Glycoprotein protein-carbohydrate links

A further relevance of glycoproteins to collagen is the nature of the protein-carbohydrate link found. Blumenfeld and Gallop (1962b) have shown that aspartic acid is involved in the ester-like links of collagen which may be binding hexose molecules. Aspartic acid also constitutes the point of attachment of carbohydrate in most glycoproteins studied to date. 

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. 

Glycoconjugates are biomolecules in which carbohydrate is covalently linked to either proteins or lipids. Proteoglycans are composed of relatively large amounts of carbohydrate (GAG units) covalently linked to small polypeptide components. Glycoproteins are proteins covalently linked to carbohydrate through N- or O-linkages. 

Fig. 30.11. Structure of salivary mucin. The sugars form hydrogen bonds with water. Sialic acid provides a negatively charged carboxylate group. The protein is extremely large, and the negatively charged sialic acids extend the carbohydrate chains so the molecules occupy a large space. All of the salivary glycoproteins are 0-linked. NANA is a sialic acid.

The glycoprotein has 0-linked carbohydrate residues, and in most such proteins the sugars are attached to the side chains of serine or threonine residues. Were the sugars N-linked instead, you would expect to find one or more asparagine residues in the glycoprotein. 

Section 23.25 Carbohydrates linked to proteins, termed glycoproteins, are one example of... 

In addition to pure proteins, which are constructed entirely from amino acid units, compounds (conjugated proteins) also exist that contain some non-amino acid residues linked to the protein component. These are usually classified according to the nature of the nonprotein component as chromoproteins (with metals), glycoproteins (with carbohydrates), lipoproteins (with lipids), and nucleoproteins (with nucleic acids). 

Being secreted proteins, most of the cellulases from T. reesei have proven to be glycoproteins. Their carbohydrate structure may be deduced from detailed analysis with CBH I and in part EG I it is composed both of N- as well as O-linked chains most of the O-glycosylation occurs in the B ( hinge ) region of the protein, whereas N-glycosylation is restricted to the core domain, most... 

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