Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

TRANSPORT FROM THE ER THROUGH THE GOLGI apparaTUS
719
N-LINKED GLYCOSYLATION
protein backbone
C
O-LINKED GLYCOSYLATION
protein backbone
C
Figure 13–32 N- and O-linked
glycosylation. In each case, only the single
sugar group that is directly attached to the
protein chain is shown.
CH 2
C O
Asparagine
H 3 C
CH
O
Threonine
NH
HOH 2 C
H O
H N-acetylglucosamine
OH H
O H
H NHCOCH 3
remainder of
oligosaccharide
side chain
CH 2 OH
O O
H
N-acetylgalactosamine
OH H
H H
H NHCOCH 3
remainder of
oligosaccharide side chain
or, in some cases—such as collagens—to hydroxylated proline and lysine side
chains. This O-linked glycosylation (Figure 13–32), like the extension of N-linked
oligosaccharide chains, is catalyzed by a series of glycosyl transferase enzymes
that use the sugar nucleotides MBoC6 in m13.32/13.32
the lumen of the Golgi apparatus to add sugars
to a protein one at a time. Usually, N-acetylgalactosamine is added first, followed
by a variable number of additional sugars, ranging from just a few to 10 or more.
The Golgi apparatus confers the heaviest O-linked glycosylation of all on
mucins, the glycoproteins in mucus secretions, and on proteoglycan core proteins,
which it modifies to produce proteoglycans. As discussed in Chapter 19,
this process involves the polymerization of one or more glycosaminoglycan chains
(long, unbranched polymers composed of repeating disaccharide units; see Figure
19–35) onto serines on a core protein. Many proteoglycans are secreted and
become components of the extracellular matrix, while others remain anchored
to the extracellular face of the plasma membrane. Still others form a major component
of slimy materials, such as the mucus that is secreted to form a protective
coating on the surface of many epithelia.
The sugars incorporated into glycosaminoglycans are heavily sulfated in the
Golgi apparatus immediately after these polymers are made, thus adding a significant
portion of their characteristically large negative charge. Some tyrosines
in proteins also become sulfated shortly before they exit from the Golgi apparatus.
In both cases, the sulfation depends on the sulfate donor 3ʹ-phosphoadenosine-5ʹ-phosphosulfate
(PAPS) (Figure 13–33), which is transported from the cytosol
into the lumen of the trans Golgi network.
What Is the Purpose of Glycosylation?
There is an important difference between the construction of an oligosaccharide
and the synthesis of other macromolecules such as DNA, RNA, and protein.
Whereas nucleic acids and proteins are copied from a template in a repeated
series of identical steps using the same enzyme or set of enzymes, complex carbohydrates
require a different enzyme at each step, each product being recognized
as the exclusive substrate for the next enzyme in the series. The vast abundance
of glycoproteins and the complicated pathways that have evolved to synthesize
them emphasize that the oligosaccharides on glycoproteins and glycosphingolipids
have very important functions.
N-linked glycosylation, for example, is prevalent in all eukaryotes, including
yeasts. N-linked oligosaccharides also occur in a very similar form in archaeal
cell wall proteins, suggesting that the whole machinery required for their synthesis
is evolutionarily ancient. N-linked glycosylation promotes protein folding in
two ways. First, it has a direct role in making folding intermediates more soluble,
thereby preventing their aggregation. Second, the sequential modifications of the
N-linked oligosaccharide establish a “glyco-code” that marks the progression of
O CH 2 P
N
N
H N N H O O
O O S O –
NH 2
H
H
HO
H
O
O – O
– O
P
O –
3′-phosphoadenosine-5′-phosphosulfate
(PAPS)
Figure 13–33 The structure of PAPS.
O