Structural Studies on the Carbohydrate Chains of Glycoproteins

Abstract

In the past 10 years, the approach of the structure elucidation of carbohydrates and glycoconjugates has changed considerably. The availability of advanced physical techniques has now led to many applications in the carbohydrate field. For the carbohydrate chains of glycoproteins this development rendered possible determinations of numerous new primary structures. Furthermore, many details were disclosed of carbohydrate-microheterogeneity as a general feature. The introduction of high-resolution lH- NMR spectroscopy for these purposes has shown to be extremely useful. For the asparagine-linked carbohydrates it turns out to be possible to define the members of the oligomannose type of compounds occurring in higher organisms, by lH- NMR parameters merely. The N-acetyllactosamine type of chains can be easily recognized as to the type of branching of the core mannotriose on the basis of the lH-NMR spectra. In a vast number of cases, the details of t he internal and peripheral parts also can be characterized in this way. However, the repeating elements like in polylactosamine type of structures are much more difficult to describe in terms of NMR spectroscopic data (1). For the carbohydrates which are O-linked to the polypeptide backbone through serine or threonine, a wide variety in structures exists. For these compounds a large collection of lH- NMR data could be obtained only recently (2). It turns out that in this case too the variability can adequately be defined in structural terms in cases when differences in sugar composition are involved . The location of branching points and the identity of carbohydrate chains can be elucidated. However, it may be clear that the occurrence of repeating units may complicate the picture. To gain insight into the functioning of carbohydrate chains i t is essential to get information on the spatial structure of such chains in solution. It is particularly relevant to know for glycoproteins which part of the carbohydrate chain is to exposed to the teins which part of the carbohydrate chain is exposed to the solution and thereby available for interaction and recognition processes . The influence of the protein part on the presentation of the carbohydrate chains to the solutions till needs further study .