Protein–Surfactant Interaction: Sodium Dodecyl Sulfate-Induced Unfolding of Ribonuclease A

Abstract

Protein-surfactant interaction is widely studied to understand stability and structural changes in proteins. In this Article, we have investigated SDS-induced unfolding of RNase A using absorbance, intrinsic fluorescence of the protein, anisotropy, TNS fluorescence, and near- and far-UV circular dichroism. Unfolding titration curves obtained from the absorbance and fluorescence changes were fitted into a five-state protein unfolding model by assuming formation of three intermediate states. Free energy changes and m-values of all four transitions between the native and unfolded state were evaluated. The transitions are categorized into two different regions. Region I, up to 0.5 mM of SDS, involves ionic interaction between the protein and SDS where the secondary and tertiary structure of the protein is altered to a less extent. In region II, hydrophobic interaction dominates and has two distinct transitions. The first transition arises from the aggregation of surfactant molecules around the protein hydrophobic sites. In the following transition, the micelles probably expand more, and a few more hydrophobic sites are occupied by the surfactant. In this region, the tertiary contacts are completely broken, and almost 50% of the secondary structure is lost. The aggregation of SDS around the protein starts well below the CMC. These conformational changes can be explained by the necklace and beads model, and the free energy of formation of such a complex for the RNase A-SDS system is found to be 5.2 (±1.0) kcal mol(-1). The probable interaction sites and the mechanism of unfolding have been discussed in detail.

Quantification of Membrane Protein-Detergent Complex Interactions

“…The FP spectroscopy was previously used to inspect: (i) the interactions of mild 14 and harsh 15 detergents with water-soluble proteins, (ii) the harsh detergent-induced unfolding 16 and resistance of soluble proteins to denaturation, 17 (iii) the detergent-mediated oligomerization of hydrophobic proteins into proteomicelles, 18 and (iv) the impact of detergent on conformational changes 14 and enzymatic activity 19 of soluble proteins.…”

Interrogating Detergent Desolvation of Nanopore-Forming Proteins by Fluorescence Polarization Spectroscopy

“…This assay relied on the use of fluorescence polarization (FP) spectroscopy. 15–18 In the past, FP spectroscopy was employed for inspecting the PDC interactions with either mild 19 or harsh detergents 20–22 and water-soluble proteins. In contrast to this work, prior sodium dodecyl sulfate (SDS)-protein interaction studies were focused on the mechanistic understanding of harsh detergent-induced protein unfolding 22 and resistance of proteins to denaturation 21 under diverse environmental conditions.…”

Molecular Insight into Human Lysozyme and Its Ability to Form Amyloid Fibrils in High Concentrations of Sodium Dodecyl Sulfate: A View from Molecular Dynamics Simulations

“…Previous studies have shown that lysozyme can easily form amyloid fibrils under different conditions, such as high temperatures and appropriate pH conditions [ 37 – 40 ]. Many amyloidogenic proteins interact with the surfaces of basement membranes; SDS has the potential to mimic this condition [ 29 , 41 – 45 ]. Furthermore, SDS surfactants mimic the bio-membrane environment and can provide anionic conditions through their anionic head groups [ 42 , 46 ].…”