Effect of Limited Proteolysis on the Enzymatic Phosphorylation of Soy Protein

The effects of limited proteolysis on the enzymatic phosphorylation of commercial soy protein isolate were evaluated for several proteases. Proteases were chosen on the basis of their ability to hydrolyze peptide bonds between specific amino acids and included trypsin (basic), chymotrypsin (aromatic), endoproteinase Glu-C from Staphylococcus aureus, strain V8 (acidic), and Pronase E (nonspecific). Hydrolyzed soy isolate was phosphorylated with the catalytic subunit of bovine cardiac muscle protein kinase, and phosphorylation was followed by incorporation of 32 P into the protein. Phosphorylation decreased with increased proteolysis for all proteases. Decreased phosphorylation was most pronounced for Pronase E-hydrolyzed protein and least pronounced for trypsin-hydrolyzed soy isolate. The solubility−pH profile of the trypsin-hydrolyzed, phosphorylated soy isolate was compared to those of both the untreated and the hydrolyzed profiles. There was no significant difference in solubility among the profiles over the pH 3−6 range. Keywords: Polypeptide; modification; functionality; hydrolysis; interaction