Among the adhesion molecules there are proteins belonging to the immunoglobulin superfamily . Its members (also called immunoglobulin-like cell adhesion molecules, ICAMs) possess a common structural motif, an immunoglobulin fold composed of about 70-110 amino acids that form two anti-parallel b-plated sheets stabilized by disulphide bonds [1, 21]. Structurally, these Ig superfamily share a composition of constant/variable domains with disulfide bonds followed by a transmembrane domain (Fig. 2.14).
Figure 2.14 Schematic structures of two exemplary members of Ig superfamily: (a) CD4 co-receptor, and (b) T-cell antigenic receptor.
Immunoglobulins function as cell adhesion and signaling receptors that transduce extracellular signals from neighboring cells or the extracellular matrix to the intracellular signaling machinery. Most of the members of this superfamily participate in the cell-cell recognition, immunological processes, and also in cancer metastasis.
Many of the integral membrane proteins, on the extracellular side of the membrane, contain one (rarely) or more oligosaccharide components (due to the origin also called carbohydrate moieties). An individual oligosaccharide chain, covalently bound to a protein, is called a glycan and such proteins are called glycoproteins.
Glycans are complex carbohydrates composed of single sugar units that are usually referred to the carbohydrate moiety attached either to proteins or to lipids . Depending on the bond structure, there are two types of glycans: O- and N-linked ones (Fig. 2.15).
Figure 2.15 The chemical structures of N- and O-linked glycosidic bonds accompanied with the schematic view of resulting N- and O-linked glycans. Specific sugars are usually represented by colored geometric symbols as presented in the legend (Gal-galactose; Man-mannose; Fuc-fucose; Sia-sialic acid; GlcNAc-N-acetylglucosamine; GalNAc-N-acetylgalactosamine; Ser-serine; Thr-threonine, Asn-aspargine).
The O-linked glycans (O-glycans) are linked to the hydroxyl group of serine (Ser) or threonine (Thr) side chain. Usually they are short and not significantly branched oligosaccharides, composed of few monosaccharide residues, such as N- acetylgalactosamine, galactose, N-acetylglucosamine, fucose, and N-acetylneuraminic acid, also called sialic acid . The N- linked glycans form the bond with an amide group of the side chain asparagine (Asn). Oppositely to O-linked glycans, N- linked glycans are often much larger (longer, more branched) and contain N-acetylglucosamine, mannose, galactose, fucose, N-acetylgalactosamine, and N-acetylneuraminic acid. Their size varies between 6-15 monosaccharide units, arranged in 2 to 5 antennae structures.
The glycans fulfill many important, structural and functional roles. They are responsible for increased solubility and stability of a number of proteins, e.g., as in the case of blood plasma proteins. Oligosaccharides differ from proteins and nucleic acids in a few characteristics: they are usually highly branched and their monomeric units are bound by variety of bonds. Due to the branching and numerous alternative linkages, these types of oligosaccharides are able to carry more information than other biological molecules. Therefore, their varying structural motives serve as a part of a recognition system as in antigen-antibody and receptor-hormone reactions, bacterial infection, cell-cell, and cell-extracellular protein interactions . Unique glycans are also involved in promoting the progression of various carcinomas, for example, some tumor antigens consist of truncated O-linked glycans. Their accumulation in many tumors correlates with invasion. Also, certain glycosphingolipids containing sialic- acids (called gangliosides) have been correlated with tumor growth.