Biomedical Engineering Reference
In-Depth Information
calorimetry experiments are suitable for comparative studies of different glycans but do not
lead to accurate calculation of affinity constants (Ahmad et al., 2004b). Another way to
determine the direct interaction of soluble galectins and glycans is the use of
hemagglutination assays, but those are limited to multivalent glycans or the inhibition of
interactions between galectins and multivalent glycans or erythrocytes (Ahmad et al., 2004a;
Ahmad et al., 2004b; Appukuttan, 2002; Giguere et al., 2008).
3.2 Determined fine specificity of galectin-1, -3 and the two galectin-8 CRDs
Although the general fold of all galectins is highly conserved, single galectins are
characterised by specific binding interactions with single carbohydrate ligands. Differences
in fine specificity have been analysed using different binding assays (as mentioned above).
Moreover extensive theoretical evaluation of the putative interactions between single amino
acids and functional groups of the bound glycan has been done by modelling and
calculation. Some specific ligands with high affinity for the single galectin CRDs are
mentioned in Table 1.
The recognition of galactose is common for all galectins but the interaction with the
monosaccharide alone is very weak (Carlsson et al., 2007; Knibbs et al., 1993; Salameh et al.,
2010). Disaccharides containing galactose β-glycosidic bound to GlcNAc, Glc or GalNAc are
bound with significantly increased affinities. Different galectins thereby show high affinity
to specific disaccharides. Galectin-3, galectin-1 and the C-terminal CRD of galectin-8 bind
preferentially LacNAc units of type I and type II while the N-terminal CRD of galectin-8
shows highest affinity for lactose (Carlsson et al., 2007; Ideo et al., 2011; Salomonsson et al.,
2010).
Extensions of the bound galactose moiety effect glycan binding in dependence on the galectin.
Galectin-3 tolerates due to its enlarged binding pocket extensions at the galactose 3`-OH-group
for example repetitive LacNAc (type II) -structures (poly-LacNAc), showing even higher
affinities for repetitive LacNAc structures compared to single LacNAc units (Hirabayashi et
al., 2002; Rapoport et al., 2008; Salomonsson et al., 2010). In contrast galectin-1 recognises
single LacNAc units presented at the non-reducing terminus of glycans not showing
preference for extended poly-LacNAc glycans (Leppänen et al., 2005). Most authors agree that
galectin-1 is not able to bind internal galactose moieties in poly-LacNAc-glycans (GlcNAc-β3-
Gal
-β4-GlcNAc) (Leppänen et al., 2005; Stowell et al., 2004; Stowell et al., 2008a) but
depending on the assay set-up some publications report affinity to this sugar unit (Di Virgilio
et al., 1999; Zhou & Cummings, 1993). These different results prove the importance of
evaluation of the test set-up and critical examination of the measured binding data.
Other extensions at the 3`-OH-group of galactose such as sulphate or neuraminic acid increase
the affinity of galectin-3, galectin-1 and especially galectin-8 N-CRD to the core disaccharide
(Carlsson et al., 2007; Sörme et al., 2002; Stowell et al., 2008a). In contrast the C-terminal
galectin-8 domain fails to bind 3`-sulfated or 3`-sialylated galactose (Ideo et al., 2003).
Modification at the 6`-OH-group for example with neuraminic acid reduces binding of all
four discussed galectin CRDs (Ideo et al., 2003; Stowell et al., 2008a). Therefore α6-sialylation
is discussed as regulatory modification for galectin-mediated functions (Zhuo & Bellis,
2011). Galectin-3 and galectin-8 C-CRD show high affinity for blood-group antigens
(Hirabayashi et al., 2002; Yamamoto et al., 2008)
