A statistical analysis of N- and O-glycan linkage conformations from crystallographic data

We have generated a database of 639 glycosidic linkage structures by an exh austive survey of the available crystallographic data for isolated oligosac charides, glycoproteins, and glycan-binding proteins. For isolated oligosac charides there is relatively little crystallographic data available, A much larger number of glycoprotein and glycan-binding protein structures have n ow been solved in which two or more linked monosaccharides can be resolved. In the majority of these cases, only a few residues can be seen. Using the 639 glycosidic linkage structures, we have identified one or more distinct conformers for all the linkages, The O5-C1-O-C(x)' torsion angles for all these distinct conformers appear to be determined chiefly by the exo-anomer ic effect. The Man alpha 1-6Man linkage appears to be less restrained than the others, showing a wide degree of dispersion outside the ranges of the d efined conformers. The identification of distinct conformers for glycosidic linkages allows "average" glycan structures to be modeled and also allows the easy identification of distorted glycosidic linkages; Such an analysis shows that the interactions between IgG Fc and its own N-linked glycan resu lt in severe distortion of the terminal Gal beta 1-4GlcNAc linkage only, in dicating the strong interactions that must be present between the Gal resid ue and the protein surface. The applicability of this crystallographic base d analysis to glycan structures in solution is discussed. This database of linkage structures should be a very useful reference tool in three-dimensio nal structure determinations.