Modification of tryptophan side chains of soybean agglutinin (SBA) with N-bromosuccinimide results in a loss of the hemagglutinating and carbohydrate binding activities of the protein. One residue/subunit is probably essential for the binding activity. Modification leads to a large decrease in the fluorescene of the protein accompained by a blue shift. Iodide ion quenching of the protein fluorescence shows that saccharide binding results in a decreased accessibility of some of the tryptophan side chains. These results strongly point towards the involvement of tryptophan residues in the active site of SBA.
Abbreviations SBA, soybean agglutinin; NBS, N-bromosuccinimide; dansyl, N-dimethyl 5-amino-naphthalene 1-sulphonyl; GalNAc, N-acetyl D-galactosamine