Abstract
Linear polysaccharides are typically composed of repeating mono- or disaccharide units and are ubiquitous among living organisms. Polysaccharide diversity arises from chain-length variation, branching, and additional modifications. Structural diversity is associated with various physiological functions, which are often regulated by cognate polysaccharide-binding proteins. Proteins that interact with linear polysaccharides have been identified or developed, such as galectins and polysaccharide-specific antibodies, respectively. Currently, data is accumulating on the three-dimensional structure of polysaccharide-binding proteins. These proteins are classified into two types: exo-type and endo-type. The former group specifically interacts with the terminal units of polysaccharides, whereas the latter with internal units. In this review, we describe the structural aspects of exo-type and endo-type protein-polysaccharide interactions. Further, we discuss the structural basis for affinity and specificity enhancement in the face of inherently weak binding interactions.
Highlights
Compared with protein–protein interactions, the binding between proteins and individual, simple carbohydrate ligands is weak
Further analyses of the structure of polysialic acid-binding proteins in complex with their ligands and their dynamics will help to clarify the conformation of polysialic acid chains
Data is currently accumulating on the three-dimensional structure of polysaccharide-binding proteins, showing how the proteins bind to the carbohydrate ligands
Summary
Compared with protein–protein interactions, the binding between proteins and individual, simple carbohydrate ligands is weak. Exo-type lectins interact with terminal units of polysaccharides, and bind glycan chains in partially sealed clefts. Endo-type lectins enhance binding affinity to polysaccharide ligands by three mechanisms: (1) Multiple-site interactions; (2) Repeated binding; (3) Recognition of ordered/higher-ordered polysaccharide structure. The apparent affinity is enhanced by slowing the dissociation of the protein This mechanism can be identified by kinetic analysis, using techniques such as surface plasmon resonance. This mode of affinity enhancement requires that the protein has ligand-binding pockets open at both ends. The third mechanism is called the conformational epitope hypothesis, but remains speculative In this interaction, a polysaccharide of a certain chain length forms a higher-ordered conformation, such as a helix, that assists in protein recognition and tighter binding. We highlight four types of soluble oligo/polysaccharides—polylactosamine, hyaluronan, short β1–3 glucans, and α2–8 polysialic acids—and summarize their three-dimensional (3D) structures and modes of protein interaction
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