title: Weak and Saturable Protein-Surfactant Interactions in the Denaturation of Apo-α-Lactalbumin by Acidic and Lactonic Sophorolipid.
abstract: Biosurfactants are of growing interest as sustainable alternatives to fossil-fuel-derived chemical surfactants, particularly for the detergent industry. To realize this potential, it is necessary to understand how they affect proteins which they may encounter in their applications. However, knowledge of such interactions is limited. Here, we present a study of the interactions between the model protein apo-α-lactalbumin (apo-aLA) and the biosurfactant sophorolipid (SL) produced by the yeastStarmerella bombicola. SL occurs both as an acidic and a lactonic form; the lactonic form (lactSL) is sparingly soluble and has a lower critical micelle concentration (cmc) than the acidic form [non-acetylated acidic sophorolipid (acidSL)]. We show that acidSL affects apo-aLA in a similar way to the related glycolipid biosurfactant rhamnolipid (RL), with the important difference that RL is also active below the cmc in contrast to acidSL. Using isothermal titration calorimetry data, we show that acidSL has weak and saturable interactions with apo-aLA at low concentrations; due to the relatively low cmc of acidSL (which means that the monomer concentration is limited to ca. 0-1 mM SL), it is only possible to observe interactions with monomeric acidSL at high apo-aLA concentrations. However, the denaturation kinetics of apo-aLA in the presence of acidSL are consistent with a collaboration between monomeric and micellar surfactant species, similar to RL and non-ionic or zwitterionic surfactants. Inclusion of diacetylated lactonic sophorolipid (lactSL) as mixed micelles with acidSL lowers the cmc and this effectively reduces the rate of unfolding, emphasizing that SL like other biosurfactants is a gentle anionic surfactant. Our data highlight the potential of these biosurfactants for future use in the detergent and pharmaceutical industry.
authors.affiliation: Interdisciplinary Nanoscience Center, Department of Molecular Biology and Genetics, Aarhus UniversityAarhus, Denmark; Agro Business Park A/STjele, Denmark.
authors.forename: Kell K
authors.initials: KK
authors.lastname: Andersen
authors.affiliation: Interdisciplinary Nanoscience Center, Department of Molecular Biology and Genetics, Aarhus University Aarhus, Denmark.
authors.forename: Brian S
authors.initials: BS
authors.lastname: Vad
authors.affiliation: Centre for Industrial Biotechnology and Biocatalysis - InBio.be, Faculty of Bioscience Engineering, Ghent University Ghent, Belgium.
authors.forename: Sophie
authors.initials: S
authors.lastname: Roelants
authors.affiliation: Centre for Industrial Biotechnology and Biocatalysis - InBio.be, Faculty of Bioscience Engineering, Ghent University Ghent, Belgium.
authors.forename: Inge N A
authors.initials: IN
authors.lastname: van Bogaert
authors.affiliation: Interdisciplinary Nanoscience Center, Department of Molecular Biology and Genetics, Aarhus University Aarhus, Denmark.
authors.forename: Daniel E
authors.initials: DE
authors.lastname: Otzen
keywords: acidic form; binding stoichiometry; lactonic form; protein denaturation; sophorolipid; unfolding kinetics