19.When an oligosaccharide is covalently bonded
to an asparagine (or other amino acid. side
chain, the resulting product is a(n)
A glycoprotein.
B oligoprotein.
C glyocophorin
D protein glycoside,
Answers
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Answer:
Carbohydrate groups are covalently attached to many different proteins to form glycoproteins. Carbohydrates are a much smaller percentage of the weight of glycoproteins than of proteoglycans. Many glycoproteins are components of cell membranes, where they play a variety of roles in processes such as cell adhesion and the binding of sperm to eggs.
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11.3.1. Carbohydrates May Be Linked to Proteins Through Asparagine (N-Linked) or Through Serine or Threonine (O-Linked) Residues
In glycoproteins, sugars are attached either to the amide nitrogen atom in the side chain of asparagine (termed an N-linkage) or to the oxygen atom in the side chain of serine or threonine (termed an O-linkage), as shown in Figure 11.18. An asparagine residue can accept an oligosaccharide only if the residue is part of an Asn-X-Ser or Asn-X-Thr sequence, in which X can be any residue. Thus, potential glycosylation sites can be detected within amino acid sequences. However, which of these potential sites is actually glycosylated depends on other aspects of the protein structure and on the cell type in which the protein is expressed. All N-linked oligosaccharides have in common a pentasaccharide core consisting of three mannose and two N-acetylglucosamine residues. Additional sugars are attached to this core to form the great variety of oligosaccharide patterns found in glycoproteins (Figure 11.19).
Figure 11.18. Glycosidic Bonds between Proteins and Carbohydrates.
Figure 11.18
Glycosidic Bonds between Proteins and Carbohydrates. A glycosidic bond links a carbohydrate to the side chain of asparagine (N-linked) or to the side chain of serine or threonine (O-linked). The glycosidic bonds are shown in red.
Figure 11.19. N -linked oligosaccharides.
Figure 11.19
N -linked oligosaccharides. A pentasaccharide core (shaded yellow) is common to all N-linked oligosaccharides and serves as the foundation for a wide variety of N-linked oligosaccharides, two of which are illustrated: (A) high-mannose type; (B) complex