The oligosaccharide components of glycoproteins are generally branched heteropolymers composed primarily of D-hexoses, with the addition in some cases of neuraminic acid (a nonose) and of L-fucose, a 6-deoxyhexose.
OLIGOSACCHARIDE STRUCTURE
The oligosaccharide
components of glycoproteins are generally branched heteropolymers composed
primarily of D-hexoses, with the addition in some cases of neuraminic acid (a
nonose) and of L-fucose, a 6-deoxyhexose.
The oligosaccharide may
be attached to the protein through an N- or an O-glycosidic link. In the former
case, the sugar chain is attached to the amide group of an asparagine side
chain and, in the latter case, to the hydroxyl group of either a serine or
threonine side chain. [Note: In the case of collagen, there is an O-glycosidic
linkage between galactose or glucose and the hydroxyl group of hydroxylysine.]
A glycoprotein may
contain only one type of glycosidic linkage (N- or O-linked) or may have both
types within the same molecule.
1. O-Linked oligosaccharides: The O-linked oligosaccharides may
have one or more of a wide variety of sugars arranged in either a linear or a
branched pattern. Many O-linked oligosaccharides are found in extracellular
glycoproteins or as membrane glycoprotein components. For example, O-linked
oligosaccharides on the surface of red blood cells help provide the ABO blood
group determinants.
2. N-linked oligosaccharides: The N-linked oligosaccharides fall
into two broad classes: complex oligosaccharides and high-mannose
oligosaccharides. Both contain the same pentasaccharide core shown in Figure
14.14, but the complex oligosaccharides contain a diverse group of additional
sugars, for example, GlcNAc, GalNAc, L-fucose, and NANA, whereas the
high-mannose oligosaccharides contain primarily mannose.
Figure 14.14 Complex (top) and
high-mannose (bottom) N-linked oligosaccharides. [Note: Members of each class
contain the same pentasaccharide core.] NANA = N-acetylneuraminic acid; Gal =
galactose; GlcNAc = N-acetylglucosamine; Man = mannose; Fuc = fucose; Asn =
asparagine.
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