NMR Analysis of Carbohydrate-Protein Interactions

Carbohydrate-protein interactions are frequently characterized by dissociation constants in the μM to mM range. This is normally associated with fast dissociation rates of the corresponding complexes, in turn leading to fast exchange on the nuclear magnetic resonance (NMR) chemical shift time scale and on the NMR relaxation time scale. Therefore, NMR experiments that take advantage of fast exchange are well suited to study carbohydrate-protein interactions. In general, it is possible to analyze ligand binding by observing either protein signals or ligand resonances. Because most receptor proteins to which carbohydrates bind are rather large with molecular weights significantly exceeding 30 kDa, the analysis of the corresponding protein spectra is not trivial, and only very few studies have been addressing this issue so far. We, therefore, focus on NMR experiments that employ observation of free ligand, that is, carbohydrate signals to analyze the bound state. Two types of NMR experiments have been extremely valuable to analyze carbohydrate-protein interactions at atomic resolution. Whereas transferred nuclear Overhauser effect (NOE) experiments deliver bioactive conformations of carbohydrates binding to proteins, saturation transfer difference (STD) NMR spectra provide binding epitopes and valuable information about the binding thermodynamics and kinetics. We demonstrate the power of a combined transfer NOE/STD NMR approach for the analysis of carbohydrate-protein complexes using selected examples.