Efficient degradation of soybean protein B3 subunit in soy sauce by ultrasound-assisted prolyl endopeptidase and its primary mechanism

Abstract

Prolyl endopeptidase can partially degrade soybean protein B₃ subunit and alleviate soy sauce secondary precipitate. In this study, the influences of ultrasound-assisted prolyl endopeptidase on the degradation of soybean protein B₃ subunit of soy sauce and primary mechanism were investigated using SDS-PAGE, MALDI-TOF-MS, circular dichromatic spectrometer, fluorescence spectra, etc. Results showed that ultrasound-assisted prolyl endopeptidase enhanced 72% degradation rate of B₃ subunit and reduced soy sauce secondary precipitate remarkably, meanwhile significantly increased content of organic taste compounds of soy sauce compared with control (p < 0.05). Sonication markedly reduced percentage of α-helix and increased percentage of random coil, made hydrophobic amino acids inside prolyl endopeptidase exposed to its surface and enhanced its flexibility, which facilitated the binding of prolyl endopeptidase active center with B₃ subunit and finally enhanced the latter's degradation rate and appearance quality of soy sauce. This work laid a foundation for solving soy sauce secondary precipitate.

Keywords: Aspergillus niger; Aspergillus oryzae; Prolyl endopeptidase; Soy sauce; Soybean protein; Ultrasound.