Emulsifying performance of modular beta-sandwich proteins: the hydrophobic moment and conformational stability

Our understanding of protein emulsifying properties is largely based on analysis of emulsifiers found in milk and seed. The 9th-10th type III fibronectin domain pair retains full biological activity following emulsification-encapsulation into polyester microspheres, for controlled delivery, but the conformational criteria determining emulsification efficiency (EE) are unknown. Here, we have generated a series of mutants of this beta-sandwich protein, changing the hydrophobic moment and conformational stability, to investigate the structure-emulsification relationship. Predictive modelling of the hydrophobic moment of beta-strands and mutations known to increase conformational stability were used to generate the series. The proteins were tested for their emulsion stability and EE for oil-in-water mixtures. We show that the stabilization of emulsions by beta-sandwich proteins is best predicted by conformational stability during equilibrium denaturation in ionic surfactant. In contrast, the EE of these proteins is inversely related to an increase in their surface hydrophobicity following unfolding in surfactant. We also describe a novel beta-sandwich emulsifier with strong EE. The requirement for interdomain flexibility to achieve maximum emulsion stability and EE is also shown. This work increases our understanding of the mechanisms involved in protein emulsification and will be of use to the microencapsulation of proteins into polyester microspheres via emulsion-extraction protocols.