Carbohydrate specificity and salt-bridge mediated conformational change inacidic winged bean agglutinin

Structures of two crystal form of the dimeric acidic winged bean agglutinin (WBAII) complexed with methyl-alpha-D-galactose have been determined at 3. 0 Angstrom and 3.3 Angstrom resolution. The subunit structure and dimerisat ion of the lectin are similar to those of the basic lectin from winged bean (WBAI) and the lectin from Erythrina corallodendron (EcorL). The conformat ion of a loop and its orientation with respect to the rest of the molecule in WBAII are, however, different from those in all the other legume lectins of known structure. This difference appears to have been caused by the for mation of two strategically placed salt bridges in the former. Modelling ba sed on the crystal structures provides a rationale for the specificity of t he lectin, which is very different from that of WBAI, for the H-antigenic d eterminant responsible for O blood group reactivity. It also leads to a qua litative explanation for the thermodynamic data on sugar-binding to the lec tin, with special emphasis on the role of a tyrosyl residue in the variable loop in the sugar-binding region in generating the carbohydrate specificit y of WBAII. (C) 2000 Academic Press.