Dataset Information

Myoglobin and ?-Lactalbumin Form Smaller Complexes with the Biosurfactant Rhamnolipid Than with SDS.

ABSTRACT: Biosurfactants (BSs) attract increasing attention as sustainable alternatives to petroleum-derived surfactants. This necessitates structural insight into how BSs interact with proteins encountered by current chemical surfactants. Thus, small-angle x-ray scattering (SAXS) has been used for studying the structures of complexes made of the proteins ?-Lactalbumin (?LA) and myoglobin (Mb) with the biosurfactant rhamnolipid (RL). For comparison, complexes between ?LA and the chemical surfactant sodium dodecyl sulfate (SDS) were also investigated. The SAXS data for pure RL micelles can be described by prolate core-shell structures with a core radius of 7.7 Å and a shell thickness of 12 Å, giving an aggregation number of 11. The small core radius is attributed to RL's complex hydrophobic tail. Data for the ?LA-RL complex agree with a 12-molecule micelle with a single protein molecule in the shell. For Mb-RL, the analysis gives complexes of two connected micelles, each containing 10 RL and one protein in the shells. ?LA-RL and Mb-RL form surfactant-saturated complexes above 5.6 and 4.7 mM RL, respectively, leaving the remaining RL in free micelles. The SAXS data for SDS agree with oblate-shaped micelles with a core of 20 Å, core eccentricity 0.7, and shell thickness of 5.45 Å, with an aggregation number of 74. The ?LA-SDS complexes contain a prolate micelle with a core radius of 11-14 Å and a shell of 8-12 Å with up to 3 ?LA per particle and up to 43 SDS per ?LA, both considerably larger than for RL. Unlike the RL-protein complexes, the number of surfactant molecules in ?LA-SDS complexes increases with surfactant concentration, and saturate at higher surfactant concentrations than ?LA-RL complexes. The results highlight how RL and SDS follow similar overall rules of self-assembly and interactions with proteins, but that differences in the strength of protein-surfactant interactions affect the formed structures.

SUBMITTER: Mortensen HG

PROVIDER: S-EPMC5770973 | biostudies-literature | 2017 Dec

REPOSITORIES: biostudies-literature

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Myoglobin and α-Lactalbumin Form Smaller Complexes with the Biosurfactant Rhamnolipid Than with SDS.

Mortensen Henriette Gavlshøj HG Madsen Jens Kvist JK Andersen Kell K KK Vosegaard Thomas T Deen G Roshan GR Otzen Daniel E DE Pedersen Jan Skov JS

Biophysical journal 20171201 12

Biosurfactants (BSs) attract increasing attention as sustainable alternatives to petroleum-derived surfactants. This necessitates structural insight into how BSs interact with proteins encountered by current chemical surfactants. Thus, small-angle x-ray scattering (SAXS) has been used for studying the structures of complexes made of the proteins α-Lactalbumin (αLA) and myoglobin (Mb) with the biosurfactant rhamnolipid (RL). For comparison, complexes between αLA and the chemical surfactant sodium ...[more]

PMID: 29262357

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Project description:Interactions between proteins and surfactants are both of fundamental interest and relevant for applications in food, cosmetics and detergency. The anionic surfactant sodium dodecyl sulfate (SDS) denatures essentially all proteins. Denaturation typically involves a number of distinct steps where growing numbers of SDS molecules bind to the protein, as seen in multidisciplinary approaches combining several complementary techniques. We adopt this approach to study the SDS-induced unfolding of Ca2+-depleted ?-lactalbumin (aLA), a protein particularly sensitive toward denaturation by surfactants. By combining stopped-flow mixing of protein and surfactant solutions with stopped-flow synchrotron small-angle X-ray scattering (SAXS), circular dichroism (CD) and Trp fluorescence, together with information from previous calorimetric studies, we construct a detailed picture of the unfolding process at the level of both protein and surfactant. A protein-surfactant complex is formed within the dead time of mixing (2.5 ms). Initially a cluster of SDS molecules binds asymmetrically, i.e., to one side of the protein, after which aLA redistributes around the SDS cluster. This occurs in two kinetic steps where the complex grows in number of both SDS and protein molecules, concomitant with protein unfolding. During these steps, the core-shell complex undergoes changes in shell thickness as well as core shape and radius. The entire process is very sensitive to SDS concentration and completes within 10 s at an SDS:aLA ratio of 9, decreasing to 0.2 s at 60 SDS:aLA. The number of aLA molecules per SDS complex drops from 1.9 to 1.0 over this range of ratios. While both CD and Trp kinetics reveal a fast and a slow conformational transition, only the slow transition is observed by SAXS, indicating that the protein-SDS complex (which is monitored by SAXS) adjusts to the presence of the unfolded protein. We attribute the rapid unfolding of aLA to its predominantly ?-helical structure, which persists in SDS (albeit as isolated helices), enabling aLA to unfold without undergoing major secondary structural changes unlike ?-sheet rich proteins. Nevertheless, the overall unfolding steps are broadly similar to those of the more ?-rich protein ?-lactoglobulin, suggesting that this unfolding model is representative of the general process of SDS-unfolding of proteins.

Dataset Information

Myoglobin and ?-Lactalbumin Form Smaller Complexes with the Biosurfactant Rhamnolipid Than with SDS.

Publications

Myoglobin and α-Lactalbumin Form Smaller Complexes with the Biosurfactant Rhamnolipid Than with SDS.

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