The structure and function of IgG have been studied in detail.

Table 2.4. Properties of human immunoglobulins

Different classes of immunoglobulins vary significantly not only in molecular weight but also in blood concentration; evidence also suggests differences in their biological properties.

The structure of IgG has been studied in detail. It has a Y-shaped, tetramer structure consisting of two identical light (L) chains and two identical heavy (H) chains with molecular weights of 23,000–24,000 and 50,000–70,000, respectively. Each of these chains is known to contain Two Types of domains: variable (V) regions, consisting of 108 amino acid residues, and constant (C) regions, consisting of 110 and 350 amino acid residues in the L- and H-chains, respectively (Fig. 2.5).

Other glycoproteins performing crucial biological functions include all plasma proteins (except albumins), transferrin, ceruloplasmin, gonadotropic and follicle-stimulating hormones, certain Enzymes, as well as glycoproteins found in saliva (mucin), Cartilage and Bone tissue, and egg white (ovomucoid). In addition to their signaling function, carbohydrate components significantly enhance the stability of their host molecules against various chemical and physical factors and protect them from proteinase degradation, thereby defining the Biological Role of glycoproteins. As integral components of The cell membrane, glycoproteins also participate in immunological reactions, ion transport, intercellular adhesion processes, and more.

Fig. 2.5. Structure of human IgG. The light (L) and heavy (H) chains, Disulfide Bonds, and variable V (red) and constant C (light) regions are shown.