Transglutaminase Cross-Linking of Whey Proteins Affects Alginate Complexation

Abstract

Cross-linked proteins can serve as ingredients generating new food matrix interactions. Generally, cross-linking of whey proteins requires harsh conditions, but here microbial transglutaminase (MTG) cross-linked whey proteins and the major constituent β-lactoglobulin (β-Lg) effectively under mild alkaline conditions without prior heating or reducing agent addition. Cross-linked β-Lg spanned 18 to >240 kDa and mass spectrometry identified 6 of 9 glutamines reacting with 8 of 15 lysines. The initial isopeptide bond formation caused loss of β-Lg native structure with t 1/2 = 3 h, while the polymerization occurred with t 1/2 = 10 h, as shown by fluorescence, circular dichroism and size exclusion chromatography analyses. Complexation with alginate was monitored by dynamic light scattering, turbidimetry and isothermal titration calorimetry and indicated that β-Lg cross-linking decreased onset of particle formation, hydrodynamic diameter, stoichiometry (β-Lg/alginate) and dissociation constant. Temperature dependent enthalpy and entropy changes suggested hydrophobic interactions were important for alginate-cross-linked β-Lg complexation.

Keywords: LC-MS/MS cross-link identification, size exclusion chromatography, dynamic light scattering, far- and near-UV CD, intrinsic and ANS fluorescence spectra, molecular dynamics inter-residue distance analysis