Glycoconjugates

TABLE 10.5 Consensus sites for protein phosphorylation by kinases.  

When post-translational modifications involve fatty acylation or glycosylation, it may also be of interest to determine the structure of the modification itself, as well as the specific site on the protein. In such cases (as described below) similar chemical and enzymatic strategies have been developed. 

In cases in which both a serine and threonine residue are present in a peptide fragment that is phosphorylated, these can be distinguished by reaction of the peptide with ethanethiol in 5N NaOH, which converts phosphoserine to 5-ethylcysteine  

One of the most common forms of protein modification is glycosylation, the addition of generally large carbohydrate structures to specific amino acid residues, which in some cases form the major portion of the molecular mass of these glycoproteins. 

Oligosaccharides may be attached to peptides by amide or acyl linkages. The N-linked oligosaccharides are generally attached to asparagine residues found in the consensus sequence Asn-X-Ser/Thr. These commonly involve an N-acetyl-glucosamine (GlcNAc) residue  

The differences in the rates of hydrolysis of various linkage types by a particular glycosidase can be used to provide information about this aspect of structure. Jack-bean a-D-mannosidase cleaves a-(l- 2) and a-(l- 6) linkages much faster than a-(l - 3). Oligosaccharides, obtained by endo-N-acetyl-/J-D-glucosaminidase hydrolysis of ovalbumin, were subjected to acetolysis, which selectively cleaved the a-(l - 6) bonds. A tetrasaccharide isolated after this treatment was then incubated with jack-bean o-d- 

D-Galactose oxidase identifies nonreducing (terminal) D-galactosyl groups by selective oxidation.3323 

The release of ceramide monohexoside and an oligosaccharide having a d.p. of 9-11 on enzymic hydrolysis favored structure 83. 

N-Acetyl-/ -D-glucosaminide-(l - 4)-/J-D-galactosyltransferase, from bovine colostrum, first substituted trisaccharide 90 (which is the partial structure of the branching point in blood-group I, antigenic structures) at the 

The oligosaccharide structures responsible for the ABO blood-group system have been related by using the appropriate glycosyltransferases for interconversion, and the results have been reviewed.223,373 Blood-group H substance (91) was converted374 into an A-active substance (92) with 

In the first half of the twentieth century, proteins were purified until all nonpro-teinaceous materials had been removed, otherwise they were regarded as impure mixtures. Likewise, polysaccharides were purified until all proteinaceous material had been removed. Despite Neuberger s discovery in 1938, proteins were still considered in the 1950s to be just polypeptides composed of a specific sequence of amino acids, and the presence of carbohydrate was considered to be due to contamination. As analytical techniques improved, it appeared that carbohydrate and proteinaceous materials could not be completely removed from each other. 

Essentials of Carbohydrate Chemistry Springer-Verlag New York Inc. 1998 

The sialic acids make up a family of carbohydrates that is composed of D-neuraminic acid substituted at different positions by a limited number of substituents. 

The amino group is substituted by an acetyl or glycoloyl group, and the hydroxyl groups at C-4, -7, -8, and -9 may be methylated or esterified by acetyl, lactyl, sulfate, or phosphate groups [2]. See Fig. 9.1 for the position specificity for neuraminic acid substitution. Sometimes several of the substituents are present on the same molecule. Unsubstituted D-neuraminic acid has not been found in nature. Twenty different sialic acids have been observed, showing a wide degree of structural diversity [2]. 

Among some biological scientists, the carbohydrates have acquired as much importance and interest as proteins and nucleic acids, and many structures of the oligosaccharide chains of glycoproteins and glycolipids have been determined. 

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Glycoconjugates. Another class of carbohydrates are the glycoconjugates (14), composed of glycoproteins, proteoglycans, peptidoglycans, and glycohpids. 

Glycoconjugate (Section 25.6) A biological molecule in which a carbohydrate is linked through a glycoside bond to a lipid or protein. 

Glycoconjugate, 991 Glycogen, function of, 1001 structure of, 1001 Glycol, 234, 662 Glycolic acid, pKa of, 756 structure of, 753 Glycolipid, 991... 

Goldstein DB, Hungund BL, Lyon RC Increased surface glycoconjugates of synaptic membranes in mice during chronic ethanol treatment. Br J Pharmacol 78 8-10, 1983... 

Three 3-deoxynonulosonic acids containing amino groups are known. The most abundant of these is 5-amino-3,5-deoxy-D- /yc ro-D-ga/acto-nonulosonic acid (neuraminic acid, 27), which occurs in different extracellular polysaccharides. Some of these, like colominic acid from E. coli K1, are homopolysaccharides. Neuraminic acid is generally A-acetylated and, as in the animal glycoconjugates, has only been found in the a-pyranosyl form (27). It also occurs in some LPS, for example those from some Rhodobacter... 

Other disorders in which glycoproteins have been implicated include hepatitis B and C, Creutzfeldt-Jakob disease, and diarrheas due to a number of bacterial enterotoxins. It is hoped that basic smdies of glycoproteins and other glycoconjugates (ie, the field of glycobiology) will lead to effective treatments for diseases in which these molecules are involved. Already, at least two disorders have been found to respond to oral supplements of sugars. 

In the author s laboratory, it has never been found necessary to resort to matrices other than glycerol or 1-thioglycerol for the analysis of saccharides and glycoconjugates. Nevertheless, alternative matrices are often equally effective, and, in some laboratories, they are preferred. The most widely used include tetraethyleneglycol (4) and its higher-molecular-weight relatives, the poly(ethyleneglycol)s, and such basic matrices as N,N -bis(2-aminoethyl)ethylenediamine ( triethylenetetramine, 5), 2,2 -... 

The synthesis of glycoconjugates opens the route to one of the most important class of biomolecules which play an active role in relevant biological reactions [26]. One way to do so is to use enzymes, which, however, suffer from instability and slow reaction rates. 

Dondoni, A. Massi, A. (2006) Design and Synthesis of New Classes of Heterocyclic C-Glycoconjugates and Carbon-Linked Sugar and Heterocyclic Amino Acids by Asymmetric Multicomponent Reactions (AMCRs). Accounts of Chemical Research, 39, 451M63. 

Franceschini V., Lazzari M. and Ciani F. (1996). Identification of surface glycoconjugates in the olfactory system of turtle. Brain Res 725, 81-87. 

Takami S., Getchell M. and Getchell T. (1995). Resolution of sensory and mucoid glycoconjugates with terminal O-galactose residues in the mucomicrovillar complex of the vomeronasal sensory epithelium by dual confocal laser-scanning microscopy. Cell Tiss Res 280, 211-216. 

Tobet S.A., Crandall J. and Schwarting G. (1992). Relationship of migrating LHRH neurons to unique olfactory system glycoconjugates in embryonic rats. Dev Biol 155, 471-482. 

Stefansson, M. and Westerlund, D., Ion-pair LC of carbohydrates at alkaline pH. Separation and retention behavior of glycoconjugates, Chromatographia, 35, 55, 1993. 

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