Protein-saccharide complexes

The ability of water to act as both donor and acceptor of hydrogen bonds makes it an excellent mediator in protein-carbohydrate complexes. Results of numerous X-ray crystallographic studies of protein-carbohydrate complexes attest to the pervasiveness of water-mediated hydrogen bonds in protein-carbohydrate interactions. Lectin-bound water molecules are proposed to act as an extension of the protein surface itself [1]. This idea is supported by the fact that water molecules occupy conserved sites in the structures of related lectins [24]. In addition, water molecules can mediate similar hydrogen bonding interactions in the complexes of different proteins with a specific carbohydrate ligand or in complexes of the same lectin with different saccharide ligands [1]. 

MIO. Mason, K. M., Protein-pol) saccharide complexes from adult human tracheal cartilage. Biochetn. J. 119, 599-601 (1970). 

Aqueous solutions of proteins and polysaccharides may exhibit phase separation at finite concentrations. Two types of behavior may be recognized, namely coacer-vation and incompatibility. Complex coacervation involves spontaneous separation into solvent-rich and solvent-depleted phases. The latter contains the protein-poly-saccharide complex that is caused by nonspecific attractive protein-polysaccharide interaction, e.g. opposite charge interaction. Incompatibility is caused by spontaneous separation into two solvent-rich phases, one composed of predominantly protein and the other predominantly polysaccharide. Depending on the interactions, a gel formed from a mixture of two biopolymers may contain a coupled network, an interpene-... 

FIGURE 9.26 The carbohydrate tnoiedes of glycoproteins may be linked to the protein via (a) serine or threonine residues (in the O-linked saccharides) or (b) asparagine residues (in the N-linked saccharides), (c) N-Linked glycoproteins are of three types high mannose, complex, and hybrid, the latter of which combines structures found in the high mannose and complex saccharides. 

The modification of molecules with saccharides also has the effect of increasing the hydrophilicity of the resultant complex due to the presence of multiple hydroxyl groups. Native glycan modification of proteins functions in much the same manner, because the carbohydrate... 

The following method for carbohydrate conjugation to dendrimers may be used to couple a variety of reducing sugars to amine-dendrimers, including saccharides, longer-chain carbohydrates, and even complex glycans after release from a protein (see Chapter 1, Section 4.6). 

The interest in using saccharide-substituted polymers to bind and cluster cell surface proteins arises from studies of L-selectin, a protein involved in inflammation. L-selectin binds glycoproteins that display complex carbohydrates, and... 

This great structural variety, however, complicates the specific biosynthesis of complex oligosaccharides. In general, the formation of each saccharide linkage requires specific enzymes ( one linkage—more than one enzyme ) and thus, in comparison with the enzymic synthesis of proteins and nucleic acids, much more effort is needed. 

The reactions that take place in the complement system can be initiated in several ways. During the early phase of infection, lipopoly-saccharides and other structures on the surface of the pathogens trigger the alternative pathway (right). If antibodies against the pathogens become available later, the antigen-antibody complexes formed activate the classic pathway (left). Acute-phase proteins (see p. 276) are also able to start the complement cascade lectin pathway, not shown). 

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