The mannose/N-acetylgalactosamine-4-SO4 receptor displays greater specificity for multivalent than monovalent ligands

Recognition of carbohydrates on glycosylated molecules typically requires m ultivalent interactions with receptors, Monovalent forms of terminal saccha rides engaged by the receptor binding sites typically display weak affiniti es in the mM range and poor specificity, In contrast, multivalent forms of the same saccharides are bound with strong affinity (10(-7)-10(-9) M) and s ignificantly greater specificity. Although multivalency can readily account for increased affinity, the molecular basis for enhanced specificity is no t well understood. We have examined the specificity of the cysteine-rich do main of the mannose/GalNAc-4-SO4 receptor using monovalent and multivalent forms of the trisaccharide GalNAc beta1,4GlcNAc beta1,2Man alpha (GGnM) sul fated at either the C4 (S4GGnM) or C3 (S3GGnM) hydroxyl of the terminal Gal NAc. Monovalent S4GGnM and S3GGnM have K-i values Of 25.8 and 16.2 muM, res pectively. Multivalent conjugates of the same GalNAc-4-SO4- and GalNAc-3SO( 4)-bearing trisaccharides (6.7 mol of trisaccharide/mol of bovine serum alb umin) have Ki values of 0.013 and 0.170 mum, respectively. The 2000-fold ve rsus 95-fold change in affinity seen for the multivalent forms of these 4-s ulfated and 3-sulfated trisaccharides reflects a difference in the impact o f conformational entropy, A large fraction of the SO4-3-GalNAc structures e xists in a form that is not favorable for binding to the Cys-rich domain. T his reduces the effective concentration of SO4-3-GalNAc as compared with SO 4-4-GalNAc under the same conditions and results in a markedly lower associ ation rate. This difference in association rate accounts for the 12-fold di fference in the rate of clearance from the blood seen with S4GGnM-BSA and S 3GGnM-BSA in vivo.