Mild method of simultaneous methionine grafting and phosphorylation of soybean globulins improves their functional properties

Abstract

Soybean 7S and 11S globulins were phosphorylated to different extent as a function of the molar ratios of POCl 3 used (75, 150 and 300 mol POCl 3 /mol protein) in the presence of equivalent amounts of triethylamine. Simultaneous addition of L-methionine during the phosphorylation yielded methionylated globulins. In some experiments, the basic amino acid arginine was used as the sole base of the reaction, instead of triethylamine, at a ratio of 6 mol arginine/mol POCl 3 . Adding L-methionine in the course of the phosphorylation reaction reduced slightly the extent of protein phosphorylation, with increasing L-methionine content in the phosphorylated globulin up to about 280% of its original level. The use of L-arginine as the reaction base led to lower phosphorylation yields but could raise simultaneously both L-methionine and L-arginine to about 143% and 155% of their original levels, respectively. Isofocusing electrophoresis showed lower isoelectric points of the phosphorylated samples, compared to the starting aliquots. SDS-PAGE did not show major changes in the subunit distribution of the modified soybean globulins. Phosphorylation and amino acylation changed the solubility of the modified samples and also other related functional properties e.g. foaming and emulsifying which were remarkably improved, especially for the samples with grafted L-methionine.