Oligosaccharide chains biosynthesis

Numerous lectins have been purified and are commercially available three plant lectins that have been widely used experimentally are listed in Table 47-7. Among many uses, lectins have been employed to purify specific glycoproteins, as tools for probing the glycoprotein profiles of cell surfaces, and as reagents for generating mutant cells deficient in certain enzymes involved in the biosynthesis of oligosaccharide chains. 

Elbien, A.D. (1987) Inhibitors of the biosynthesis and processing of N-linked oligosaccharide chains. Annual Review of Biochemistry 56, 497-534. 

The biosynthesis of O-Iinked oligosaccharides follows a different scheme. In contrast with the dolichyl pyrophosphate-mediated synthesis of N-linked oligosaccharides, the glycosyltransferases necessary for the synthesis of O-linked oligosaccharides are located in the Golgi apparatus [9]. Monosaccharide residues are added sequentially to the growing oligosaccharide chain. 

The term monomeric mechanism will be used for the mechanism depicted in the left-hand part of Scheme 2 (sequence a). In this case, the monosaccharide residues are transferred consecutively from the corresponding glycosyl donors (Z-A or Z -B) onto a membrane-bound glycosyl acceptor. The acceptor is generally a monosaccharide residue, which may be a fragment of an oligosaccharide chain linked to a hydrophobic molecule embedded in a cell membrane. In many instances, the acceptor that is used for assembly of the polymeric chain (Y) is not identical to the final acceptor (X) of the chain, and further transfer of the chain from Y to X, or liberation of the polysaccharide molecule in the case of exocellular polysaccharides, is a necessary step in the biosynthesis. 

Many secretory and membrane proteins are modified in the lumen of the ER. Protein glycosylation, the addition of covalently bound oligosaccharide chains, is a common reaction in the ER lumen and the Golgi complex. Most proteins in the ER lumen destined for secretion from the cell or for transport to other intracellular sites are glycoproteins. The carbohydrate content of glycoproteins can vary from 0.5 to 80% or more of the glycoprotein mass. Glycoprotein structures were described in Chapter 9, and their biosynthesis are covered in Chapter 20. 

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