Abstract

The strong and usually denaturing interaction between anionic surfactants and proteins/enzymes has both benefits and drawbacks: For example, it is in good use in electrophoretic mass determinations (SDS-PAGE) but limits enzyme efficiency in detergent formulations. Therefore, studies of the interactions between proteins and anionic surfactants as well as non-ionic surfactants are of both basic and applied relevance. The anionic surfactants, sodium dodecyl sulfate (SDS), denatures and unfolds globular proteins under most conditions. In contrast, it has been shown that the non-ionic surfactant, octaethylene glycol monododecyl ether, (C₁₂E₈) protects bovine serum albumin (BSA) from unfolding in SDS. Small-angle X-ray scattering (SAXS) is an ideal technique for studying the structures formed in these systems. Since the protein has a positive excess electron density and the alkyl chains of the surfactant have a negative excess contrast, the protein and surfactant components can be distinguished in the modelling of the data. We have investigated whether globular proteins unfolded by SDS can be refolded upon addition of C₁₂E₈. Several proteins[i, ii], including BSA, α-lactalbumin, (αLA), lysozyme (LYZ), and β-lactoglobulin (βLG), were studied by laboratory-based SAXS and both near- and far-UV circular dichroism (CD). All proteins and their complexes with SDS were attempted refolded by the addition of C₁₂E₈. The addition of C₁₂E₈ to the protein-SDS samples, except aLA, resulted in refolding of the tested proteins and dissociation from surfactant micelles. It concluded that C₁₂E₈ competes with globular proteins for association with SDS, making it possible to release and refold SDS-denatured proteins by adding sufficient amounts of C₁₂E₈. In the last part of the talk, recent work employing synchrotron radiation SAXS in combination with stopped-flow techniques on the kinetics of unfolding an refolding will be shown[iii, iv].

[i] Kaspersen JD, Søndergaard A, Madsen DJ, Otzen DE, Pedersen JS. Refolding of SDS-Unfolded Proteins by Nonionic Surfactants. Biophys. J. 2017, 112, 1609– 1620

[ii] Rasmussen HØ, Enghild JJ, Otzen DE, Pedersen JS. Unfolding and partial refolding of a cellulase from the SDS-denatured state: From β-sheet to α-helix and back. B B A - General Subjects. 2020, 1864(1):129434 1-12.

[iii] Pedersen JN, Lyngsø J, Zinn T, Otzen D, Pedersen JS. A complete picture of protein unfolding and refolding in surfactants. Chemical Science. 2020;11(3):699-712.

[iv] Jensen GV, Pedersen JN, Otzen DE, Pedersen JS. Multi-Step Unfolding and Rearrangement of α-Lactalbumin by SDS Revealed by Stopped-Flow SAXS. Frontiers in Molecular Biosciences. 2020 ;7. 125.