Combining Sites

Information about the combining sites of lectins comes from X-ray crystallographic studies of their complexes with ligands, from binding experiments with sugars and their derivatives, from site-directed mutagenesis, and to a limited extent also from molecular modelling. Such studies have shown that like the lectins themselves, the sites are diverse, even when their specificity is the same, although within a given lectin family the sites may be similar (Sharon, 1993). The sites appear to be preformed, since few conformational changes occur upon ligand binding; recently however evidence for their possible plasticity has been presented (Swaminathan, C. P. et al., 2000). They are mostly in the form of shallow depressions on the surface of the protein, where typically only one or two edges or faces of the ligand are bound, and are thus similar to those of anti-carbohydrate antibodies or glycosidases such as lysozyme (Kabat, 1962; Quiocho, 1993; Bundle, 1998). In a few lectins the combining sites are however in the form of deep clefts.