Liquid storage stability of the Class II hydrophobin HFBI: Effect of pH on functional properties

Class II hydrophobins, a family of highly amphiphilic proteins, are well known to stabilize foams and emulsions at remarkably low concentrations. In this study, the liquid storage stability of the Class II hydrophobin HFBI from Trichoderma reesei was investigated at different pH conditions and the effect on the foam and emulsion activity was studied. Reversed phase high performance liquid chromatography analysis during storage visualised the degradation of HFBI at neutral (pH 7) and slightly alkaline (pH 9) conditions. Tryptic digestion and peptide separation using liquid chromatography tandem mass spectrometry confirmed that the degradation was caused by ammonia loss and deamidation of the asparagine residues present in HFBI. The loss of one or more amino acids at the N-terminal end of HFBI was revealed by electron spray ionisation time of flight mass spectrometry. The results give valuable new insights in the stability and functionality of HFBI as the post-translational modification of HFBI at neutral and alkaline pH only had a limited effect on the hydrophobic patch of HFBI while the foam stability and emulsion activity of HFBI was also not directly influenced. The results suggest that the function of dissolved HFBI is retained regardless of the detected degradation.