Glycoproteins carbohydrate moieties

Many proteins found in nature are glycoproteins because they contain covalently linked oligo- and polysaccharide groups. The list of known glycoproteins includes structural proteins, enzymes, membrane receptors, transport proteins, and immunoglobulins, among others. In most cases, the precise function of the bound carbohydrate moiety is not understood. 

Maras, M., Saelens, X., Laroy, W. et al. (1997) In vitro conversion of the carbohydrate moiety of fungal glycoproteins to mammalian-type oligosaccharides - evidence for JV-acetylglucosaminyltransferase-I-accepting glycans from Trichoderma reesei. European Journal of Biochemistry, 249 (3), 701-707. 

Premier, C., Mach, L., Glossl, J. and Marz, L. (1992) The antigenicity of the carbohydrate moiety of an insect glycoprotein, honey-bee (Apis mellifera) venom phospholipase A2. The role of al,3-fucosylation of the asparagine-bound IV-acetylglucosamine. Bio chemicalJournal 284, 377-380. 

The neuraminic acids (also called sialic acids), which occur in the terminal positions of carbohydrate moieties of many glycoproteins and ganglio-sides, must be hydrolyzed under very mild conditions. For example, hydrolysis with 0.01 M hydrochloric acid for 30 min at 100° causes 20% decomposition of N-acetylneuraminic acid. Sialic acids are more tolerant to methanolysis they may also be removed enzymically. 

The carbohydrate moiety of glycoproteins is added to a preformed protein molecule. Thus, the structure of the protein part is determined by the corresponding DNA structure, but the structure of the carbohydrate part is determined by the specificity of the enzymes (the structure of which is ultimately encoded in DNA as well) that catalyze the decoration of the protein once formed. The carbohydrate structures are generally quite complex and their formation is the product of several enzymes acting with several substrates. 

K Schmid. Human plasma a,-acid glycoprotein—biochemical properties, the amino acid sequence and the structure of carbohydrate moiety, variants and polymorphism. In P Baumann, CB Eap, WE Muller, JP Tillement, eds. Al-pharacid glycoprotein genetics, biochemistry, physiological function, and pharmacology. Alan R. Liss, New York, 1989, pp 7-22. 

Immunoglobulins are glycoproteins. The carbohydrate moiety is attached to the heavy chain (usually the Ch2 domain) via an N-linked glycosidic bond. Removal of the carbohydrate group has no effect upon antigen binding but does affect various antibody effector functions and alters its serum half-life. 

The spiropyran derivative incorporated RMs showed good photo-controUed extraction of zwitter-ionic amino acid under UV irradiation and release under visible irradiation Unlike native enzyme, the modified counterpart tended to form reversible oligomeric structures in RMs, which allowed in realizing the role of carbohydrate moieties in the functioning of glycoproteins... 

In this Section, the nature of the linkages between the protein chain and the carbohydrate moieties in glycoproteins will be considered, and some indication of their occurrence will be given. It will become clear that, in a number of examples, a given polypeptide chain is glycosidically substituted at more than one point. Sometimes, the amino acid residue at two such points is of a different type, so that more than one type of carbohydrate-peptide bond occurs in the same protein, It is likely that more examples of this kind of structure will be found as more features of the constitution of glycoproteins involved in membranes and connective tissue are determined. 

The possibility of categorizing glycoproteins according to the nature of the carbohydrate-peptide bond contained has been suggested.38 To some extent, this method is possible, but, as was pointed out, it does not result in a rigid classification, for examples are known in which more than one type of carbohydrate moiety is attached to a given polypeptide chain. 

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