Specificity of glycosaminoglycan-protein interactions.

Glycosaminoglycans (GAGs) interact with a variety of proteins with important functions in development and homeostasis. Most of these proteins bind to heparin in vitro, a highly sulfated GAG species, although heparan sulfate and/or chondroitin/dermatan sulfate are more frequent physiological ligands. Binding affinity and specificity are determined by charge distribution, mainly due to sulfate and carboxylate groups and by GAG chain conformation. Interactions may be nonspecific, essentially reflecting charge density or highly specific, dependent on rare GAG-structural features. Yet other GAG epitopes bind protein ligands with intermediate specificity and variable affinity. Studies of heparan sulfate biosynthesis point to stochastic but strictly regulated, cell-specific polymer modification. Together, these features allow for graded modulation of protein functional response.