Lectins: Analytical Tools from Nature

This chapter introduces lectins and their analytical techniques. Lectins are proteins of non-immune origin that recognize and bind to specific carbohydrate structural epitopes without modifying them. This group of carbohydrate-binding proteins function as central mediators of information transfer in biological systems and perform their duties by interacting with glycoproteins, glycolipids, and oligosaccharides. Whether extracted from natural sources or expressed in cell cultures, lectins provide models for the study of protein-carbohydrate interactions and exquisite tools for the analysis of carbohydrates, in either free form or bound to lipids or proteins. Lectins can be identified based on functional assays (for instance, hemagglutination) or by amino acid sequence homology (putative lectins) with known lectin sequences. Only a small fraction of the lectins that have been discovered to date have been carefully characterized with respect to protein structure, binding affinities for extended carbohydrates, binding thermodynamics, and other properties. The discovery of new lectins in biological systems means that reliable techniques for determining lectin properties are needed. Because of the complex nature of protein-carbohydrate interactions, no single technique can provide universal characterization. X-ray crystallography of purified lectins in complex with saccharides can provide high-resolution structural data and a visual tool to probe protein-carbohydrate interactions. NMR of lectin-carbohydrate complexes has been proven to be a useful alternative technique for three-dimensional structure determination. Further characterization of the specificity and energetics of the binding site in solution can be accomplished elegantly with isothermal calorimetry.