Interactions of β-Lactoglobulin with Bovine Submaxillary Mucin vs. Porcine Gastric Mucin: The Role of Hydrophobic and Hydrophilic Residues as Studied by Fluorescence Spectroscopy.

Hilal Yılmaz,Seunghwan Lee,Ioannis S Chronakis

doi:10.3390/molecules26226799

Hilal Yılmaz, Seunghwan Lee + Show 1 more

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https://doi.org/10.3390/molecules26226799

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Abstract

The aim of this study was to investigate binding interactions between β-lactoglobulin (BLG) and two different mucins, bovine submaxillary mucins (BSM) and porcine gastric mucin (PGM), using intrinsic and extrinsic fluorescence spectroscopies. Intrinsic fluorescence spectra showed an enhanced decrease of fluorescence intensity of BLG at all pH conditions when BLG was mixed with PGM rather than with BSM. We propose that, unlike BSM, the tertiary structure of PGM changes and the hydrophobic regions are exposed at pH 3 due to protonation of negatively charged residues. Results suggest that PGM also facilitated the structural unfolding of BLG and its binding with PGM by a hydrophobic interaction, especially at acidic pH, which was further supported by extrinsic fluorescence spectroscopy. Hydrophobic interaction is suggested as the dominant interaction mechanism between BLG and PGM at pH 3, whereas electrostatic interaction is the dominant one between BLG and BSM.

Highlights

Proteins are important ingredients for food products to provide desirable textural, sensory, and nutritional properties
between β-lactoglobulin (BLG) molecules exist mainly as dimers at neutral pH and room temperature, whereas they dissociate into monomers at pH < 3, and partly exist as octamers at its isoelectric point [7,8]
BLG has a nonpolar interior region and two Tryptophan (Trp) residues, [9] i.e., Trp-19, which is located in the more hydrophobic environment at the bottom of the calyx formed by the antiparallel β-strands, while Trp-61 is positioned adjacent to the strand involved in antiparallel interaction of the dimer [10]

Summary

Introduction

Proteins are important ingredients for food products to provide desirable textural, sensory, and nutritional properties. The interactions of β-lactoglobulin (BLG), a major whey protein, and mucins, as the major macromolecular component in saliva or gastric fluids, are drawing increasing attentions in the context of understanding the oral processing or digestion of dairy food on the molecular level. Bovine BLG has been one of the most extensively studied proteins, [1,2,3,4,5] mainly due to its abundance in cow’s milk. Extensive studies regarding the structure and functions of BLG have provided important information about pH-dependent conformational transitions [11] and binding with various hydrophobic and amphiphilic ligands [9]

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