Biomedical Engineering Reference
In-Depth Information
1-O-4
M - M
GlcNAc - M
M
M - GlcNAc - GlcNAc - Asn
M - GlcNAc - GlcNAc - Asn
1-O-6
M - M
Glc - Glc - Glc - M - M - M
GlcNAc - M
Fucosyl-transferase
GlcNAc - M
Glycosidases
M - GlcNAc - GlcNAc - Asn
|
F
M - M
M
GlcNAc - M
M - GlcNAc - GlcNAc - Asn
M - M
M - M - M
Fucosyl-transferase
GlcNAc
M
Mannocosidases
M - GlcNAc - GlcNAc - Asn
|
F
M
GlcNAc
M
M
M - GlcNAc - GlcNAc - Asn
M
GlcNAc
M
or
GlcNAc-transferase I
GlcNAc
M
M
M
M - GlcNAc - GlcNAc - Asn
|
F
GlcNAc
M - GlcNAc - GlcNAc - Asn
M
GlcNAc - M
GlcNAc - M
-mannosidase II
GlcNAc-
transferase II
Galactosyl-transferase
Gal - GlcNAc
M
M
M - GlcNAc - GlcNAc - Asn
M - GlcNAc - GlcNAc - Asn
|
F
Gal - GlcNAc
GlcNAc - M
Gal - GlcNAc - M
GlcNAc-
transferase II
Fucosyl-transferase
-2,6-sialyl-transferase
-2,3-sialyl-transferase
M
M - GlcNAc - GlcNAc - Asn
|
F
Sial - Gal - GlcNAc
M
GlcNAc - M
GlcNAc-ase
M - GlcNAc - GlcNAc - Asn
|
F
Sial - Gal - GlcNAc
M
Sial - Gal - GlcNAc - M
M - GlcNAc - GlcNAc - Asn
|
F
or isomers
M
-1,6-mannosidase II
M
M - GlcNAc - GlcNAc - Asn
|
F
FIGURE 10.11
Example of a N-linked glycosylation pathway (Glc
¼
glucose residue, M
¼
mannose residue,
GlcNAc
sialic acid residue).
The oligosaccharide side-chain is bound to an asparagine (Asn) of the protein. The upper (and main) arms are
connected via
a
-1-O-4 connection and the lower ones via
a
-1-O-6. The GlcNAc-ase step is important in insect cells
but not mammalian cells.
¼
N-acetylglucosamine residue, F
¼
fucose residue, Gal
¼
galactose residue, Sial
¼
display a range of glycoforms; a single form is not observed. A simple sequence of three
amino acids, of which asparagine must be one, is required for attachment of N-linked sugars
and amino sugars. The sequence at the attachment site is Asn-Xaa-Ser/Thr, where Xaa is any
amino acid and the third amino acid in the sequence must be serine or threonine.
The process of N-linked glycosylation begins in the ER, where a preformed branched
oligosaccharide (the dolichol pyrophosphate-oligosaccharide) with 14 sugars is transferred to
the amino group of asparagine. The 14-sugar residue is first “trimmed” by a set of specific
glycosidases. In yeast, oligosaccharide processing often stops in the ER, leading to simple gly-
coforms (or high mannose or oligomannose forms). The initial trimming takes place in the ER,
followed by transfer to the Golgi apparatus where final trimming occurs, followed by addi-
tion of various sugars or amino sugars. These units are added through the action of various
glycosyltransferases using nucleotide-sugar cosubstrates as sugar donors. In insect cells, high
levels of N-acetylglucosaminidase activity results typically in dead-end structures with
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