Structural features of the combining site region of Erythrina corallodendron lectin: Role of tryptophan 135

Abstract The role of Trp 135 and Tyr 108 in the combining site of Erythrina corallodendron lectin (ECorL) was investigated by physicochemical characterization of mutants obtained by site‐directed mutagenesis, hemagglutination‐inhibition studies, and molecular modeling, including dynamics simulations. The findings demonstrate that Trp 135 in ECorL: (1) is required for the tight binding of Ca 2+ and Mn 2+ to the lectin because mutation of this residue into alanine results in loss of these ions upon dialysis and concomitant reversible inactivation of the mutant; (2) contributes to the high affinity of methyl α‐ N ‐dansylgalactosaminide (MeαGalNDns) to the lectin; and (3) is solely responsible for the fluorescence energy transfer between the aromatic residues of the lectin and the dansyl group in the ECorL‐MeαGalNDns complex. Docking of MeαGalNDns into the combining site of the lectin reveals that the dansyl moiety is parallel with the indole of Trp 135, as required for efficient fluorescence energy transfer, in one of the two possible conformations that this ligand assumes in the bound state. In the W135A mutant, which still binds MeαGalNDns strongly, the dansyl group may partially insert itself into the place formerly occupied by Trp 135, a process that from dynamics simulations does not appear to be energetically favored unless the loop containing this residue assumes an open conformation. However, a small fraction of the W135A molecules must be able to bind MeαGalNDns in order to explain the relatively high affinity, as compared to galactose, still remaining for this ligand. A model for the molecular events leading to inactivation of the W135A mutant upon demetallization is also presented in which the cis ‐ trans isomerization of the Ala 88‐Asp 89 peptide bond, observed in high‐temperature dynamics simulations, appears not to be a required step.