Glycoprotein attachment points

Figure 1.5 The four polar amino acids. The arrows show the attachment points for carbohydrate residues on glycoproteins.

Figure 1.34 Common attachment points for polysaccharide chains on glycoproteins.

Core pentasaccharide Man(a1-6)[Man(a1-3)]Man()J1-4)GlcNAc(/S1-4)GlcNAc of JV-glycoproteins attached to the consensus sequence Asn-Xaa-Ser/Thr. Solid arrows indicate the points of attachment of the outer ami saccharides forming carbohydrate chains, caiied antennae, in addition, the inner-core may be substituted by several monosaccharides dashed arrows)... 

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. 

Differences in the structure of glycoproteins can manifest themselves in a number of ways. Perhaps the first point to be emphasized is that the nature of the polypeptide chain to which oligosaccharide moieties become attached is not always the same. This is a situation that we have already discussed in connection with certain of the glycosaminoglycans (see p. 440). 

Some membrane proteins are covalently linked to complex arrays of carbohydrate. For example, in gly-cophorin, a glycoprotein of the erythrocyte plasma membrane, 60% of the mass consists of complex oligosaccharide units covalently attached to specific amino acid residues. Ser, Thr, and Asn residues are the most common points of attachment (see Fig. 7-31). At the other end of the scale is rhodopsin of the rod cell plasma membrane, which contains just one hexasac-charide. The sugar moieties of surface glycoproteins influence the folding of the proteins, as well as their sta-... 

The Bik glycoprotein has an isoelectric point (p/) of 2.1 and is composed of a peptide and two glycoconjugate portions [30]. The predicted peptide sequence of Bik is helpful to understanding protein function. The peptide has Kunitz-binding domains I and II attached to the N-terminal peptide tail [31-33] (Fig. 2). Protein mass spectrometry by surface-enhanced laser desorption ionization (SELDI) in combination with detection by Bik antibodies has demonstrated that substantial variation exists in the Bik molecule [13,14]. 

A final point of general organization concerns the carbohydrate. All transferrins so far characterized, except apparently for one fish transferrin (84), are glycoproteins. There is, however, no pattern to the sites of attachment of the carbohydrate chains on different proteins—they appear almost randomly distributed over the protein surface (Fig. 4), strengthening the view that the carbohydrate plays no direct role in function. Rabbit serum transferrin, for example, has one carbohydrate chain, on its C-lobe (residue 490) human serum transferrin has two, both on the C-lobe (residues 416 and 611) human lactoferrin has two, one on each lobe (at residues 137 and 478) and bovine lactoferrin has four, one on the N-lobe (residue 233) and three on the C-lobe (residues 368, 476, and 545). 

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. 

How is it possible to determine the structure of a glycoprotein—the oligosaccharide structures and their points of attachment Most approaches make use of enzymes that cleave oligosaccharides at specific types of linkages. 

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