Nonstatistical binding of a protein to clustered carbohydrates

Abstract

Carbohydrate-derivatized self-assembled monolayers (SAMs) are used as a model system to address issues involving cell-surface carbohydrate-protein interactions. Here we examine the influence of carbohydrate surface density on protein-binding avidity. We show that the binding selectivity of Bauhinia purpurea lectin switches from one carbohydrate ligand to another as the surface density of the carbohydrate ligands increases from values of sugar Ϸ 0.1-1.0. Polyvalent binding is possible at all surface densities investigated; hence, the switch in selectivity is not due simply to the achievement of a critical density that permits polyvalent contacts. Instead, secondary interactions at high surface densities promote a switch in carbohydrate-binding selectivity. These findings may have implications for how changes in the composition and the density of cell-surface carbohydrates influence biological recognition processes and regulatory pathways. The patterns of expression of cell-surface carbohydrates change during development and differentiation (1-3). Changes in the composition of cell-surface carbohydrates also occur during oncogenesis and metastasis (4, 5). Although the biological functions of cell-surface carbohydrates are still incompletely defined, it is believed that they are somehow involved in signaling pathways that determine the temporal and spatial identity of cells. How changing patterns of cell-surface carbohydrates are linked to cell growth and differentiation is not understood.The early steps in cell-surface carbohydrate-mediated cellular processes are presumed to involve binding events between carbohydrates and receptors in the plasma or on the surface of other cells. Because these binding events are linked to specific cellular responses, one might expect them to be highly specific. However, studies in solution have shown that carbohydrateprotein binding interactions have dissociation constants that are typically in the mM range and that different carbohydrate ligands have similar affinities for the same protein receptor. If carbohydrates bind weakly and with poor selectivity, how can they mediate specific cellular processes?This paradox has been partially explained by the phenomenon of polyvalency. Carbohydrate-protein binding events usually involve several simultaneous contacts between carbohydrates that are clustered on cell surfaces and protein receptors that contain multiple carbohydrate-binding sites. Numerous studies on model systems have demonstrated that polyvalent displays of carbohydrates can lead to remarkably high binding avidities (6-16). Because avidity and specificity are not necessarily correlated (6, 7), however, polyvalency does not completely resolve the paradox of how marginally selective carbohydrate-binding interactions can produce highly specific responses.Two mechanisms have been proposed to explain how specificity can be achieved in cell-surface carbohydrate-binding interactions. In one mechanism, polyvalent carbohydrate-protein interactions facilitate the adhesion of cells. T...

Unraveling functional significance of natural variations of a human galectin by glycodendrimersomes with programmable glycan surface

“…Also, multivalency can be mimicked by local clusters as in branched glycans (35)(36)(37)(38)(39)(40)(41)(42). Parallel experiments on monolayers to characterize cis interactions plus consideration of presence of cholesterol and sphingolipids for microdomain generation would further broaden the scope, together with consideration of secondary effects (43). In this way, structurally programmed amphiphilic Janus glycodendrimers or mixtures of respective preparations have the enormous potential to shed light on fundamental aspects of structure-activity relationships within the process of how specific carbohydratelectin interactions are realized and enhanced.…”

Synthetic Multivalent Ligands as Probes of Signal Transduction

“…Solid Supports-Receptor binding, activation, and endocytosis have been studied using functionalized beads and surfaces. [127][128][129][130] The beads employed in these investigations vary in composition; they can be derived polystyrene, latex, polysaccharides, or other insoluble materials. The typical reactions used to conjugate binding epitopes to beads are straightforward and general, though rarely are these processes chemoselective or regioselective.…”

“…[9,129,[216][217][218] In this way, a study addressing the mechanism by which T cell responses are enhanced by the co-receptor CD28 was undertaken. [87] Ligand binding to the TCR is necessary and sufficient for activation of T cells; however, co-stimulation of CD28 yields a more potent response.…”

The Mannose/N-Acetylgalactosamine-4-SO4Receptor Displays Greater Specificity for Multivalent than Monovalent Ligands

“…Investigators have previously observed greater specificity on the part of lectins for polyvalent ligands as compared with their monovalent counterparts (15)(16)(17)(18)(19)(20)(21)(22). The specificity for multivalent ligands has been attributed to a requirement for unique spacing patterns (15)(16)(17)(18), secondary protein-protein and protein-carbohydrate interactions (19,20), and amplification in the polyvalent state of small differences in the affinities between monovalent forms (21). One prediction of a model in which small differences in affinity of monovalent ligands are amplified in the polyvalent state is that the affinities obtained for Mu11 binding to immobilized S4GGnM-BSA and S3GGnM-BSA would differ but that the B max obtained at saturation would be the same.…”