Carbohydrates on Cell Surfaces

A We re characterizing carbohydrates on cell surfaces. This is important because many diseases are brought on by cell-to-cell inter-... 

Sugar based polymers, which are obtained by polymerization of vinyl sugars, have recently received increased attention from two viewpoints. One is the development of environmentally friendly material from renewable resources. Another is physiologically active material that mimics carbohydrate on cell surface. This article provides an overview of known sugar based polymers and the recent advances of the poly(vinylalcohol) (PVA) with sugar pendants. 

Such sugar-dendrimer complexes ( sugar balls ) have been used to inhibit the interactions of viruses with cell surfaces. Many viruses bind to particular carbohydrate residues on cell surfaces, which in turn facilitate their entry into cells and the resultant infection process. A virus particle presents a multi-dentate surface consisting of many carbohydrate-binding proteins able to interact with multiple cell-surface carbohydrates. The surface of a dendrimer that is modified with... 

Lex-antigenic trisaccharide (a-L-Fuc(l-3)-[p-D-Gal-(l-4)-p-D-GlcNAc]) and its sialylated structure (SLex) are terminal components of a nnmber of glycoconjugates on cell surfaces [31]. SLex serves as a ligand for the endothelial leukocyte molecule-1 (E-selectin) [32], which mediates the initial stages of adhesion of leukocytes to activated endothelial cells, and pays a critical role during inflammatory responses [33]. Lex-based carbohydrates have shown promise in therapeutic investigations related to the inflammatory process. As a result, extensive efforts have been directed toward the synthesis of SLex and related molecules [34 also see Chap. 15],... 

Lectins, or proteins with specific binding sites for carbohydrates, can be used as targeting molecules to localize particular glycoconjugates such as glycoproteins or glycolipids on cell surfaces (Fig. 373). Labeled with gold particles, lectins are important probes for detection of cell surface components and intracellular receptors and in immunological or biochemical assay procedures (Bog-Hansen et al., 1978 Kimura et al., 1979 Nicolson, 1978 Roth, 1983 Benhamou et al., 1988 Nakajima et al., 1988). 

Mucus. The cells of internal epithelia throughout the body are surrounded by an intercellular ground substance known as mucus. The principal components of mucus are complexes composed of proteins and carbohydrates. These complexes may be free of association or may be attached to certain regions on cell surfaces. This matrix may play a role in cell-cell adhesion, as well as acting as a lubricant, allowing cells to move relative to one another.99 Moreover, mucus is believed to play a role in bioadhesion of mucoadhesive drug delivery systems.100... 

This time you will be engaged in the synthesis of blood group determinants. Besides the described glycoconjugates there are carbohydrate antigens of the A, B, H (O) and Lewis families on cell surfaces too. Because of their biological significance, these glycoconjugates are used to stimulate antibody production. 

Figure 11.9. Modified Monosaccharides. Carbohydrates can be modified by the addition of substituents (shown in red) other than hydroxyl groups. Such modified carbohydrates are often expressed on cell surfaces.

The diverse carbohydrate structures displayed on cell surfaces are well suited to serve as interaction sites between cells and their environments. Proteins termed lectins (from the Latin legere, "to select") are the partners that bind specific carbohydrate structures. Lectins are ubiquitous, being found in animals, plants, and microorganisms. We have already seen that some lectins, such as calnexin, function as chaperones in protein folding (Section 11.3.6). 

The ability of viruses to infect specific cell types is dictated in part by the ability of these viruses to bind to particular structures or receptors on the surfaces of cells. In some cases, these receptors are carbohydrates. For example, influenza virus recognizes sialic acid residues present on cell-surface glycoproteins. The viral protein that binds to these sugars is called hemasslutinin(Fisure 11.31). 

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