The crystal structures of Man(alpha 1-3)Man(alpha 1-0)Me and Man(alpha 1-6)Man(alpha 1-0)Me in complex with concanavalin A

The crystal structures of concanavalin A in complex with Man(alpha 1-6)Man( alpha 1-O)Me and Man(alpha 1-3)Man(alpha 1-O)Me were determined at resoluti ons of 2.0 and 2.8 Angstrom respectively. In both structures, the O-1-linke d mannose binds in the conserved monosaccharide-binding site. The O-3-linke d mannose of Man(alpha 1-3)Man(alpha 1-O)Me binds in the hydrophobic subsit e formed by Tyr-12, Tyr-100, and Leu-99. The shielding of a hydrophobic sur face is consistent with the associated large heat capacity change. The O-6- linked mannose of Man(alpha 1-6)Man(alpha 1-)Me binds in the same subsite f ormed by Tyr-12 and Asp-16 as the reducing mannose of the highly specific t rimannose Man(alpha 1-3)[Man(alpha 1-6)]Man(alpha 1-O)Me. However, it is mu ch less tightly bound. Its O-2 hydroxyl makes no hydrogen bond with the con served water 1. Water 1 is present in all the sugar-containing concanavalin A structures and increases the complementarity between the protein-binding surface and the sugar, but is not necessarily a hydrogen-bonding partner. A water analysis of the carbohydrate-binding site revealed a conserved wate r molecule replacing O-4 on the alpha 1-3-linked arm of the trimannose. No such water is found for the reducing or O-6-linked mannose. Our data indica te that the central mannose of Man(alpha 1-3)[Man(alpha 1-6)]Man(alpha 1-O) Me primarily functions as a hinge between the two outer subsites.