Chemistry Reference
In-Depth Information
Info Box 1
Similarities in the process of
N
-glycosylation in all three primary lineages of
life strongly suggest common founder events in which the Asn sequon was
defi ned and general principles were arranged.
N
- Glycosylated proteins in pro-
karyotic cells are rare nowadays, but they still exist in the plasma membrane
of Gram-negative bacteria and some Archaea (see Chapter 8.1). In prokaryotes,
diverse preassembled oligosaccharide precursors are externalized across the
cell membrane, further modifi ed and then used for polypeptide chain glycosyl-
ation. In the eukaryotes, however, glycan diversity is created on the glycopro-
teins themselves, liberating epigenetic pressure on the LLO precursors.
Therefore, intracellular
N
-glycans could be structurally conserved, and they
have become the basis for the evolution of additional functions.
The bipartite pathway for synthesizing LLOs was already established in cells
in which synthesis of the cytosolic intermediates could use soluble donor
substrates. For the extracellular extension of the pentamannosyl LLO, non-
diffusible membrane- anchored donor substrates (Dol - PP - Man/Glc) had to be
utilized to prevent their loss by diffusion into the periplasmic space. Thus,
Glc
1
Man
(7 - 9)
GlcNAc
2
-PP-Dol has to be considered as the archetypical LLO,
already established before the luminal organization of the rough ER evolved
from the periplasmic space. The sole use of a sugar nucleotide inside the ER
(UDP-Glc) by UGGG1 is an exception that proves the rule, insofar as this
process evolved long after compartmentalization had been introduced. The
evolutionary origin of the Golgi apparatus is still a mystery, but the use of acti-
vated small compounds as donor substrates points to a late appearance of this
organelle. Its structural and functional relation with microtubules or actin fi la-
ments suggests a time point long after cell polarity was established.
Figure 6.2
Illustration of the biosynthetic and
catabolic ER pathway of the
N
- glycan precursor
Glc
3
Man
9
GlcNAc
2
. (a) Enzymes are named
according to their primary entry name in Swiss-
Prot ( www.expasy.org). Transferases are shown
in light-gray boxes and glycosidases in darker
gray boxes. Monospecifi c enzymes (middle
gray) act on defi ned positions. Polyspecifi c
enzymes (darker gray) hydrolyze specifi c glyco-
sidic linkages at different positions along the
N
-glycan. Enzymatic reactions (arrows) are
named according to the enzyme commission
( www.brenda.org ). Preformed
N
- glycan precur-
sors serve as acceptor and activated sugar
nucleotides as donor substrates for glycosyl-
transferase reactions. (b) Detailed view of the
GCS2
AB
- UGGG1 de - and reglycosylation cycle
(dashed box B in C). Glycoprotein
UMP by ENTP5 and exported from the lumen
of the ER (see Figure 6.3b). Inset in (b) shows
the different human ectonucleoside diphospha-
tases in the lumen of ER and Golgi apparatus.
(c) All glycosyltransferases (light-gray boxes)
and glycosidases (middle-dark gray boxes)
engaged in the
N
- glycan biosynthesis and trim-
ming reactions are mentioned. Note that for
the biosynthesis, the scheme has to be read
from right to left. It starts with the transfer of
GlcNAc to Dol-PP by ALG7-DPAGT1. The left
inset indicates the enzymes for Dol-PP mono-
saccharide synthesis, the donor substrate for
ER-luminal transferases. After transfer to
polypeptide, the precursor oligosaccharide is
trimmed by mono- (middle-gray boxes) and
poly - specifi c (dark - gray boxes) glycosidases.
All glycosidases are exoglycosidases with the
exception of MANEA and NGLY1. Gray arrows
1 - 2 - gluco-
sidase cleaves glucose, whereas UDP-glucose:
α
