Heteroprotein complex between soy protein isolate and lysozyme: Protein conformation, lysozyme activity, and structural characterization.

Heteroprotein complexes are formed by electrostatic interactions of oppositely charged proteins in a purely aqueous environment. Understanding the relationship between their structural and functional properties will contribute to their tailor-made applications. Therefore, this study investigated the protein conformation, assembling structure, and enzyme activity of soy protein isolate/lysozyme (SPI/LYS) complexes at mass ratios of 2:1 (soluble complex) and 1:1.3 (stoichiometric ratio). Electrostatic complexation increased the surface hydrophobicity of complexes. Their surface hydrophobicity decreased with increasing NaCl concentrations and reached the theoretical values at the critical salt concentration of 200 mM NaCl. Electrostatic complexation did not decrease the LYS activity (∼43,000 units/mg). SPI/LYS complexes exhibited flocculated structures in which the two proteins were unevenly distributed; these were typical amorphous complexes. High dilution disassembled these complexes over 5 μm into particles of ∼100 nm, and NaCl reduced the size of these particles. Immobilized water was detected in the complexes formed by particle flocculation.