Page 62 - Academic Press Encyclopedia of Physical Science and Technology 3rd BioChemistry

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Encyclopedia of Physical Science and Technology EN006P-81 June 29, 2001 21:48

846 Glycoconjugates and Carbohydrates

5 GDP
5 GDP-

α(1,2)
α(1,2)
UDP
CYTOSOL
α(1,3) α(1,6)
UDP-
β(1,4)
UMP
β(1,4) β(1,4)
Tunicamycin
GDP UDP
UDP-
Amphomycin
P P P
Translocation GDP- UDP-
P P P P P P
Dol
ER Membrane P P P P
P P
(x 4) (x 3)
LUMEN OF ER β(1,4) β(1,4)
β(1,4) β(1,4)
α(1,6) α(1,3) α(1,6) α(1,3)
= GlcNAc
α(1,6) α(1,3) α(1,2)
= Mannose α(1,2)
= Glucose α(1,2) α(1,2) α(1,2) α(1,2)
α(1,3)
α(1,3)
α(1,2)

FIGURE 26 Biosynthesis of the 14-sugar, lipid-linked oligosaccharide, the universal precursor for N-linked
glycosylation.

varies widely, from single sugars to extended polysac- lion with a hundred or more saccharide units covalently
charide chains. The following discussion highlights key attached.
features of these types but is not intended to provide full Glycosylation of tyrosine residues is unusual but a key
details. step in the biosynthesis of glycogen, the major storage
Onemajorcategoryof O-linkedglycosylationistermed glucan of liver and muscle. The core protein, glycogenin,
mucin type. This is characterized by linkage of the sugar is able to autoglucosylate and attaches a series of glucosyl
(N-acetylgalactosamine in the alpha configuration) to ser- residues to a single tyrosine in the protein. When the glu-
ine or threonine hydroxyl groups (Fig. 27). There is no cose chain has reached four (or more) units (all linked
identifiable consensus amino acid sequence known which α-1-4), the resulting saccharide moiety is then recognized
targets specific residues to be substituted. The saccha- by glycogen synthase for continuation of glycogen forma-
ride units range from di- to intermediate size oligosaccha- tion. The final polysaccharide may have several thousand
rides (up to 10 sugars) and are very diverse. Additional glucose residues.
sugars present include galactose, N-acetylglucosamine, Currently about 20 proteins have been identified as col-
L-fucose, and sialic acid; some of the saccharide units lagens. Criteria for this classification include the pres-
may be sulfated. Mannose is characteristically absent. ence of a triple helical domain (“collagen helix”) and the
These molecules are often found in epithelial secretions; presence of hydroxyproline and hydroxylysine residues.
the protein cores may be quite large with a single glyco- The latter may also be glycosylated with either a sin-
protein having an aggregate molecular weight of one mil- gle galactose residue or a disaccharide (glucopyranosyl