Carbohydrate specificity of pea lectin revealed by X-ray analysis

Structures of two new crystal complexes of pea lectin (Pisum sativum, dimer, molecular mass ca. 50 kDa) with D-glucopyranose (P2₁2₁2₁ space group; unit cell parameters: a 73.4, b 107.7, and c 64.6 Å) and D-mannopyranose (P2₁2₁2₁; a 62.6, b 135.1, and c 54.9 Å) were solved by the molecular replacement approach at 2.3 and 1.98 Å resolutions and refined to the R factor values of 0.155 and 0.217, respectively. The comparative analysis of the structures found and those determined earlier showed pea lectin to bind the α- and β-forms of monosaccharides in a ratio close to that in solution. Stereochemical features of the carbohydrate specificity of pea lectin were studied, and a statistical-dynamic model of the carbohydrate binding was developed using energy computation methods. This model enables the evaluation of the region of the monosaccharide interaction with the protein carbohydrate-binding site and the ensemble of the carbohydrate orientations providing for its productive binding. Amino acid substitutions, Asn39 → Gln and Asn39 → Lys, capable of causing directed changes in the lectin-binding specificity toward the cognate carbohydrates were proposed.