Albert Neuberger

Studies on the chemical modification of soybean agglutinin

Abstract Soybean agglutinin was subjected to various chemical treatments in order to detect which amino acid residues are involved in its carbohydrate binding activity. Modification of tyrosine and histidine residues was demonstrated but had no effect on the haemagglutinating activity. Modification of amino groups with citraconic anhydride in the absence or presence of 0.1 m d -galactose or 0.1 m 2-acetamido-2-deoxy- d -galactose decreased the lectin activity, and treatment with cyclohexane-1,2-dione which modified arginyl groups had a similar effect. Reaction of carboxyl groups, activated by carbodiimide, with α-aminobutyric acid methyl ester led to the incorporation of ester groups giving an inactive material. Modification of tryptophan residues carried out in native and denaturing conditions with N -bromosuccinimide led to a complete loss of haemagglutinating activity. Treatment in the presence of the sugars partially protected the tryptophan residues from oxidation and some activity was retained. The results indicated that tryptophan groups are involved but that tyrosine and histidine groups are not involved in the binding site of the lectin. It is proposed that the side chains of lysine, arginine, glutamate, and aspartate are not involved in the active site, but that the charged groups are important for maintaining a fully active conformation. An alternative explanation is that these amino acid residues are in fairly close proximity to the active site.