Roseman D S, Baenziger J U
Department of Pathology, Washington University School of Medicine, St. Louis, Missouri 63110, USA.
J Biol Chem. 2001 May 18;276(20):17052-7. doi: 10.1074/jbc.M101027200. Epub 2001 Mar 6.
Recognition of carbohydrates on glycosylated molecules typically requires multivalent interactions with receptors. Monovalent forms of terminal saccharides engaged by the receptor binding sites typically display weak affinities 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 significantly greater specificity. Although multivalency can readily account for increased affinity, the molecular basis for enhanced specificity is not well understood. We have examined the specificity of the cysteine-rich domain of the mannose/GalNAc-4-SO4 receptor using monovalent and multivalent forms of the trisaccharide GalNAcbeta1,4GlcNAcbeta1,2Manalpha (GGnM) sulfated at either the C4 (S4GGnM) or C3 (S3GGnM) hydroxyl of the terminal GalNAc. Monovalent S4GGnM and S3GGnM have K(i) values of 25.8 and 16.2 microm, respectively. Multivalent conjugates of the same GalNAc-4-SO4- and GalNAc-3-SO4-bearing trisaccharides (6.7 mol of trisaccharide/mol of bovine serum albumin) have K(i) values of 0.013 and 0.170 microm, respectively. The 2000-fold versus 95-fold change in affinity seen for the multivalent forms of these 4-sulfated and 3-sulfated trisaccharides reflects a difference in the impact of conformational entropy. A large fraction of the SO4-3-GalNAc structures exists in a form that is not favorable for binding to the Cys-rich domain. This reduces the effective concentration of SO4-3-GalNAc as compared with SO4-4-GalNAc under the same conditions and results in a markedly lower association rate. This difference in association rate accounts for the 12-fold difference in the rate of clearance from the blood seen with S4GGnM-BSA and S3GGnM-BSA in vivo.
对糖基化分子上碳水化合物的识别通常需要与受体进行多价相互作用。受体结合位点所结合的单糖形式的末端糖类通常在毫米范围内显示出弱亲和力且特异性较差。相比之下，相同糖类的多价形式则以强亲和力（10^(-7)-10^(-9) M）结合且特异性显著更高。尽管多价性能够很容易地解释亲和力的增加，但增强特异性的分子基础尚未得到很好的理解。我们使用在末端GalNAc的C4（S4GGnM）或C3（S3GGnM）羟基处硫酸化的三糖GalNAcbeta1,4GlcNAcbeta1,2Manalpha（GGnM）的单价和多价形式，研究了甘露糖/GalNAc-4-SO4受体富含半胱氨酸结构域的特异性。单价的S4GGnM和S3GGnM的K(i)值分别为25.8和16.2微摩尔。相同的带有GalNAc-4-SO4和GalNAc-3-SO4的三糖的多价缀合物（6.7摩尔三糖/摩尔牛血清白蛋白）的K(i)值分别为0.013和0.170微摩尔。这些4-硫酸化和3-硫酸化三糖的多价形式所观察到的亲和力变化2000倍与95倍反映了构象熵影响的差异。很大一部分SO4-3-GalNAc结构以不利于与富含半胱氨酸结构域结合的形式存在。这与相同条件下的SO4-4-GalNAc相比，降低了SO4-3-GalNAc的有效浓度，并导致结合速率显著降低。这种结合速率的差异解释了体内S4GGnM-BSA和S3GGnM-BSA从血液中清除速率的12倍差异。
