Moothoo D N, Naismith J H
Centre for Biomolecular Sciences, The University, St. Andrews, Scotland, United Kingdom.
Glycobiology. 1998 Feb;8(2):173-81. doi: 10.1093/glycob/8.2.173.
Carbohydrate recognition by proteins is a key event in many biological processes. Concanavalin A is known to specifically recognize the pentasaccharide core (beta-GlcNAc-(1-->2)-alpha- Man-(1-->3)-[beta-GlcNAc-(1-->2)-alpha-Man-(1-->6)]-Man) of N-linked oligosaccharides with a Ka of 1.41 x 10(6 )M-1. We have determined the structure of concanavalin A bound to beta-GlcNAc-(1-->2)-alpha-Man-(1-->3)-[beta-GlcNAc-(1-->2)-alpha-Man- (1-->6)]-Man to 2.7A. In six of eight subunits there is clear density for all five sugar residues and a well ordered binding site. The pentasaccharide adopts the same conformation in all eight subunits. The binding site is a continuous extended cleft on the surface of the protein. Van der Waals interactions and hydrogen bonds anchor the carbohydrate to the protein. Both GlcNAc residues contact the protein. The GlcNAc on the 1-->6 arm of the pentasaccharide makes particularly extensive contacts and including two hydrogen bonds. The binding site of the 1-->3 arm GlcNAc is much less extensive. Oligosaccharide recognition by Con A occurs through specific protein carbohydrate interactions and does not require recruitment of adventitious water molecules. The beta-GlcNAc-(1-->2)-Man glycosidic linkage PSI torsion angle on the 1-->6 arm is rotated by over 50 degrees from that observed in solution. This rotation is coupled to disruption of interactions at the monosaccharide site. We suggest destabilization of the monosaccharide site and the conformational strain reduces the free energy liberated by additional interactions at the 1-->6 arm GlcNAc site.
蛋白质对碳水化合物的识别是许多生物过程中的关键事件。已知伴刀豆球蛋白A能特异性识别N - 连接寡糖的五糖核心（β - GlcNAc - (1→2) - α - Man - (1→3) - [β - GlcNAc - (1→2) - α - Man - (1→6)] - Man），其解离常数Ka为1.41×10⁶ M⁻¹。我们已确定了与β - GlcNAc - (1→2) - α - Man - (1→3) - [β - GlcNAc - (1→2) - α - Man - (1→6)] - Man结合的伴刀豆球蛋白A的结构，分辨率达到2.7埃。在八个亚基中的六个中，所有五个糖残基都有清晰的电子密度以及一个有序的结合位点。该五糖在所有八个亚基中采用相同的构象。结合位点是蛋白质表面一个连续的延伸裂隙。范德华相互作用和氢键将碳水化合物固定在蛋白质上。两个GlcNAc残基都与蛋白质接触。五糖1→6臂上的GlcNAc形成了特别广泛的接触，包括两个氢键。1→3臂上GlcNAc的结合位点则不那么广泛。伴刀豆球蛋白A对寡糖的识别通过特定的蛋白质 - 碳水化合物相互作用发生，不需要额外水分子的参与。1→6臂上β - GlcNAc - (1→2) - Man糖苷键的ψ扭转角比在溶液中观察到的旋转了超过50度。这种旋转与单糖位点处相互作用的破坏相关联。我们认为单糖位点的不稳定和构象应变降低了1→6臂上GlcNAc位点额外相互作用所释放的自由能。
