Carbohydrate glycosylations

Polysaccharides are formed by glycosidically linked carbohydrate (glycosyl) residues. As expected, the nomenclature of polysaccharides is based on the nomenclature of carbohydrates. The recommendations of the International Union of Pure and Applied Chemistry-International Union of Biochemistry and Molecular Biology Joint Commission on Biochemical Nomenclature (lUPAC-IUB JCBN) have been pub-lished and are open to the public at http //www. chem.qmul.ac.uk/iupac/2carb/39.html. 

In the molecular engineering of carbohydrates, glycosyl trichloroacetimidates play an important role as effective glycosyl donor molecules they are generally prepared by trapping anomeric alcohols by treatment with trichloroacetonitrile in the presence of a strong base [73]. However, purification is sometimes diminished by the acid lability of trichloroacetimidate functionalities. To reduce such an inconvenience to remove trace... 

Structure—function relationships of prolactin among a variety of species have been pubUshed (17,18). Only one gene for prolactin appears to exist (19). Although classically placed in the category of simple protein hormones, prolactin can be glycosylated. Carbohydrate attachment occurs at Asn-31, where the consensus glycosylation sequence Asn—X—Ser is found. 

Fig. 3. Fluman LH, FSH, and TSH a suburnt [69431-84-1]. Amino acid numbering is relative to maximum homology between species (48). Note the 4 amino acid deletion in human a suburnt between positions 6 and 9. Consensus glycosylation sites are at Asn-56 and 82. GHO = carbohydrate chain.

Fig. 3. Human CG, hLH, and equine CG (eCG) P-subunits. Amino acid numbeiing is relative to maximum homology between the three subunits. Consensus glycosylation sites ate at Asn-13 and 30. = same amino acid as hCG/3. Underlined Asn residues indicate attachment of N-linked carbohydrate chains. Serines at positions 121, 127, 132, and 138 of hCGP are underlined to indicate sites of O-linked carbohydrate attachment. Residues 115—118,...

Figure 25.7 Glycoprotein formation occurs by initial phosphorylation of the starting carbohydrate to a glycosyl phosphate, followed by reaction with UTP to form a glycosyl uridine 5 -diphosphate. Nucleophilic substitution by an -OH (or -NH2) group on a protein then gives the glycoprotein.

A conceptually new direct oxidative glycosylation with glycal donors, employing a reagent combination of triflic anhydride and diphenyl sulfoxide, has recently been reported by Gin [83], This new 3-glycosylation method works very well with hindered hydroxy nucleophiles, including sterically shielded carbohydrate hydroxy systems, and can be run on large scales. 

Glycosydation AChE and BChE carry 3 and 9, respectively, N-glycosylation consensus sequences attaching carbohydrate residues to the core protein via asparagines. Different molecular forms of the enzymes in various tissues, show different number and composition of carbohydrate residues. N-glycosylation at all sites was shown to be important for effective biosynthesis, secretion and clearance of ChEs from the circulation. Altered patterns of AChE glycosylation have been observed in the brain and cerebrospinal fluid of Alzheimer s disease (AD) patients, with potential diagnostic value. 

Carbohydrate-coupling or glycosylation, is a major synthesis method in carbohydrate preparation. Silver silicates and Ag(I)-exchanged zeolite A - so-called insoluble Ag(I) - have been advocated as promoting agents, applied in more than stoichiometric amount (Fig. 9). All hydroxyl groups except the attacking one are suitably protected. 

Science 200T,21(5512) 2263. (This issue contains a special section entitled Carbohydrates and Glycobiology. It contains articles on the synthesis, structural determination, and functions of sugar-containing molecules and the roles of glycosylation in the immune system). 

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