Carbohydrate Metabolism III: Glycoproteins, Glycolipids, GPI Anchors, Proteoglycans, and Peptidoglycans
M an — P — G IcN A c
Proposed mechanism for the phosphorylation of high-mannose oligosaccharides on glycoproteins (primarily acid
hydrolases) being directed to lysosomes. The enzymes catalyzing the reactions are I, UDP-N-acetylglucosamine:
lysosomal enzyme N-acetylglucosaminyl-I -phosphotransferase and
R = GlcNAc/31
> 4GlcNAc —
- Asn. * Additional mannose residues that are phosphorylated in other isomers.
has no discernible effect on biological function. Informa-
tion regarding the function of carbohydrate side chains
has been obtained either after their removal from glyco-
proteins by glycosidases or by inhibition of some step in
the glycosylation pathway.
Most inhibitors affect Asn-linked glycoprotein syn-
thesis. Some have antiviral activity (tunicamycin, deoxy
sugars) or inhibit peptidoglycan synthesis in bacteria
(tunicamycin, streptovirudin); others are isolated from
plants that are toxic to humans or animals (swainsonine,
castanospermine). The range of substances includes in-
hibitors of dolichol phosphate synthesis, of initiation
or polymerization of the lipid-linked oligosaccharide, of
processing of the oligosaccharide after transfer to glyco-
protein, and of elongation of the processed side chain.
Inhibitors of dolichol phosphate formation may in-
terfere with the synthesis of polyprenyl diphosphate or
prevent recycling or phosphorylation of polyisoprenols.
Dolichols are synthesized from precursors of cholesterol
synthesis (Chapter 19). A regulatory enzyme in the precur-
sor synthesis is /l-hydroxy-/8-methylglutaryl-CoA (HMG-
CoA) reductase. Inhibition of HMG-CoA reductase by
25-hydroxycholesterol and drugs such as mevinolin pre-
vents synthesis of dolichol and inhibits glycosylation in
some systems. Cholesterol synthesis is also inhibited,
which is disruptive to cell function. Bacitracin inhibits
dephosphorylation of dolichol diphosphate, pre-
venting recycling of dolichol phosphate. Glycosylation
stops in the absence of dolichol phosphate.
tracin does not affect glycoprotein biosynthesis, presum-
ably because it does not enter cells.
The transfer of GlcNAc-P from UDP-GlcNAc to
dolichol phosphate is inhibited by tunicamycin, strep-
tovirudin, and amphomycin. Tunicamycin may be a com-
petitive inhibitor of GlcNAc-1-phosphate transferases
and inhibits the initial step in peptidoglycan synthesis and
in the synthesis of the oligosaccharide units linking tei-
choic acids to peptidoglycans. Initiation is also inhibited
by the antibiotics diumycin and (at high concentrations)
showdomycin. Diumycin has a greater inhibitory effect on
the synthesis of Man-P-Dol, whereas showdomycin blocks
formation of Glc-P-Dol. Both prevent polymerization of
the lipid-linked oligosaccharide.
Sugar analogues and deoxy sugar inhibitors include 2-
deoxy-D-glucose, 2-deoxy-2-fluoro-D-glucose, 2-deoxy-
2-amino-D-glucose, and 2-deoxy-2-fluoro-D-mannose, no
two of which have the same effect on glycosylation. Many
are metabolized to UDP- or GDP-sugars and may be incor-
porated into oligosaccharides, making addition of further
sugars impossible, thereby causing synthesis of truncated
oligosaccharides. Others may inhibit one or more glyco-
syltransferase, and more than one step is often affected.
Another class of inhibitors that affect processing or
“trimming” of the glucosylated, high-mannose side chains
have in common the ability to inhibit one or more of the
specific processing glycosidases. Deoxynojirimycin and