Weak and saturable protein-surfactant interactions in the denaturation of apo-α-lactalbumin by acidic and lactonic sophorolipid

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-alpha-lactalbumin (apo-aLA) and the biosurfactant sophorolipid (SL) produced by the yeast Starmerella 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

Role of deficits in pathogen recognition receptors in infection susceptibility

This work was supported by the Northern Portugal Regional Operational Programme (NORTE 2020), under the Portugal 2020 Partnership Agreement, through the European Regional Development Fund (FEDER) (NORTE-01-0145-FEDER-000013), and the Fundação para a Ciência e Tecnologia (FCT) (IF/00735/2014 to A.C. and SFRH/BPD/96176/2013 to C.C.

A Kinetic Analysis of the Folding and Unfolding of OmpA in Urea and Guanidinium Chloride: Single and Parallel Pathways

The outer membrane protein OmpA from <i>Escherichia coli</i> can fold into lipid vesicles and surfactant micelles from the urea-denatured state. However, a complete kinetic description of the folding and unfolding of OmpA, which can provide the basis for subsequent protein engineering studies of the protein’s folding pathway, is lacking. Here we use two different denaturants to probe the unfolding mechanism of OmpA in the presence of the surfactant octyl maltoside (OM). Unfolding of OmpA in the presence of micelles, achieved with the potent denaturant guanidinium chloride (GdmCl), leads to single-phase unfolding. In contrast, OmpA unfolds in urea only below OM’s critical micelle concentration, and this occurs in different phases, which we attribute to the existence of states that have bound different amounts of surfactant, from completely “naked” to partly covered by surfactant. Multiple parallel refolding phases are attributed to different levels of collapse prior to folding. Kinetic results used to derive the stability of OmpA in surfactant, using either urea or GdmCl as the denaturing agent, give comparable results and indicate a minimalist three-state folding scheme involving denatured state D, folding intermediate I, and native state N. N and I are stabilized by 15.6 and 2.6 kcal/mol, respectively, relative to D. The periplasmic domain of OmpA does not contribute to stability in surfactant micelles. However, BBP, a minimalist transmembrane β-barrel version of OmpA with shortened loops, is destabilized by ∼10 kcal/mol compared to OmpA, highlighting loop contributions to OmpA stability