Secondary Structure and Subunit Composition of Soy ProteinIn VitroDigested by Pepsin and Its Relation with Digestibility

Abstract

In the present study,in vitrodigestibility and structure of soybean protein isolates (SPIs) prepared from five soybean varieties were investigated in simulated gastric fluid (SGF), using FT-IR microspectroscopy and SDS-PAGE. The result indicated thatβ-conformations were prone to be hydrolyzed by pepsin preferentially and transformed to unordered structure duringin vitrodigestion, followed by the digestion ofα-helix and unordered structure. A negative linear correlation coefficient was found between theβ-conformation contents of five SPIs and theirin vitrodigestibility values. The intensities of the protein bands corresponding to 7S and 11S fractions were decreased and many peptide bands appeared at 11~15 kDa during enzymatic hydrolysis.β-conglycinin was poorly hydrolyzed with pepsin, especially theβ-7S subunit. On the other hand, basic polypeptides of glycinin degraded slower than acidic polypeptides and represented a large proportion of the residual protein after digestion. 11S-A3of all SPIs disappeared after 1 h digestion. Moreover, a significant negative linear correlation coefficient (r=-0.89) was found between theβ-7S contents of five SPIs and theirin vitrodigestibility values. These results are useful for further studies of the functional properties and bioactive properties of these varieties and laid theoretical foundations for the development of the specific functional soy protein isolate.