Heat sensitive protein-heat stable protein interaction: Synergistic enhancement in the thermal co-aggregation and gelation of lactoferrin and α-lactalbumin

Abstract

The synergistic enhancement in the thermal co-aggregation and gelation of lactoferrin (LF), a heat sensitive protein, and α-lactalbumin (ALA), a heat stable protein, was investigated at pH 7.0. Heating temperatures (70 °C and 90 °C; 30 min) and ALA concentrations (0-0.5 mM) significantly affected the structural characteristics of the resultant thermal aggregates and gels. The turbidity and size of LF-ALA thermal aggregates were increased with increasing ALA concentration; at a low total protein concentration of 0.57 mM, LF-ALA gels can be formed. The presence of ALA led to some hydrophobic residues originally located in the interior of LF to be exposed further during heating. New intermolecular disulfide bonds, mainly unstable ones, were formed between LF molecules and/or ALA molecules during co-heating. The aggregation of LF and ALA was a gradual denaturation process, accompanied by an increase in β-sheet content and decrease in α-helix content. Random spherical aggregates with large size (1-5 μm) were observed by transmission electron microscopy, clearly confirming the nucleation and growth of LF with ALA. There existed strong rheological synergism between LF and ALA, thereby leading to a large reduction in gelation times (4-11 min) with increasing ALA concentration and heating temperature. Considering these data, LF and ALA played different and indispensable roles in thermal aggregation and gelation: LF engaged readily in thermal aggregation, while ALA mainly assisted the LF thermal aggregation. Three types of mechanisms (co-fusion, nucleation and growth) involved in the aggregation and gelation processes. In all, the data of the current study has enhanced the comprehension of heat sensitive protein-heat stable protein thermal aggregation and gelation, and may help to design LF-based new ingredients for the control of food textures and delivery systems for food and pharmaceutical applications.

Keywords: Lactoferrin; Nucleation mechanism; Protein-protein interaction; Synergistic interaction; Thermal aggregation.