Kinetic studies on the interaction of ferricytochrome c with anionic surfactants

Kinetics of Structural Transitions Induced by Sodium Dodecyl Sulfate in α-Chymotrypsin

The temporal changes in circular dichroism at 222 and 260 nm were recorded by using stopped-flow spectroscopy after mixing α-chymotrypsin solutions with sodium dodecyl sulfate solutions. Simultaneously with the circular dichroism signal, the fluorescence emission was recorded. Changes in the secondary and tertiary structures of chymotrypsin induced by sodium dodecyl sulfate are characterized by either three or four one-way reactions with relaxation amplitudes and times precisely determined by an advanced numerical procedure of Kuzmič. Quantitatively, transitions within the secondary and tertiary structures of the protein are significantly different. Moreover, changes in the tertiary structure depend on the type of recorded signal (either circular dichroism or fluorescence) and the wavelength of the incident radiation. The latter observation is particularly interesting as it indicates that the contributions of protein's different tryptophans to the total recorded fluorescence depend on the excitation wavelength. We present several results justifying this hypothesis.

Measuring Structural Changes in Cytochrome c under Crowded Conditions Using In Vitro and In Silico Approaches

It is known from in vitro studies that macromolecular crowding in the cell effects protein structure, stability and function; but predictive studies are relatively unexplored. There are few reports where the effect of various crowder mixtures has been exploited to discern their combined effect on the structural stability of proteins. These studies are more significant because their effect can mimicked with in vivo conditions, where the environment is heterogeneous. Effects of two crowders, polyethylene glycol (PEG 400 Da), and its monomer ethylene glycol (EG) alone and in mixture on the structural stability of cytochrome c (cyt c) were determined using various spectroscopic and bioinformatics tools. The main conclusions of our study are (i) the monomer EG has a kosmotropic effect on the protein (stabilizes the protein), and has no significant effect on the tertiary structure; (ii) PEG 400 destabilizes the structure as well as the stability of the protein; and (iii) EG counteracts the destabilizing effect of PEG 400. From this investigation, it seems evident that proteins may fold or unfold in the crowded environment of the cell where various interactions assist them to maintain their structure for their functions. Bioinformatics approaches were also used to support all of the in vitro observations. Cyt c is functional protein; if the structure of the protein is modulated due to change in the environment its nature of function will also change. Our research addresses the question by modulating the environment around the protein, and the macromolecule (protein) conformation dynamics and interaction study via in vitro and in silico approaches which indirectly compares with that of the environment in-cellular milieu, which is highly crowded.

Interaction of polyethylene glycol with cytochrome c investigated via in vitro and in silico approaches

One of the significant proteins that have attracted research groups due to virtue of being a potent selective anticancer drug target and property of triggering apoptosis upon release in cytoplasm is cytochrome c (cyt c). The mechanical transformations due to the macromolecular crowding in membrane in the mammalian cell are proposed to be useful inductors of changes in volume. It is very interesting to know that mitochondrial function were observed to be improved by polyethylene glycol (PEG) interaction, which in turn inhibits the cyt c (a pro-apoptotic cell death factor). In this work, the effect of polyethylene glycol of molecular weight 4 kilo Dalton (PEG 4 kDa) was investigated to highlight the structural transformations (tertiary and secondary structure) in cyt c using a choice of spectroscopic techniques (including UV-Vis absorption, near-UV, far-UV and Soret circular dichroism and fluorescence spectroscopy), which shows noteworthy shifts in the secondary and tertiary structures at higher concentrations of PEG 4 kDa with small changes in the heme-globular interactions. The size distribution changes of native protein treated with various concentrations of the crowder were observed and analyzed by dynamic light scattering (DLS). The interaction studies of the crowder with the protein was observed and analyzed by FTIR, isothermal titration calorimetry, time resolved fluorescence and molecular docking. The investigations suggested that the structural changes in the protein occurred due to soft interactions of PEG 4 kDa, which usually destabilizes proteins. The experimental evidence in this study proposed that crowding could be another approach to mechanical super-competition and free of certain markers that could aid in the identification and control of various diseases. This study suggests that crowders at specific concentrations, which softly interact with proteins, can be exploited as remedy for various diseases.

A study on the interaction of horse heart cytochrome c with some conventional and ionic liquid surfactants probed by ultraviolet-visible and fluorescence spectroscopic techniques

The interactions of a protein cytochrome c with some selected conventional and ionic liquid surfactants have been investigated at pH7.4 using ultraviolet-visible and fluorescence spectroscopic techniques. We used four conventional surfactants - cetyltrimethylammonium bromide (CTAB), dodecyltrimethylammonium bromide (DTAB), sodium N-dodecanoylsarcosinate (SDDS), and N-decanoyl-N-methylglucamine (Mega 10), and a surface active ionic liquid 1-hexadecyl-3-methylimidazolium chloride (C₁₆MeImCl). All the investigated surfactants were found to induce an unfolding of the protein cytochrome c. In presence of CTAB, SDDS and C₁₆MeImCl, the heme iron atom was found to loose methionine from its axial position. Differential binding of the surfactant monomers and their micelles to the protein molecules was inferred. The ionic surfactants were found to be more effective than the nonionic one in unfolding the investigated protein. However, the extent of binding of CTAB/C₁₆MeImCl to cytochrome c reaches a plateau past the critical micellization concentration (cmc) of the surfactant. For each of the cytochrome c-DTAB, cytochrome c-SDDS and cytochrome c-Mega 10 system, although there exists an inflection in the surfactant-binding, saturation point could not be detected. It has been demonstrated from the ultraviolet-visible spectral studies that the oxidation state of iron in cytochrome c does not change when the protein binds with the investigated surfactants.

SDS induced dissociation of STY3178 oligomer: experimental and molecular dynamics studies

STY3178 the yfdX Salmonella Typhi protein dissociates reversibly in presence of sodium dodecyl sulphate from trimer to monomer.

Effect of Bioconjugation on the Reduction Potential of Heme Proteins

The modification of protein surfaces employing cationic and anionic species enables the assembly of these biomaterials into highly sophisticated hierarchical structures. Such modifications can allow bioconjugates to retain or amplify their functionalities under conditions in which their native structure would be severely compromised. In this work, we assess the effect of this type of bioconjugation on the redox properties of two model heme proteins, that is, cytochrome c (CytC) and myoglobin (Mb). In particular, the work focuses on the sequential modification by 3-dimethylamino propylamine (DMAPA) and 4-nonylphenyl 3-sulfopropyl ether (S1) anionic surfactant. Bioconjugation with DMAPA and S1 are the initial steps in the generation of pure liquid proteins, which remain active in the absence of water and up to temperatures above 150 °C. Thin-layer spectroelectrochemistry reveals that DMAPA cationization leads to a distribution of bioconjugate structures featuring reduction potentials shifted up to 380 mV more negative than the native proteins. Analysis based on circular dichroism, MALDI-TOF mass spectrometry, and zeta potential measurements suggest that the shift in the reduction potentials are not linked to protein denaturation, but to changes in the spin state of the heme. These alterations of the spin states originate from subtle structural changes induced by DMAPA attachment. Interestingly, electrostatic coupling of anionic surfactant S1 shifts the reduction potential closer to that of the native protein, demonstrating that the modifications of the heme electronic configuration are linked to surface charges.

Ionic liquid-induced all-α to α + β conformational transition in cytochrome c with improved peroxidase activity in aqueous medium

Choline dioctylsulfosuccinate [Cho][AOT] (a surface active ionic liquid) has been found to induce all-α to α + β conformational transition in the secondary structure of enzyme cytochrome c (Cyt c) with an enhanced peroxidase activity in its aqueous vesicular phase at pH 7.0.

Interactions of anionic surfactants with methemoglobin

Interactions of two anionic surfactants, sodium dodecyl sulphate (SDS) and sodium bis(2-ethylhexyl) sulfosuccinate (AOT) at concentrations below and above critical micelle concentration with methemoglobin (metHb) have been investigated by conventional as well as by stopped-flow absorption and fluorescence spectroscopy. The absorption spectra of metHb in AOT reverse micelles have been also analyzed. Both surfactants in their monomeric form convert metHb to reversible hemichrome. This is connected with a diminution of peroxidase-like activity of metHb and with an increase of the susceptibility of heme for a damage by H(2)O(2). In micellar solutions of AOT and SDS as well as in AOT reverse micelles pentacoordinated ferric species seems to be the predominant form of this protein. It has been concluded, basing on a kinetic analysis, that conformational changes in the heme environment of metHb as induced by both surfactants occur independently of the alterations in the tertiary structure of this protein.

Interaction between the natural lipopeptide [Glu1, Asp5)] surfactin-C15 and hemoglobin in aqueous solution

The interaction between natural lipopeptide [Glu(1), Asp(5)] surfactin-C15 (surfactin) and hemoglobin (Hb) has been studied. Surface tension measurements show that the critical micelle concentration (cmc) of surfactin increases from 1.54 x 10(-5) to 3.86 x 10(-5) mol/L with Hb. The UV spectra display that the effect of surfactin on Hb exhibits strong concentration-dependent fashion and the aquometHb convert to hemichrome at high surfactin concentration. Small-angle neutron scattering (SANS) and freeze-fracture transmission electron microscopy (FF-TEM) measurements show that surfactin result in the formation of a fractal structure representing a "necklace model" of micelle-like clusters randomly distributed along the protein polypeptide chain at high surfactin concentration. Far-UV circular dichroism (CD) results confirmed that surfactin can disrupt the helical structure of protein at high concentrations, although the enhanced native-like behavior of protein by low concentration of surfactin was observed. The microenvironment change around Phe amino residues and disulfide bonds of Hb was obtained from near-UV CD spectra.

On the mechanism of SDS-induced protein denaturation

To understand the mechanism of ionic detergent-induced protein denaturation, this study examines the action of sodium dodecyl sulfate on ferrocytochrome c conformation under neutral and strongly alkaline conditions. Equilibrium and stopped-flow kinetic results consistently suggest that tertiary structure unfolding in the submicellar and chain expansion in the micellar range of SDS concentrations are the two major and discrete events in the perturbation of protein structure. The nature of interaction between the detergent and the protein is predominantly hydrophobic in the submicellar and exclusively hydrophobic at micellar levels of SDS concentration. The observation that SDS also interacts with a highly denatured and negatively charged form of ferrocytochrome c suggests that the interaction is independent of structure, conformation, and ionization state of the protein. The expansion of the protein chain at micellar concentration of SDS is driven by coulombic repulsion between the protein-bound micelles, and the micelles and anionic amino acid side chains.

Interaction of giant extracellular Glossoscolex paulistus hemoglobin (HbGp) with zwitterionic surfactant N-hexadecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate (HPS): Effects of oligomeric dissociation

The present work focuses on the interaction between the zwitterionic surfactant N-hexadecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate (HPS) and the giant extracellular hemoglobin of Glossoscolex paulistus (HbGp). Electronic optical absorption, fluorescence emission and circular dichroism spectroscopy techniques, together with Gel-filtration chromatography, were used in order to evaluate the oligomeric dissociation as well as the autoxidation of HbGp as a function of the interaction with HPS. A peculiar behavior was observed for the HPS-HbGp interaction: a complex ferric species formation equilibrium was promoted, as a consequence of the autoxidation and oligomeric dissociation processes. At pH 7.0, HPS is more effective up to 1mM while at pH 9.0 the surfactant effect is more intense above 1mM. Furthermore, the interaction of HPS with HbGp was clearly less intense than the interaction of this hemoglobin with cationic (CTAC) and anionic (SDS) surfactants. Probably, this lower interaction with HPS is due to two factors: (i) the lower electrostatic attraction between the HPS surfactant and the protein surface ionic sites when compared to the electrostatic interaction between HbGp and cationic and anionic surfactants, and (ii) the low cmc of HPS, which probably reduces the interaction of the surfactant in the monomeric form with the protein. The present work emphasizes the importance of the electrostatic contribution in the interaction between ionic surfactants and HbGp. Furthermore, in the whole HPS concentration range used in this study, no folding and autoxidation decrease induced by this surfactant were observed. This is quite different from the literature data on the interaction between surfactants and tetrameric hemoglobins, that supports the occurrence of this behavior for the intracellular hemoglobins at low surfactant concentration range. Spectroscopic data are discussed and compared with the literature in order to improve the understanding of hemoglobin-surfactant interaction as well as the acid isoelectric point (pI) influence of the giant extracellular hemoglobins on their structure-activity relationship.

Spectroscopic, structural, and functional characterization of the alternative low-spin state of horse heart cytochrome C

The alternative low-spin states of Fe(3+) and Fe(2+) cytochrome c induced by SDS or AOT/hexane reverse micelles exhibited the heme group in a less rhombic symmetry and were characterized by electron paramagnetic resonance, UV-visible, CD, magnetic CD, fluorescence, and Raman resonance. Consistent with the replacement of Met(80) by another strong field ligand at the sixth heme iron coordination position, Fe(3+) ALSScytc exhibited 1-nm Soret band blue shift and epsilon enhancement accompanied by disappearance of the 695-nm charge transfer band. The Raman resonance, CD, and magnetic CD spectra of Fe(3+) and Fe(2+) ALSScytc exhibited significant changes suggestive of alterations in the heme iron microenvironment and conformation and should not be assigned to unfold because the Trp(59) fluorescence remained quenched by the neighboring heme group. ALSScytc was obtained with His(33) and His(26) carboxyethoxylated horse cytochrome c and with tuna cytochrome c (His(33) replaced by Asn) pointing out Lys(79) as the probable heme iron ligand. Fe(3+) ALSScytc retained the capacity to cleave tert-butylhydroperoxide and to be reduced by dithiothreitol and diphenylacetaldehyde but not by ascorbate. Compatible with a more open heme crevice, ALSScytc exhibited a redox potential approximately 200 mV lower than the wild-type protein (+220 mV) and was more susceptible to the attack of free radicals.

Crystallization of soluble proteins in vapor diffusion for x-ray crystallography

The preparation of protein single crystals represents one of the major obstacles in obtaining the detailed 3D structure of a biological macromolecule. The complete automation of the crystallization procedures requires large investments in terms of money and labor, which are available only to large dedicated infrastructures and is mostly suited for genomic-scale projects. On the other hand, many research projects from departmental laboratories are devoted to the study of few specific proteins. Here, we try to provide a series of protocols for the crystallization of soluble proteins, especially the difficult ones, tailored for small-scale research groups. An estimate of the time needed to complete each of the steps described can be found at the end of each section.

Giant extracellular Glossoscolex paulistus Hemoglobin (HbGp) upon interaction with cethyltrimethylammonium chloride (CTAC) and sodium dodecyl sulphate (SDS) surfactants: Dissociation of oligomeric structure and autoxidation

The effects of two ionic surfactants on the oligomeric structure of the giant extracellular hemoglobin of Glossoscolex paulistus (HbGp) in the oxy - form have been studied through the use of several spectroscopic techniques such as electronic optical absorption, fluorescence emission, light scattering, and circular dichroism. The use of anionic sodium dodecyl sulphate (SDS) and cationic cethyltrimethyl ammonium chloride (CTAC) has allowed to differentiate the effects of opposite headgroup charges on the oligomeric structure dissociation and hemoglobin autoxidation. At pH 7.0, both surfactants induce the protein dissociation and a significant oxidation. Spectral changes occur at very low CTAC concentrations suggesting a significant electrostatic contribution to the protein-surfactant interaction. At low protein concentration, 0.08 mg/ml, some light scattering within a narrow CTAC concentration range occurs due to protein-surfactant precipitation. Light scattering experiments showed the dissociation of the oligomeric structure by SDS and CTAC, and the effect of precipitation induced by CTAC. At higher protein concentrations, 3.0 mg/ml, a precipitation was observed due to the intense charge neutralization upon formation of ion pair in the protein-surfactant precipitate. The spectral changes are spread over a much wider SDS concentration range, implying a smaller electrostatic contribution to the protein-surfactant interactions. The observed effects are consistent with the acid isoelectric point (pI) of this class of hemoglobins, which favors the intense interaction of HbGp with the cationic surfactant due to the existence of excess acid anionic residues at the protein surface. Protein secondary structure changes are significant for CTAC at low concentrations while they occur at significantly higher concentrations for SDS. In summary, the cationic surfactant seems to interact more strongly with the protein producing more dramatic spectral changes as compared to the anionic one. This is opposite as observed for several other hemoproteins. The surfactants at low concentrations produce the oligomeric dissociation, which facilitates the iron oxidation, an important factor modulating further oligomeric protein dissociation.

Peracetylated Bovine Carbonic Anhydrase (BCA-Ac18) Is Kinetically More Stable than Native BCA to Sodium Dodecyl Sulfate

Bovine carbonic anhydrase (BCA) and its derivative with all lysine groups acetylated (BCA-Ac18) have different stabilities toward denaturation by sodium dodecyl sulfate (SDS). This difference is kinetic: BCA-Ac18 denatures more slowly than BCA by several orders of magnitude over concentrations of SDS ranging from 2.5 to 10 mM. The rates of renaturation of BCA-Ac18 are greater than those of BCA, when these proteins are allowed to refold from a denatured state ([SDS]=10 mM) to a folded state ([SDS]=0.1 to 1.5 mM). On renaturation, the yields of the correctly folded protein (either BCA or BCA-Ac18) decrease with increasing concentration of SDS. At intermediate concentrations of SDS (from 0.7 to 2 mM for BCA, and from 1.5 to 2 mM for BCA-Ac18), both unfolding and refolding of the proteins are too slow to be observed; an alternative process-probably aggregation-competes with refolding of the denatured proteins at those intermediate concentrations. Because it is experimentally impractical to prove equilibrium, it is not possible to establish whether there is a difference in the thermodynamics of unfolding/refolding between BCA and BCA-Ac18.

Cytochrome c?dimyristoylphosphatidylglycerol interactions studied by asymmetrical flow field-flow fractionation

Lipid membranes are well recognized ligands that bind peripheral and integral proteins in a specific manner and regulate their function. Cytochrome c (cyt c) is one of the partner peripheral protein that binds to the lipid membranes via electrostatic and hydrophobic interactions. In this study, asymmetrical flow field-flow fractionation (AsFlFFF) was used to compare the interactions of cyt c with the acidic phospholipid 1,2-dimyristoyl-sn-glycero-3-phospho-rac-glycerol (DMPG), oleic acid (OA), and sodium dodecyl sulfate (SDS). The influence of pH and the cyt c-lipid molar mass ratios were evaluated by monitoring the diffusion coefficients and particle diameter distributions obtained for the free and lipid-bound protein. The hydrodynamic particle diameter of cyt c (pI 10) was 4.1 nm at pH 11.4 and around 4.2 nm at pH 7.0 and 8.0. Standard molar mass marker proteins were used for calibration to obtain the molar masses of free cyt c and its complexes with lipids. AsFlFFF revealed the binding of cyt c to DMPG and to OA to be mainly electrostatic. In the absence of electrostatic interactions, minor complex formation occurred, possibly due to the extended lipid anchorage involving the hydrophobic cavity of cyt c and the hydrocarbon chains of DMPG or SDS. The possibility of the formation of the molten globule state of cyt c, induced by the interaction between cyt c and lipids, is discussed.

Effect of sodium dodecyl sulfate on folding and thermal stability of acid-denatured cytochrome c: a spectroscopic approach

The molten globule (MG) state can be an intermediate in the protein folding pathway; thus, its detailed description can help understanding protein folding. Sodium dodecyl sulfate (SDS), an anionic surfactant that is commonly used to mimic hydrophobic binding environments such as cell membranes, is known to denature some native state proteins, including horse cytochrome c (cyt c). In this article, refolding of acid denatured cyt c is studied under the influence of SDS to form MG-like states at both low concentration and above the critical micelle concentration using Fourier transform Infrared (FTIR) and ultraviolet and visible absorption as well as fluorescence and circular dichroism (CD). Thermal denaturation monitored with FTIR and CD shows distinct final high temperature states starting from MG-like states formed with different SDS/protein ratios. The results suggest that the SDS/protein ratio as well as the actual SDS (or protein) concentration affects structure and its thermal stability. Thermal denaturation monitored with CD and FTIR for cyt c at neutral pH but denatured with SDS showed that at a high SDS/protein ratio, the thermal behavior of MG-like states formed at low and neutral pH are quite similar. Based on the results obtained, the merits of two models of the protein-surfactant structure are discussed for different SDS concentrations.

1H NMR structural characterization of the cytochrome c modifications in a micellar environment

The interaction of cytochrome c with micelles of sodium dodecyl sulfate was studied by proton NMR spectroscopy. The protein/micelles ratio was found to be crucial in controlling the extent of the conformational changes in the heme crevice. Over a range of ratios between 1:30 and 1:60, the NMR spectra of the ferric form display no paramagnetic signals due to a moderately fast exchange between intermediate species on the NMR time scale. This is consistent with an interconversion of bis-histidine derivatives (His18-Fe-His26 and His18-Fe-His33). Further addition of micelles induces a high-spin species that is proposed to involve pentacoordinated iron. The resulting free binding site, also encountered in the ferrous form, is used to complex exogenous ligands such as cyanide or carbon monoxide. Attribution of the heme methyls was performed by means of exchange spectroscopy through ligand exchange or electron transfer. The heme methyl shift pattern of the micellar cyanocytochrome in the ferric low spin form is different from the pattern of both the native and the cyanide cytochrome c adduct, in the absence of micelles, reflecting a complete change of the heme electronic structure. Analysis of the electron self-exchange reaction between the two redox states of the micellar cyanocytochrome c yields a rate constant of 2.4 x 10(4) M(-1) s(-1) at 298 K, which is surprisingly close to the value observed in the native protein.

Conformational equilibria and dynamics of cytochrome c induced by binding of sodium dodecyl sulfate monomers and micelles

Circular dichroism, nuclear magnetic resonance, electron paramagnetic resonance, UV-vis absorption, and resonance Raman (RR) spectroscopic techniques were employed to study protein and heme structural changes of cytochrome c (Cyt-c) induced by sodium dodecyl sulfate (SDS) monomers and micelles via hydrophobic and electrostatic interactions, respectively. Both modes of interactions cause the transition to the conformational state B2, which is implicated to be involved in the physiological processes of Cyt-c. At sub-micellar concentrations of SDS, specific binding of only ca. three SDS monomers, which is likely to occur at the hydrophobic peptide segment 81-85, is sufficient for a complete conversion to a B2 state in which Met80 is replaced by His33 (His26). These heme pocket structural changes are not linked to secondary structure changes of the protein brought about by nonspecific binding of SDS monomers in different regions of the protein. Upon binding of micelles, B2 high-spin species can also be stabilized by electrostatic interactions. In addition, the micelle interaction domain is located on the front surface of Cyt-c, which includes a ring-like arrangement of lysine residues appropriate for binding one micelle. According to freeze-quench RR and stopped-flow experiments, state B2 is formed on the long millisecond timescale and reveals a complex dependence on the SDS concentration that can be interpreted in terms of competitive binding of monomers and micelles.