Conformational Aspects

Due to the many functions which have been associated with the different types sugar sidechains, like cell-cell recognition, protein folding (e.g. together with calnexin (Bergeron 1994 and ref. therein), proteolysis protection, antigenicity, only to name a few, one has tried to investigate the 3D structure of the sugar sidechains of proteins by X-ray analysis and molecular modelling, but so far no general scheme (code) for the oligosaccharide chains has been discovered. But Molecular Modelling has revealed that for the Man-alpha1,6 arm, with its higher degree of flexibility, only two conformations are generally available, leading to certain constraints in the overall conformation of the sugar chain. One of the important factors in determining the three dimensional structure and therefore the amount of interaction with other molecules, certainly is the formation of hydrogen bonds:

Interactions between carbohydrate binding proteins (lectins, selectins, glycosyltransferases, glycosidases), which have been found so far and which can often differentiate between even subtle differences in the sidechains, can mostly be ascribed to hydrogen bonding. The hydrogen bonds through the sugar hydroxyl groups are characteristic for each type of monosaccharide yielding very specific templates for interaction with other structures. Recent studies have shown that the interaction of carbohydrates is not limited to proteins but they can also interact with each other. This has been found for the LeX epitope, which interacts with LeX trisaccharide sequences on other cells (Eggens 1989) and for the carbohydrate chains of the Fc parts of IgG (Dwek et al. 1991).

Besides these hydrophilic effects mediated by the numerous possibilities for hydrogen bonds and which can also increase the solubility of glycoproteins, hydrophobic interactions are also possible. These can occur via N-/O-acetyl or O-methyl groups. Such hydrophobic binding effects have been associated with some lectins. (Sharon 1989, Quicho 1986) (For more information about the different types of lectins and their requirements for divalent metal ions see also Drickamer 1988 and Powell 1995 and references therein.)

Finally, the posttranslational modifications leading to sulfation and/or phosphorylation introduce charged groups into the oligosaccharide chain adding up to the overall charge of the chain which is otherwise solely determined by the number of sialic acid residues present. Therefore ionic interactions seem also to play an important role in determining the results of the contact with other molecules.

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Written by: Christian Frosch

frosch@mzdmza.zdv.uni-mainz.de

http://www.uni-mainz.de/~frosc000

Institute of Toxicology

University of Mainz

Last update: 10.07.1995