A general method for co-crystallisation of concanavalin A with carbohydrates.

A small grid of conditions has been developed for co-crystallization of the plant lectin concanavalin A (conA) and polysaccharides. Crystals have been obtained of complexes of conA with alpha 1-2 mannobiose, 1-methyl alpha 1-2 mannobiose, fructose, a trisaccharide and a pentasaccharide. The crystals diffract to resolutions of 1.75-2.7 Angstrom using a copper rotating-anode source. The crystals are grown in the presence of polyethylene glycol 6K [10=20%(w/v)] at around pH 6.0. Optimization for each particular carbohydrate requires small adjustments in the conditions; however, all complexes give some crystalline precipitate in this limited grid. The alpha 1-2 mannobiose complex crystals diffract to 1.75 Angstrom with space group I222 and cell dimensions a = 91.7, b = 86.8, c = 66.6 Angstrom . One monomer is present in the asymmetric unit. The 1-methyl alpha 1-2 mannobioside complex crystallizes in space group P2(1)2(1)2(1), cell dimensions a = 119.7, b = 119.7, c = 68.9 Angstrom and diffract to 2.75 Angstrom. One tetramer is present in the asymmetric unit. Two crystal forms of the conA-fructose complex have been obtained. The first has space group P2(1)2(1)2(1), cell dimensions a = 121.7, b = 119.9, c = 67.3 Angstrom with a tetramer in the asymmetric unit and diffracts to 2.6 Angstrom. The second crystallizes in space group C222(1), cell dimensions a = 103.3, b = 117.9, c = 254.3 Angstrom with two dimers in the asymmetric unit and diffracts to 2.42 Angstrom. Structures and crystallization of the trisaccharide-conA and pentasaccharide-conA complexes have already been reported. In all complexes, the protein is found as a tetramer, although varying combinations of non-crystallographic and crystallographic symmetry are involved in generating the tetramer. The precise packing of the tetramer varies from crystal to crystal and it is likely that this variability facilitates crystallization.