Interaction between sodium dodecyl sulfate and ferricytochrome c

Complete observation of all structural, conformational, and fibrillation transitions of monomeric globular proteins at submicellar sodium dodecyl sulfate concentrations

Although considerable information is available regarding protein-sodium dodecyl sulfate (SDS) interactions, it is still unclear as to how much SDS is needed to denature proteins. The role of protein charge and micellar surfactant concentration on amyloid fibrillation is also unclear. This study reports on equilibrium measurements of SDS interaction with six model proteins and analyzes the results to obtain a general understanding of conformational breakdown, reorganization and restructuring of secondary structure, and entry into the amyloid fibrillar state. Significantly, all of these responses are entirely resolved at much lower than the critical micellar concentration (CMC) of SDS. Electrostatic interaction of the dodecyl sulfate anion (DS^- ) with positive surface potential on the protein can completely unfold both secondary and tertiary structures, which is followed by protein chain restructuration to α-helices. All SDS-denatured proteins contain more α-helices than the corresponding native state. SDS interaction stochastically drives proteins to the aggregated fibrillar state.

Unfolding Pathway of a Globular Protein by Surfactants Monitored with Raman Optical Activity

Protein denaturation by surfactants has received increased attention in the last years due to its implications in topics such as pharmaceutics, cosmetics, paints, or biotechnology. This phenomenon is highly dependent on the physicochemical (structural) properties of the denaturing agents. In this work, we have measured for the first time the Raman optical activity (ROA) of bovine serum albumin (BSA) in the presence of three surfactants (anionic, cationic, and neutral), which has allowed us to detect new spectroscopic insights of the protein-surfactant interaction that conventional Raman spectroscopy cannot. Our work proposes two new groups of ROA marker bands to explore the unfolding of BSA induced by surfactants, which are related to "polar" (amide I and III modes) and "apolar" (methylene bending and phenyl breathing modes) protein sections. The appearance of the former groups is related to the initial attack of the surfactant, while the second groups relate to the hydrophobic unfolding.

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.

On the edge of the denaturation process: Application of X-ray diffraction to barnase and lysozyme cross-linked crystals with denaturants in molar concentrations

Structural data about the early step of protein denaturation were obtained from cross-linked crystals for two small proteins: barnase and lysozyme. Several denaturant agents like urea, bromoethanol or thiourea were used at increasing concentrations up to a limit leading to crystal disruption (>or=2 to 6 M). Before the complete destruction of the crystal order started, specific binding sites were observed at the protein surfaces, an indication that the preliminary step of denaturation is the disproportion of intermolecular polar bonds to the benefit of the agent "parasiting" the surface. The analysis of the thermal factors first agree with a stabilization effect at low or moderate concentration of denaturants rapidly followed by a destabilization at specific weak points when the number of sites increase (overflooding effect).

The interaction of ferrocytochrome c with long-chain fatty acids and their CoA and carnitine esters

Non-covalent modification of cytochrome c may have implications for electron transport and energy metabolism. We examined the interaction of various fatty acids (FAs), their coenzyme A and carnitine esters, and fatty alcohols with horse heart ferrocytochrome c. A comparison of FAs indicated a minimum chain length of 14 carbons was required for significant effect on the ferroheme chromophore and major changes in electronic spectra. Coenzyme A and carnitine esters interacted less strongly than FAs whereas long-chain alcohols did not interact with the protein. We found a single, saturable FA binding site with Kd (oleate) of 23.1 µM (by stopped-flow kinetics), 30 µM (by radiochemical binding assay), and 29 µM (by spectrophotometric assay). The binding stoichiometry was 1:1. We present evidence from electronic spectra, and proton NMR (nuclear magnetic resonance) that the S–Fe coordination (methionine 80) was disrupted by ligand binding. From molecular modeling we identify a putative binding channel flanked by lysines 72 and 73.Key words: cytochrome c, fatty acids, acyl-CoA, acyl-carnitine.

Kinetic studies on the interaction of ferricytochrome c with anionic surfactants

The kinetics of absorbance and fluorescence changes of cytochrome c as induced by an aqueous solution of the anionic surfactant sodium dodecyl sulfate (SDS) or sodium bis(2-ethylhexyl)sulfosuccinate (AOT) are studied. The results are compared with far-UV circular dichroism (CD) spectra. Both surfactants cause similar alterations in the secondary structure of cytochrome c, while their influence on the heme environment of cytochrome c is different. In the presence of AOT below and above critical micellar concentration a conversion of the low-spin native cytochrome c to a denatured low-spin protein not having methionine ligand takes place. In the presence of SDS micelles conversion of the native protein to a denatured mixed-spin form occurs. The changes in the heme group induced by both surfactants occur independently of the alterations in tertiary structure.

Statistical Effects of the Binding of Ionic Surfactant to Protein

Two different theories based on a multiple equilibrium model for analysing the binding data for ionic surfactant-protein interactions are investigated and modified, and intrinsic and statistical Gibbs free energies of binding per mole of surfactant are estimated. The characterization of the two models and interpretation of the binding process in terms of intrinsic and statistical binding free energies are discussed. These theories are applied to analysis of sodium n-dodecyl sulfate binding to ribonuclease A and lysozyme. Copyright 1997Academic Press

A change in the heme stereochemistry of cytochrome c upon addition of sodium dodecyl sulfate: electron paramagnetic resonance and electronic absorption spectral study

Electron paramagnetic resonance and electronic absorption spectral changes upon addition of sodium dodecyl sulfate (SDS) to ferric and ferrous cytochrome c have been measured at 77 degrees K and at room temperature. The spectral changes upon addition of SDS to ferric cytochrome c were performed, in two steps, from native low-spin to another low-spin spectrum and subsequently to high-spin-like spectrum. On the other hand, the spectral changes upon addition of SDS to ferrous cytochrome c proceeded, in one step, from native low-spin to high-spin spectrum. The high-spin-like spectrum of ferric cytochrome c and the high-spin spectrum of ferrous cytochrome c in the presence of high concentrations of SDS are, respectively, apparently similar to those of ferric and ferrous cytochrome c' at physiological pH in spectral features. These spectral similarities suggest the similarities in the heme stereochemistry and the ground state of heme iron. Further, the spectra of cytochrome c in the presence of SDS varied with the change of pH values. The ferric high-spin-like and ferrous high-spin spectra were stable at neutral pH and below it. Conformational changes of cytochrome c upon addition of SDS are also discussed.

Mechanism-based in vivo inactivation of lauric acid hydroxylases

The hepatic cytochrome P-450 isozymes that catalyze omega- and (omega - 1)-hydroxylation of lauric acid are specifically inactivated in vitro but not in vivo by 10-undecynoic acid. The lack of in vivo activity may result from rapid degradation of the inhibitor by beta-oxidation. Strategies for the construction of fatty acid analogues that retain the ability to inactivate fatty acid hydroxylases but are resistant to metabolic degradation have therefore been sought. Fatty acid analogues in which the carboxylic acid group is replaced by a sulfate moiety, or in which two methyl groups are placed vicinal to the carboxylic acid group, have been found to inactivate lauric acid hydroxylases in vitro and in vivo without causing time-dependent inhibition of ethoxycoumarin O-deethylation or N-methyl-p-chloroaniline N-demethylation.

Identification of heme axial ligands of cytochrome c' from Alcaligenes sp. N.C.I.B. 11015

The spectral properties of both ferric and ferrous cytochromes c' from Alcaligenes sp. N.C.I.B. 11015 are reported. The EPR spectra at 77 K and the electronic, resonance Raman, CD and MCD spectra at room temperature have been compared with those of the other cytochromes c' and various hemoproteins. In the ferrous form, all the spectral results at physiological pH strongly indicated that the heme iron(II) is in a high-spin state. In the ferric form, the EPR and electronic absorption spectra were markedly dependent upon pH. EPR and electronic spectral results suggested that the ground state of heme iron(III) at physiological pH consists of a quantum mechanical admixture of an intermediate-spin and a high-spin state. Under highly alkaline conditions, identification of the axial ligands of heme iron(III) was attempted by crystal field analysis of the low-spin EPR g values. Upon the addition of sodium dodecyl sulfate to ferric and ferrous cytochrome c', the low-spin type spectra were induced. The heme environment of this low-spin species is also discussed.

Circular dichroism study of the unfolding-refolding of a cardiotoxin from Taiwan cobra (Naja naja atra) venom

Circular dichroism spectroscopy has been used to study the unfolding-refolding process of a cardiotoxin from Taiwan cobra (Naja naja atra) venom upon addition of fluoroalcohols or sodium dodecyl sulfate (SDS) to its aqueous solution. In these experiments, the disulfide bridges remained intact. The unfolding process has been found to be reversible both for fluoroalcohols and for SDS unfolding. The reversibility of the unfolding-refolding process of cardiotoxin in aqueous mixtures of fluoroalcohols was dependent on the volume per volume ratio of alcohol to water. SDS did not unfold the secondary structures of cardiotoxin whereas its tertiary structure was affected. If the SDS concentration in aqueous solution exceeded the critical micelle concentration value of SDS, a quasi-refolded state of cardiotoxin was observed. The mechanism of unfolding-refolding is discussed in terms of molecular interactions which might govern the protein conformation in solution.

Kinetic aspects of the interaction of horse heart cytochrome c with sodium dodecyl sulfate

The sodium dodecyl sulfate (SDS) concentration dependence of spectral changes in circular dichroism (CD) and in absorbance of cytochrome c were examined in the far-ultraviolet region, aromatic region, and the Soret band. The Soret peak obtained in 0.60 mM SDS was nine times greater than that of the native state. (The critical micelle concentration, CMC, of SDS was 2.2 mM in the phosphate buffer used.) The results indicated that the drastic change at the Soret band did not accompany the corresponding large-scale change in secondary structure of the protein. In the stopped-flow measurements, two and three processes were followed at 406 nm below and above the CMC, respectively. At 289 nm only one process was observed, and this corresponded to the second process at 406 nm. Therefore, the second process at 406 nm was considered to be a change in tertiary structure around the heme group. The first process and the third process seemed to reflect a change in the heme environment; the former appeared to be due to a solvent effect and the latter due to a binding effect of a large number of dodecyl sulfate ions.

Cooperative binding of hexadecyltrimethylammonium chloride and sodium dodecyl sulfate to cytochrome c and the resultant change in protein conformation

The binding of hexadecyltrimethylammonium chloride (HTAC) and sodium dodecyl sulfate (SDS) to cytochrome c was determined by potentiometric titration and the corresponding changes in protein conformation by circular dichroism (CD). The binding isotherms were biphasic; about 20 surfactant cations or anions were bound to cytochrome c in the first phase. Another 30 or so HTA+ ions were bound in the second phase, which was below the critical micelle concentration of the surfactant, but the binding of dodecyl sulfate ions in the second phase increased sharply near the critical micelle concentration. The binding of both surfactants was highly cooperative and was endothermic; the data in the first phase fitted the Hill plot. The corresponding change in the secondary structure of cytochrome c was small; the CD spectra in the ultraviolet region showed a moderate increase in the helicity in HTAC solution and some changes in conformation in SDS solution. However, the CD spectra for the Soret band indicated a marked change in the local conformation around the heme.

Conformation of insulin and its fragments in surfactant solutions

The conformation of insulin, A and B chains and des(B23-30)octapeptide insulin (DOI) in surfactant solutions was studied by circular dichroism (CD). Addition of sodium dodecyl sulfate (NaDodSO4), dodecylammonium chloride (DodNH3Cl) or dodecyltrimethylammonium chloride (Dod(CH3)3NCl) slightly alters the CD spectrum of insulin, but not DOI. The CD of insulin in NaDodSO4 solution resembles that of DOI without the surfactant, suggesting the dissociation of the COOH-terminal antiparallel beta-form of insulin oligomers (DOI lacks B23-30 and exists as monomers). Reduced insulin and DOI lose their ordered structure in water, but their helical conformation is restored in surfactant solutions. Reduced DOI in NaDodSO4 solution appears to be more helical than DOI with the disulfide bonds intact as judged from their ellipticities. Oxidized A and B chains of insulin are unordered in aqueous solution. A chain that lacks cationic side groups remains unordered in a NaDodSO4 solution. It shows a small amount of helix in a Dod(CH3)3NCl solution and forms the interchain beta-form in a DodNH3Cl solution. In contrast, B chain becomes partially helical in all three surfactant solutions.

Effect of sodium dodecyl sulphate on the 12S fraction of mustard proteins (B. juncea)

The effect of SDS on the 12S protein fraction of mustard seed (B. juncea) has been followed by the techniques of ultracentrifugation, gel filtration, gel electrophoresis, viscosity, ultraviolet difference spectra and fluorescence spectra. At low concentrations of SDS, up to 0.1%, both aggregation and dissociation of the protein occurs. Only dissociation occurs at higher SDS concentrations and is complete at 0.5% SDS. Viscosity increases sharply up to 0.15% SDS, remains constant between 0.15 and 0.30% and then increases markedly again. SDS induces also difference spectra with minima at 280, 288 and 295 nm. Fluorescence emission intensity increases at SDS concentrations less than 0.05% and quenching occurs at higher SDS concentrations. The results suggest that SDS causes association, dissociation and denaturation of the protein molecule.

Solubilization of isolated central-nervous-system myelin preparations by the amniotic detergent sodium dodecyl sulphate

The mechanism for the solubilization of isolated central-nervous-system myelin by sodium dodecyl sulphate was studied in detail. The release of protein and phospholipid to the 100000 g x 1 h supernatant fraction is dependent on the total amount of detergent relative to the amount of membrane present and on the ionic strength of the solubilization system. Gel-filtration analysis of supernatant fractions indicate that at suboptimal concentrations of detergent these contain lipid-protein complexes. The complete dissociation of the individual protein components from lipid is dependent on the total amount of sodium dodecyl sulphate present in the system. The results indicate that for the analysis of membrane components in sodium dodecyl sulphate it is essential that sufficient detergent is present.

Differential partitioning of tRNAs between micellar and aqueous phases: a convenient gel filtration method for separation of tRNAs

Partition coefficients (Kp) of phenylalanine, glutamic acid-II and tyrosine tRNAs between water and micellar cetyltrimethylammonium chloride (CTACl) have been determined to be 5600, 520 and 3100, respectively, by gel filtration on Sephadex G-100-120. Equations for obtaining Kp values from quantitative gel filtration have also been derived. The use of this method for the separation of tRNAs is demonstrated.

Crystallographic studies of protein denaturation and renaturation. 2. Sodium dodecyl sulfate induced structural changes in triclinic lysozyme

Cross-linked triclinic lysozyme was denatured with sodium dodecyl sulfate. Removal of the denaturant resulted in a refolding of the protein to a conformation similar to but not identical with the native one. Three-dimensional x-ray diffraction data out to 3.2-A resolution were collected for two states in the refolding pathway, and appropriately weighted electron density difference maps were constructed. An analysis of these maps reveals that a sodium dodecyl sulfate molecule is trapped in the interior of the protein, and results in a separation of regions of the polypeptide chain. Our results are discussed in terms of current models for protein folding.

The interaction between Beta-lactoglobulin and sodium N-dodecyl sulphate

1. The binding of sodium n-dodecyl sulphate to beta-lactoglobulin was studied in the pH range 3.5-7.0 by equilibrium dialysis, ultracentrifugation and microcalorimetry. 2. At low binding concentrations (less than 30 bound surfactants anions per protein molecule) the complexes formed aggregates in solution. 3. At higher binding concentrations aggregation does not occur at low ionic strength (0.01 mol/litre), but continues at high ionic strength (0.1 mol/litre). 4. At 25 degrees C the enthalpy of interaction of sodium n-dodecyl sulphate with beta-lactoglobulin can be interpreted as the sum of the enthalpies of formation of a complex with 2 bound surfactant anions, with an enthalpy change of -9.5 kJ-mol-1 of bound surfactant, and complexes containing at least 22 bound surfactant anions, with limiting enthalpies per bound surfactant anion of -12.4 kJ-mol-1 at pH 3.5 and -3.25 kJ-mol-1 at pH 5.5. 5. The binding of surfactant and the enthalpy of interaction at pH 3.5 ARE NOT SIGNIFICANTLY AFFECTED BY THE ADDITION Of 8 M-urea. 6. The data indicate that at low binding concentrations the interaction is of an ionic nature, and is accompanied by a conformational change in the protein.

A theoretical approach to the binding of amphipathic molecules to globular proteins

1. A theory based on a multiple equilibrium model with stoicheiometric binding constants has been formulated. It is applicable to the interaction of amphipathic molecules with charged macromolecules. 2. The theory has been applied to the binding of sodium n-dodecyl sulphate to ribonuclease A, beta-lactoglobulin and bovine serum albumin. 3. Over the ranges of surfactant concentration where binding is non-co-operative and co-operative, the experimental data can be satisfactorily fitted with energy and co-operatively parameters which are of comparable magnitude for the three globular proteins. 4. The results imply that the energy of interaction of sodium n-dodecyl sulphate with the proteins is equivalent to the formation of approximately four CH2 hydrophobic bonds.