CONFORMATION OF BETA-METHYLMELIBIOSE BOUND TO THE RICIN B-CHAIN AS DETERMINED FROM TRANSFERRED NUCLEAR OVERHAUSER EFFECTS

Transferred nuclear Overhauser effect (TRNOE) experiments have revealed a change in the torsion angles about the alpha- 1-6 glycosidic bond of methyl beta-melibioside upon binding of the melibioside to the ricin B-chain (Rb). A full relaxation rate matrix simulation of experimental buildup curves aided in quantitative interpretation of 1D selective inversion recovery TRNOE experiments. The data are consistent with a model in which both major (omega almost-equal-to 170-degrees) and minor (omega almost-equal-to -60-degrees) conformers for methyl beta-melibioside are significantly populated in solution while the Rb/methyl beta-melibioside complex has little of the minor conformer populated. The results indicate that the ricin B-chain excludes binding of certain ligand conformations on the basis of unfavorable interactions between the protein surface and remote portions of the disaccharide system.