Glycoproteins Contain Several Types Of Oglycosidic Linkages

At least four subclasses of O-glycosidic linkages are found in human glycoproteins: (1) The GalNAc-Ser(Thr) linkage shown in Figure 47-1 is the predominant linkage. Two typical oligosaccharide chains found in members of this subclass are shown in Figure 47-2. Usually a Gal or a NeuAc residue is attached to the GalNAc, but many variations in the sugar compositions and lengths of such oligosaccharide chains are found. This type of linkage is found in mucins (see below). (2) Proteoglycans contain a Gal-Gal-Xyl-Ser trisac-charide (the so-called link trisaccharide). (3) Collagens contain a Gal-hydroxylysine (Hyl) linkage. (Subclasses [2] and [3] are discussed further in Chapter 48.) (4) Many nuclear proteins (eg, certain transcription factors) and cytosolic proteins contain side chains consisting of a single GlcNAc attached to a serine or threo-nine residue (GlcNAc-Ser[Thr]).

Table 47-7. Three plant lectins and the sugars with which they interact.1

Lectin

Abbreviation

Sugars

Concanavalin A Soybean lectin Wheat germ agglutinin

ConA WGA

Man and Glc Gal and GalNAc Glc and NeuAc

1In most cases, lectins show specificity for the anomeric nature of the glycosidic linkage (a or |); this is not indicated in the table.

1In most cases, lectins show specificity for the anomeric nature of the glycosidic linkage (a or |); this is not indicated in the table.

ch2oh I

(Protein)

' Glycine ) Ethanolamine

Mannose

CH2OH

OH H

' Glycine ) Ethanolamine

Mannose

PI-PLC

Mannose Mannose j Glucosamine Inositol

Plasma membrane

Figure 47-1. Depictions of (A) an O-linkage (^-acetylgalactosamine to serine); (B) an N-linkage (W-acetylglu-cosamine to asparagine) and (C) a glycosylphosphatidylinositol (GPI) linkage. The GPI structure shown is that linking acetylcholinesterase to the plasma membrane of the human red blood cell. The carboxyl terminal amino acid is glycine joined in amide linkage via its COOH group to the NH2 group of phosphorylethanolamine, which in turn is joined to a mannose residue. The core glycan contains three mannose and one glucosamine residues. The glucosamine is linked to inositol, which is attached to phosphatidic acid. The site of action of PI-phospholi-pase C (PI-PLC) is indicated. The structure of the core glycan is shown in the text. This particular GPI contains an extra fatty acid attached to inositol and also an extra phosphorylethanolamine moiety attached to the middle of the three mannose residues. Variations found among different GPI structures include the identity of the carboxyl terminal amino acid, the molecules attached to the mannose residues, and the precise nature of the lipid moiety.

GalNAc

GalNAc fa 2,6 NeuAc

Ser(Thr)

Ser(Thr)

Figure 47-2. Structures of two O-linked oligosaccharides found in (A) submaxillary mucins and (B) fe-tuin and in the sialoglycoprotein of the membrane of human red blood cells. (Modified and reproduced, with permission, from Lennarz WJ: The Biochemistry of Glycoproteins and Proteoglycans. Plenum Press, 1980.)

Diabetes 2

Diabetes 2

Diabetes is a disease that affects the way your body uses food. Normally, your body converts sugars, starches and other foods into a form of sugar called glucose. Your body uses glucose for fuel. The cells receive the glucose through the bloodstream. They then use insulin a hormone made by the pancreas to absorb the glucose, convert it into energy, and either use it or store it for later use. Learn more...

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