Chemistry Reference
In-Depth Information
Figure 1.7
Illustration of phosphorylated
(phosphated) and sulfated (sulfurylated) glycan
' words ' . 6 - Phosphorylation of a mannose moiety
(in the context of a mannose-rich pentasaccha-
ride) is the key section of a routing signal in
lysosomal enzymes (a), 4-sulfation of the
GalNAc
β
1 - 4GlcNAc (LacdiNAc) epitope forms
the 'postal code' for clearance from circulation
by hepatic endothelial cells of pituitary glyco-
protein hormones labeled in such a manner (b),
the HNK (human natural killer)-1 epitope (3-
sulfated GlcA
β
1 - 3Gal
β
1 - 4GlcNAc) is involved
in cell adhesion/migration in the nervous system
(c) and the encircled 3-
O
- sulfation in the penta-
saccharide's center is essential for heparin's
anticoagulant activity (d). All sugars are in their
pyranose form. Please note that the central
GlcN unit has
N,O
-trisulfation and that the 2-
sulfated IdoA, given in the
1
C
4
conformation,
can also adopt the hinge-like
2
S
O
skew - boat
structure (please see Figure 1.6; about 60% or
more for the
2
S
O
form in equilibrium depending
on the structural context) when present within
glycosaminoglycan chains of the proteoglycan
heparin. 2-Sulfation of IdoA serves two pur-
poses: favoring the hinge-like
2
S
O
conformation
and precluding reconversion to GlcA.
time [5, 9], and its structural and functional aspects are the topic of the following
chapters. That said, we have built the evidence for the following conclusions.
1.4
Conclusions
'Carbohydrates are ideal for generating compact units with explicit informational
properties, since the permutations on linkages are larger than can be achieved by
amino acids, and, uniquely in biological polymers, branching is possible. More-
over, the oligosaccharide units are not fl exible but exhibit highly specifi c structures
with only limited degrees of freedom' [10]. This statement highlights that the sugar
code has a third dimension. What this means is explained in the next chapter and
its relevance for protein - carbohydrate interactions is outlined in Chapter 13 .
