Triblock-Copolymer-Assisted Mixed-Micelle Formation Results in the Refolding of Unfolded Protein

Alkanols Regulate the Fluidity of Phospholipid Bilayer in Accordance to Their Concentration and Polarity

In spite of the widespread use of alkanols as penetration enhancers, their effect on vesicular formulations remains largely unexplored. These can affect the stability and integrity of the phospholipid bilayers. In this study, we have investigated the interaction of linear (ethanol, butanol, hexanol, octanol) and branched alkanols (t-amylol and t-butanol) with three phospholipids (soya lecithin, SL; soy L-α-phosphatidylcholine, SPC; and 1,2-dipalmitoyl-sn-glycero-3-phosphocholine, DPPC). Thermodynamic and structural aspects of these interactions were studied as a function of the alkanol concentration and chain length. Our interpretations are based on isothermal titration calorimetry (ITC) and dynamic light scattering (DLS) experiments. We observed one-site interactions wherein hydroxyl and acyl groups interacted with the polar and nonpolar regions of the phospholipid, respectively. The stability and structural integrity of bilayers appeared to be dependent upon (a) the hydrocarbon chain length and concentration of alcohols, and (b) the degree of unsaturation in the phospholipid molecule. We found that these interactions triggered a reduction in the enthalpy which was compensated by increased entropy, keeping free energy negative. Drop in enthalpy indicates reversible disordering of the bilayer which enables the diffusion of alcohol without triggering destabilization. Ethanol engaged predominantly with the interface, and it resulted in higher enthalpic changes. Interactions became increasingly unfavorable with longer alcohols - a cutoff point was recorded with hexanol. The overall sequence of membrane disordering capability was recorded as follows: ethanol < butanol < octanol < hexanol. Octanol's larger size restricted its penetration in the bilayer, and hence it caused less enthalpic changes relative to hexanol. This could also be verified from the trends in the area ratio of these vesicles obtained from the DLS data. Branched alkanols displayed a lower binding affinity with the phospholipids relative to their linear counterparts. These data are useful while contemplating the inclusion of short-chain alcohols as penetration enhancers in phospholipid vesicles.

Exploring the Specific Role of Iron Center in the Catalytic Activity of Human Serum Transferrin: CTAB-Induced Conformational Changes and Sequestration by Mixed Micelles

Conformational changes play a seminal role in modulating the activity of proteins. This concept becomes all the more relevant in the context of metalloproteins, owing to the formation of specific conformation(s) induced by internal perturbations (like a change in pH, ligand binding, or receptor binding), which may carry out the binding and release of the metal ion/ions from the metal binding center of the protein. Herein, we investigated the conformational changes of an iron-binding protein, monoferric human serum transferrin (Fe-hTF), using several spectroscopic approaches. We could reversibly tune the cetyltrimethylammonium bromide (CTAB)-induced conformation of the protein, exploiting the concept of mixed micelles formed by three sequestrating agents: (3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate) hydrate (CHAPS) and two bile salts, namely, sodium cholate (NaC) and sodium deoxycholate (NaDC). The formation of mixed micelles between CTAB and these reagents (CHAPS/NaC/NaDC) results in the sequestration of CTAB molecules from the protein environment and aids the protein in reattaining its native-like structure. However, the guanidinium hydrochloride-induced denatured Fe-hTF did not acquire its native-like structure using these sequestrating agents, which substantiates the exclusive role of mixed micelles in the present study. Apart from this, we found that the conformation of transferrin (adopted in the presence of CTAB) displays pronounced esterase-like activity toward the para-nitrophenyl acetate (PNPA) substrate as compared to native transferrin. We also outlined the impact of the iron center and amino acids surrounding the iron center on the effective catalytic activity in the CTAB medium. We estimated ∼3 times higher specific catalytic efficiency for the iron-depleted Apo-hTF compared to the fully iron-saturated Fe2-hTF in the presence of CTAB.

Fluorescence monitoring of binding of a Zn (II) complex of a Schiff base with human serum albumin

Zn (II) complexes of Schiff bases have potential applications in biomedical sciences as imaging agents, cancer therapeutics and diagnostics. Thus, it is important to understand their interaction with carrier proteins, like serum albumins. The present paper focuses on the binding interactions between Human serum albumin (HSA) and Znsalampy, making use of fluorescence spectroscopic techniques at ensemble as well as at single molecular level. An idea about the binding constant is obtained from the quenching of the single Trp (Tryptophan) residue of HSA by Znsalampy. Fluorescence correlation spectroscopy (FCS) has also been used to monitor the protein-ligand binding. The location of Znsalampy in its complex with HSA is determined by competitive binding experiments and molecular docking calculations. The binding constant obtained from the Znsalampy-HSA interaction falls in the ideal range for biological applications and the location is found to be in the proximity of Sudlow's site I. The esterase activity of HSA is retained in the presence of the Znsalampy. Hence, it is concluded that this Znsalampy may be a potential probe and biomarker in biomedical applications.

How do surfactants unfold and refold proteins?

Although the anionic surfactant sodium dodecyl sulfate, SDS, has been used for more than half a century as a versatile and efficient protein denaturant for protein separation and size estimation, there is still controversy about its mode of interaction with proteins. The term "rod-like" structures for the complexes that form between SDS and protein, originally introduced by Tanford, is not sufficiently descriptive and does not distinguish between the two current vying models, namely protein-decorated micelles a.k.a. the core-shell model (in which denatured protein covers the surface of micelles) versus beads-on-a-string model (where unfolded proteins are surrounded by surfactant micelles). Thanks to a combination of structural, kinetic and computational work particularly within the last 5-10 years, it is now possible to rule decisively in favor of the core-shell model. This is supported unambiguously by a combination of calorimetric and small-angle X-ray scattering (SAXS) techniques and confirmed by increasingly sophisticated molecular dynamics simulations. Depending on the SDS:protein ratio and the protein molecular mass, the formed structures can range from multiple partly unfolded protein molecules surrounding a single shared micelle to a single polypeptide chain decorating multiple micelles. We also have much new insight into how this species forms. It is preceded by the binding of small numbers of SDS molecules which subsequently grow by accretion. Time-resolved SAXS analysis reveals an asymmetric attack by SDS micelles followed by distribution of the increasingly unfolded protein around the micelle. The compactness of the protein chain continues to evolve at higher SDS concentrations according to single-molecule studies, though the protein remains completely denatured on the tertiary structural level. SDS denaturation can be reversed by addition of nonionic surfactants that absorb SDS forming mixed micelles, leaving the protein free to refold. Refolding can occur in parallel tracks if only a fraction of the protein is initially stripped of SDS. SDS unfolding is nearly always reversible unless carried out at low pH, where charge neutralization can lead to superclusters of protein-surfactant complexes. With the general mechanism of SDS denaturation now firmly established, it largely remains to explore how other ionic surfactants (including biosurfactants) may diverge from this path.

Chelerythrine Chloride: A Potential Rumen Microbial Urease Inhibitor Screened by Targeting UreG

Inhibition of ruminal microbial urease is of particular interest due to its crucial role in regulating urea-N utilization efficiency and nitrogen pollution in the livestock industry. Acetohydroxamic acid (AHA) is currently the only commercially available urease inhibitor, but it has adverse side effects. The urease accessory protein UreG, which facilitates the functional incorporation of the urease nickel metallocentre, has been proposed in developing urease inhibitor through disrupting urease maturation. The objective of this study was to screen natural compounds as potential urease inhibitors by targeting UreG in a predominant ruminal microbial urease. In silico screening and in vitro tests for potential inhibitors were performed using molecular docking and an assay for the GTPase activity of UreG. Chelerythrine chloride was selected as a potential urease inhibitor of UreG with an inhibition concentration IC₅₀ value of 18.13 μM. It exhibited mixed inhibition, with the K_i value being 26.28 μM. We further explored its inhibition mechanism using isothermal titration calorimetry (ITC) and circular dichroism (CD) spectroscopy, and we found that chelerythrine chloride inhibited the binding of nickel to UreG and induced changes in the secondary structure, especially the α-helix and β-sheet of UreG. Chelerythrine chloride formed a pi-anion interaction with the Asp41 residue of UreG, which is an important residue in initiating the conformational changes of UreG. In conclusion, chelerythrine chloride exhibited a potential inhibitory effect on urease, which provided new evidence for strategies to develop novel urease inhibitors targeting UreG to reduce nitrogen excretion from ruminants.

Nature of Saccharide-Induced F127 Micellar Dehydration: An Insight with FDAPT (2-Formyl-5-(4'-N,N-dimethylaminophenyl)thiophene), a Multiparametric Fluorescent Probe

FDAPT (2-formyl-5-(4'-N,N-dimethylaminophenyl)thiophene) is an efficient environment-sensitive fluorescent probe, which senses the alteration of its microenvironment with six different fluorescent parameters, namely, emission intensity, wavelength, fluorescence anisotropy, and corresponding three time-dependent parameters fluorescence lifetime, time-resolved emission spectrum, and anisotropy decay. In the present work, the nature of saccharide-induced dehydration of a F127 polymeric micelle is investigated in detail with FDAPT emission. Using a multiparametric fluorescence approach, it is observed that the saccharide molecules not only decrease the critical micellization temperature of the F127 solution but also strongly alter the physical properties inside the micellar structures. The local polarity and fluidity significantly decrease in the saccharide-induced micelle as compared to the normal F127 micelle. The probe solvation dynamics study reveals that the water content in the core as well as corona domain diminishes significantly in the saccharide-induced micelle as compared to the normal micelle. More precisely, dehydration occurs more in the core region than in the corona region. Also, the saccharide-induced dehydration alters the relative size of the core and corona regions. The extent of dehydration varies with different saccharide molecules. It is also found that the dehydration efficiency order is trisaccharide (raffinose) > disaccharide (sucrose) > monosaccharide (glucose and fructose).

Structural peculiarities of lysozyme - PLURONIC complexes at the aqueous-air and liquid-liquid interfaces and in the bulk of aqueous solution

Interaction between proteins and synthetic polymers that represent a perspective potential in drug delivery or/and already used in medicine plays a key role in biological functioning of both molecules along with a system as a whole. In present study association between hen egg white lysozyme and Pluronic triblock-copolymers (L121, P123 and F127) in the bulk of the solution as well as at the aqueous-air and liquid-liquid interfaces was analyzed by means of spectroscopic and radiochemical assay. In protein-Pluronic complexes lysozyme keeps the secondary structure (CD and SAXS data results), while fluorescence and UV-analysis indicates changes in the local surrounding of fluorophoric amino acid residues. Radiochemical assay in combination with molecular docking reveals the formation of the complexes, in which proline residues turned to the interface between water and hydrophobic medium.

Ecotoxicity and interacting mechanism of anionic surfactant sodium dodecyl sulfate (SDS) and its mixtures with nonionic surfactant fatty alcohol-polyoxyethlene ether (AEO)

This paper reports the proportion-dependent toxicity of binary surfactant mixtures containing anionic sodium dodecyl sulfate (SDS) and nonionic fatty alcohol-polyoxyethlene ether (AEO) toward Photobacterium phosphoreum. The crucial role of toxicity interactions was elucidated by spectroscopic probing the refolding of the unfolded bovine serum albumin (BSA) induced by SDS and theoretical calculating the interaction parameter of mixed surfactants based on Rubingh's model from the critical micelle concentrations. The SDS/AEO mixtures can be divided into two groups based on the toxicity response to the proportion of AEO in the mixtures: Group I contained low mass proportions of AEO, that is, SDS:AEO = 4:1, 3:1; Group II featured high AEO proportions, that is, SDS:AEO = 3:2, 1:1, 2:3, 1:4. The toxicity of SDS/AEO mixtures decreased with the enhanced proportion of AEO in Group I and then fluctuated slightly when the AEO proportion increased to that of Group II. The mixture with the mass ratio of 1:1 showed a slightly higher toxicity than the others in Group II. Scanning electron microscopy (SEM) images illustrated that the addition of AEO hindered the action of SDS against the cell membrane. Fluorescence measurement indicated that AEO could extract SDS molecules embedded in the BSA matrix, except for those bound to the highly active sites of BSA, and refold stepwise the unfolded protein. The results were in excellent analogy to the proportion-dependent toxicity of SDS/AEO mixture, indicating the formation of mixed micelles playing a key role. The interaction parameter further revealed that antagonism led to the mixture with equal mass ratio (1:1) showing higher toxicity than other mass ratios in Group II. These results can be useful for compounding SDS/AEO mixtures in application efficiently and eco-friendly.

A Novel pH-Tunable Secondary Conformation Containing Mixed Micellar System in Anticancer Treatment

In this study, for the first time, we precisely assembled the poly-γ-benzyl-l-glutamate and an amphiphilic copolymer d-α-tocopherol polyethylene glycol succinate into a mixed micellar system for the embedment of the anticancer drug doxorubicin. Importantly, the intracellular drug-releasing behaviors could be controlled by changing the secondary structures of poly-γ-benzyl-l-glutamate via the precise regulation of the buffer’s pH value. Under neutral conditions, the micellar architectures were stabilized by both α-helix secondary structures and the microcrystalline structures. Under acidic conditions (pH 4.0), the interior structures transformed into a coil state with a disordered alignment, inducing the release of the loaded drug. A remarkable cytotoxicity of the Dox-loaded mixed micelles was exhibited toward human lung cancer cells in vitro. The internalizing capability into the cancer cells, as well as the intracellular drug-releasing behaviors, were also identified and observed. The secondary structures containing Dox-loaded mixed micelles had an outstanding antitumor efficacy in human lung cancer A549 cells-bearing nude mice, while little toxicities occurred or interfered with the hepatic or renal functions after the treatments. Thus, these pH-tunable α-helix-containing mixed micelles are innovative and promising for controlled intracellular anticancer drug delivery.

Spectroscopic probing of the refolding of an unfolded protein through the formation of mixed-micelles

We report the unfolding of the globular protein, Bovine Serum Albumin (BSA) induced by anionic surfactant sodium dodecyl sulfate (SDS) and subsequently monitored the refolding of this denatured BSA using triblock copolymers F127 and P123 through the formation of mixed micelles. Our study exclusively represents the reversibility of this unfolding-refolding process using pluronic triblock copolymers F127/P123 as refolding agents. We confirm the recovery of its native state from its denatured state estimating the α-helical structure of the denatured protein from the CD data which support our steady state fluorescence spectra monitoring the fluorescence of the intrinsic Trp molecules present in BSA. Time resolved study also corroborates the stepwise recovery of the denatured BSA as well as the reversibility of the processes. Isothermal Titration Calorimetry (ITC) data explain the negligible interactions between the triblock copolymers and the native state of BSA. The high binding constant of SDS and triblock copolymers probably play the crucial role in the stepwise recovery of the unfolded BSA followed by reversibility of the refolding processes through the formation of the mixed micelles. The mechanism of mixed-micelle formation has been substantiated by the fact that the Guanidine Hydrochloride denatured BSA does not react with F127/P123 whereby no recovery of the protein was observed.