Chemistry Reference
In-Depth Information
summarized in Figure 20.2. This shows the variation in the number and order of
CTLD and non-CTLD protein domains in multidomain CTLDcps, as well as
whether they are secreted or anchored in the membrane.
20.3
Mechanism of Carbohydrate Binding
In this section we will examine how crystal structures of CTLDs with bound sugar
ligands have revealed a general molecular mechanism of carbohydrate binding at
the main binding site. Specifi cally we will look at the roles of the Ca
2+
and two
groups of residues (the '
E
P
N
' / '
Q
P
D
' and ' W
N
D' motifs) in forming the binding
site that interacts with the sugar, and at how the
E
P
N
and
Q
P
D
motifs discriminate
mannose-type and galactose-type sugars, respectively. We confi ne our attention to
this main site but note that some C-type lectins, such as the vertebrate group IV
selectins (Chapters 19 and 27), bind additional monosaccharide units of oligosac-
charide ligands such as the sialylated and sulfated Lewis
x
epitope (see Tables 7.4
and 27.2 for structures) at auxiliary sites of the CTLD (see Figure 16.1 h). We also
note that it is this site that has been modifi ed in many non - carbohydrate - binding
CTLDs to select other ligands (protein, ice, calcium carbonate), reinforcing our
developing view that this site is unusually adaptable by evolution.
The architecture of the primary monosaccharide-binding site for mannose- type
ligands is illustrated in Figure 20.3a. This is based on the crystal structure for the
CTLD of rat MBP-A in complex with the
N
- glycan Man
6
- GlcNAc
2
- Asn; see Chap-
ters 6 and 8 for
N
-glycan structures and Figure 1.6 for structures of monosaccha-
rides. The complex is stabilized by a network of coordination and hydrogen bonds.
Oxygen atoms from the 3
-hydroxyls of the mannose form two coordination
bonds with the Ca
2+
ion and four hydrogen bonds with residues - Glu185 and
Asn187 (
E
P
N
), Asn205 (W
N
D) and Glu193 - whose carbonyl side - chains coordi-
nate the Ca
2+
-binding site. This bonding pattern is fundamental for CTLD/Ca
2+
/
monosaccharide complexes, and is observed in all known structures. The
E
P
N
and
W
N
D motifs are in the long-loop region and
′
and 4
′
4 strand of the CTLD structure,
respectively (Figure 20.1). Asp206 of the W
N
D motif contributes another Ca
2+
coordination bond, while the Trp204 residue is highly conserved and contributes
to the hydrophobic core [1, 4].
The arrangement of the hydrogen-bond donors and acceptors and coordination
bonds in the binding site, as summarized in Figure 20.3b, has two important
features. First, it determines the overall positioning and orientation of the sugar
in the binding site. However, as shown in Figure 20.3b, the site has a 2-fold
symmetry axis relating the sugar hydroxyls which would allow the sugar to be
rotated by 180 ° without introducing any changes to the bonding scheme. Indeed,
examples are now known. The structure of the rat MBP-C complex with mannose
showed this hexapyranose bound in the opposite orientation. Also, structures
of a galactose-binding mutant of MBP-A and CEL-I, a C-type lectin from the
echinoderm sea cucumber, showed galactose bound in the opposite orientation
β
