Oligosaccharide-protein interactions: a three-dimensional view

For carbohydrates to serve as recognition elements in cellular function, there must be 'receptors' which are capable of distinguishing between the multitude of oligosaccharide structures generated by a cell. Generally these receptors are assumed to be proteins, and the plant lectins have been used as model systems to examine the molecular basis for specificity in such interactions. Three aspects of the specificity of oligosaccharide-protein interactions will be discussed: (1) the conformational flexibility of oligosaccharides will be demonstrated through a quantitative analysis of nuclear magnetic resonance measurements; (2) a comparison of the measured and calculated values for the entropy barrier to oligosaccharide binding will be used to argue that the barrier arises from a loss of this conformational flexibility upon binding to the lectin (this conclusion is also supported by X-ray crystallographic studies); and (3) the thermodynamic model can be extended to the binding of glycoproteins to receptors and the high affinity of these interactions explained by either multivalency or fixation of the oligosaccharide in the 'correct' three-dimensional structure through interaction with the protein moiety.