Molten globule-like state of cytochrome c induced by polyanion poly(vinylsulfate) in slightly acidic pH

Singlet oxygen quenching as a probe for cytochrome c molten globule state formation

Singlet oxygen refers to the nonradical metastable excited state of molecular oxygen that readily oxidizes various cellular components. Its behavior in different biological systems has been studied for many years. Recently, we analyzed the effect of singlet oxygen quenching by heme cofactor in cytochrome c (cyt c). Here, we have exploited this effect in the investigation of conformational differences in the molten globule states of cyt c induced by different sodium anions, namely sulfate, chloride and perchlorate. The high efficiency of heme toward quenching singlet oxygen enabled us to use this property for the analysis of the otherwise experimentally difficult-to-determine parameter of heme upon exposure to solvents as highly similar conformational states of cyt c in the molten globule states are induced by different salts at acidic pH. Our results from singlet oxygen quenching experiments correlate well with other spectroscopic methods, such as circular dichroism and fluorescence measurements, and suggest increasing availability of heme in the order: perchlorate < chloride < sulfate. Based on our findings we propose that singlet oxygen phosphorescence measurements are useful in determining the differences in the protein conformation of their heme regions, particularly regarding the relative heme exposure to the solvent.

Heme is responsible for enhanced singlet oxygen deactivation in cytochrome c

The deactivation of singlet oxygen, the lowest electronic excited state of molecular oxygen, by proteins is usually described through the interaction of singlet oxygen with certain amino acids. Changes in accessibility of these amino acids influence the quenching rate and the phosphorescence kinetics of singlet oxygen. In the cellular environment, however, numerous proteins with covalently bound or encapsulated cofactors are present. These cofactors could also influence the deactivation of singlet oxygen, and these have received little attention. To confront this issue, we used cytochrome c (cyt c) and apocytochrome c (apocyt c) to illustrate how the heme prosthetic group influences the rate constant of singlet oxygen deactivation upon acidic pH-induced conformational change of cyt c. Photo-excited flavin mononucleotide (FMN) was used to produce singlet oxygen. Our data show that the heme group has a significant and measurable effect on singlet oxygen quenching when the heme is exposed to solvents and is therefore more accessible to singlet oxygen. The effect of amino acids and heme accessibility on the FMN triplet state deactivation was also investigated.

Monomeric banana lectin at acidic pH overrules conformational stability of its native dimeric form

Banana lectin (BL) is a homodimeric protein categorized among jacalin-related family of lectins. The effect of acidic pH was examined on conformational stability of BL by using circular dichroism, intrinsic fluorescence, 1-anilino-8-napthalene sulfonate (ANS) binding, size exclusion chromatography (SEC) and dynamic light scattering (DLS). During acid denaturation of BL, the monomerization of native dimeric protein was found at pH 2.0. The elution profile from SEC showed two different peaks (59.65 ml & 87.98 ml) at pH 2.0 while single peak (61.45 ml) at pH 7.4. The hydrodynamic radii (R h) of native BL was 2.9 nm while at pH 2.0 two species were found with R h of 1.7 and 3.7 nm. Furthermore at, pH 2.0 the secondary structures of BL remained unaltered while tertiary structure was significantly disrupted with the exposure of hydrophobic clusters confirming the existence of molten globule like state. The unfolding of BL with different subunit status was further evaluated by urea and temperature mediated denaturation to check their stability. As inferred from high Cm and ΔG values, the monomeric form of BL offers more resistance towards chemical denaturation than the native dimeric form. Besides, dimeric BL exhibited a Tm of 77°C while no loss in secondary structures was observed in monomers even up to 95°C. To the best of our knowledge, this is the first report on monomeric subunit of lectins showing more stability against denaturants than its native dimeric state.

Alkaline transition of horse heart cytochrome c in the presence of ZnO nanoparticles

The effect of zinc oxide nanoparticles (ZnO NPs) on cytochrome c (cyt c) in alkaline pH was studied with absorption spectroscopy and UV circular dichroism (CD). Spectral data from UV-vis spectroscopy and circular dichroism indicate only small changes in the native structure of the protein at neutral pH after the interaction with ZnO nanoparticles. The stability around the heme crevice of cyt c and therefore the switch of the axial ligand Met80 to Lys which occurs in conditions of higher pH was proven following the interaction of cytochrome c with ZnO nanoparticles. The formation of cyt c-ZnO NPs complex based on electrostatic attraction was accompanied by a significant increase in the apparent pKa constant of the alkaline transition of cyt c.

Deconstructing time-resolved optical rotatory dispersion kinetic measurements of cytochrome c folding: from molten globule to the native state

The far-UV time-resolved optical rotatory dispersion (TRORD) technique has contributed significantly to our understanding of nanosecond secondary structure kinetics in protein folding and function reactions. For reduced cytochrome c, protein folding kinetics have been probed largely from the unfolded to the native state. Here we provide details about sample preparation and the TRORD apparatus and measurements for studying folding from a partly unfolded state to the native secondary structure conformation of reduced cytochrome c.

The non-native conformations of cytochrome c in sodium dodecyl sulfate and their modulation by ATP

To understand the interaction of cytochrome c (cyt c) with membranes, a systematic investigation of sodium dodecyl sulfate (SDS)-induced conformational alterations in native horse heart ferricytochrome c (pH 7.0) was carried out using heme absorbance, tryptophan fluorescence and circular dichroism (CD) spectroscopy. ATP interaction with membrane-bound cyt c is known to regulate the process of apoptosis. To understand the effect of nucleotide phosphates on membrane-bound cyt c, we also carried out studies of the interaction of ATP with cyt c in the presence of SDS. Fluorescence and UV-Vis data suggest that SDS induces two different transitions (F to C1, C1 to C2) in cyt c, one in the pre-micellar region and the other in the post-micellar region. The fluorescence data further indicated the increase in distance between Trp 59 and heme in the intermediates in both the regions, suggesting loosening up of cyt c on titration with SDS. The far-UV and near-UV CD data suggest partial loss of secondary and tertiary structure in C1, but complete loss of tertiary structure and no further loss of secondary structure in C2. On titration of C1 and C2 with ATP, the secondary structure is restored. However, the heme ligation pattern and heme exposure change only for C2, but not for C1 on the addition of ATP.

Characterization of the polyanion-induced molten globule-like state of cytochromec

Cytochrome c (cyt c) undergoes a poly(vinylsulphate) (PVS)-induced transition at slightly acidic pH into a molten globule-like state that resembles the effect that negatively charged membrane surfaces have on this protein. In this work, the thermodynamic properties of the molten globule-like state of cyt c in complex with PVS are studied using differential scanning calorimetry, circular dichroism, fluorescence, and absorbance spectroscopy. The temperature-induced transition of the molten globule-like state of cyt c in the complex with PVS is characterized by a significantly lower calorimetric enthalpy than in the "typical" molten globule state of cyt c, i.e. free protein at pH 2.0 in high ionic strength. Moreover, the thermally-denatured state of cyt c in the complex at pH < 6 contains nearly 50% of the native secondary structure. The dependence of the transition temperature on the pH indicates a role for histidine residues in the destabilization of the cyt c structure in the PVS complex and in stabilization of the denatured state with the residual secondary structure. A comparison of the effects of small anions and polyanions demonstrates the importance of cooperativity among the anions in the destabilization of cyt c. Predictably, other hydrophilic flexible polyanions such as heparin, polyglutamate, and polyadenylate also have a destabilizing effect on the structure of cyt c. However, a correlation between the properties of the polyanions and their effect on the protein stability is still unclear.

Comparative study of effects of polyols, salts, and alcohols on trichloroacetic acid-induced state of cytochrome c

A systematic investigation of the effect of polyethylene glycols, salts, and alcohols on the trichloroacetic acid (TCA)-induced state of ferricytochrome c was made using various spectroscopic techniques. Native cytochrome c (Cyt c) has a fluorescence maximum at 335 nm, whereas the TCA-induced state of Cyt c has a red shift of 7 nm with enhanced fluorescence intensity. The near- and far-UV CD spectra showed a significant loss of tertiary and secondary structure, although the protein is relatively less unfolded as compared with a conformation at pH 2.0. Addition of 70% (v/v) polyols to TCA (3.3 mM)-induced state of Cyt c resulted in increased 1-anilino-8-naphthalene sulfonate binding and increased mean residue ellipticity at 222 nm, indicating increase in compactness with enhanced exposure of hydrophobic surface area. Also, the stabilizing effect of salts and alcohols on the TCA-induced state was studied and compared with their effect on trifluoroacetic acid-unfolded state of Cyt c. Among all the polyols, salts, and alcohols studied, PEG-400, K3[Fe(CN)6], and butanol were the most efficient in inducing secondary structure in TCA-induced state as examined by the above-mentioned spectroscopic techniques. For salts, the efficiency in inducing the secondary structure followed the order K3[Fe(CN)6] > KClO4 > K2SO4 > KCl. For alcohols, this order was found to be as follows: butanol > propanol > ethanol > methanol.

Reversibility of Structural Transition of Cytochrome c on Interacting with and Releasing from Alternating Copolymers of Maleic Acid and Alkene

The interaction of cytochrome c (cyt c) with poly(isobutylene-alt-maleic acid) (PIMA) and poly(1-tetradecene-alt-maleic acid) (PTMA) was studied using circular dichroism, absorption spectroscopy, and atomic force microscopy to investigate the electrostatic and hydrophobic influence of the copolymers on the structure of cyt c. At pH 7.4, the interaction of PIMA with cyt c can only partly disturb the integrity of the heme pocket, while PTMA has very intensive influence on the structure of cyt c. After adding 0.15 M NaCl, PIMA-cyt c complexes dissociate, and the released cyt c recovers its native structure, whereas NaCl has no significant influence on PTMA-cyt c complexes. GuHCl (0.5 M) destroys PTMA-cyt c complexes, forming GuHCl-PTMA precipitates; the cyt c released from the complexes regenerates its native structure. In comparison with electrostatic interaction, hydrophobic interaction leads to more stable polymer-cyt c complexes and more intensive influence on cyt c structure, but cyt c can recover its native state after release.

Unusual effect of salts on the homodimeric structure of NADH oxidase from Thermus thermophilus in acidic pH

The unusual salt-dependent behavior of the homodimeric flavoenzyme NADH oxidase from Thermus thermophilus in acidic pH has been studied using circular dichroism (CD) and sedimentation velocity. The native-like secondary and quaternary structures in acidic low ionic strength conditions were significantly perturbed by the addition of salts. The peptide region of the CD spectra showed a major salt-induced conformational change in the protein secondary structure. Sedimentation velocity experiments showed dissociation of the homodimeric structure of NADH oxidase in the presence of salt (>1 M). The new acidic conformation of the protein was stabilized by high ionic strength as indicated by a salt-induced increase in the melting temperature of the protein, and by a shift in the apparent pK(a) values of the conformational transition to a less acidic pH. Distortion of the dominant alpha-helical signal was expressed as the disappearance of the parallel polarized Moffitt exciton band at 208 nm without an accompanying loss of amplitude of n-->pi* electronic transitions at 222 nm. The unusual CD spectra correlated qualitatively with the theoretically calculated CD spectra of short alpha-helical structures and/or twisted beta-sheets. Differences between the experimentally obtained CD spectra and theoretical calculations (AGADIR) of the alpha-helical content of NADH oxidase indicate a role for non-local interactions in the protein conformation at high ionic strength and low pH. These findings indicate the importance of the homodimeric interface and electrostatic interactions for maintaining the structural integrity of this thermophilic protein.

Prevention of thermal induced aggregation of cytochromec at isoelectric pH values by polyanions

Differential scanning calorimetry, viscometry, optical and CD spectroscopy were used to characterize the influence of two polyanions, poly(vinylsulfate) (PVS), and poly(4-styrene-sulfonate) (PSS) on thermal transition reversibility of ferricytochrome c at or near isoelectric pH. In these conditions, both PVS and PSS enhance the thermal transition reversibility of cytochrome c by preventing the aggregation of denatured protein molecules. Data indicate that the polyanions are in complex with cytochrome c that is stabilized by synergistic effect of Coulombic and non-Coulombic interactions.

pH-Dependent interaction of cytochrome c with mitochondrial mimetic membranes: the role of an array of positively charged amino acids

The interaction of cytochrome c (cyt c) with mitochondrial mimetic vesicles of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine, 1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine, and heart cardiolipin (PCPECL) was investigated over the 7.4-6.2 pH range by means of turbidimetry and photon correlation spectroscopy. In the presence of cyt c, the decrease of pH induced an increase in vesicle turbidity and mean diameter resulting from vesicle fusion as determined by a rapid decrease in the excimer/monomer ratio of 2-(10-(1-pyrene)-decanoyl)-phosphatidylcholine (PyPC). N-acetylated cyt c and protamine, a positively charged protein, increased vesicle turbidity in a pH-independent manner, whereas albumin did not affect PCPECL vesicle turbidity. pH-dependent turbidity kinetics revealed a role for cyt c-ionizable groups with a pK(a)((app)) of approximately 7.0. The carbethoxylation of these groups by diethylpyrocarbonate prevented cyt c-induced vesicle fusion, although cyt c association to vesicles remained unaffected. Matrix-assisted laser desorption ionization time-of-flight analysis revealed that Lys-22, Lys-27, His-33, and Lys-87 cyt c residues were the main targets for carbethoxylation performed at low pH values (<7.5). In fact, these amino acid residues belong to clusters of positively charged amino acids that lower the pK(a). Thus, at low pH, protonation of these invariant and highly conserved amino acid residues produced a second positively charged region opposite to the Lys-72 and Lys-73 region in the cyt c structure. These two opposing sites allowed two vesicles to be brought together by the same cyt c molecule for fusion. Therefore, a novel pH-dependent site associating cyt c to mitochondrial mimetic membranes was established in this study.

Characterization of molten globule state of cytochrome c at alkaline, native and acidic pH induced by butanol and SDS

In our earlier communications, we had studied the acid induced unfolding of stem bromelain, glucose oxidase and fetuin [Eur. J. Biochem. 269 (2002) 47; Biochem. Biophys. Res. Comm. 303 (2003) 685; Biochim. Biophys. Acta 1649 (2003) 164] and effect of salts and alcohols on the acid unfolded state of alpha-chymotrypsinogen and stem bromelain [Biochim. Biophy. Acta 1481 (2000) 229; Arch. Biochem. Biophys. 413 (2) (2003) 199]. Here, we report the presence of molten globule like equilibrium intermediate state under alkaline, native and acid conditions in the presence of SDS and butanol. A systematic investigation of sodium dodecyl sulphate and butanol induced conformational alterations in alkaline (U(1)) and acidic (U(2)) unfolded states of horse heart ferricytochrome c was examined by circular dichroism (CD), tryptophan fluorescence and 1-anilino-8-napthalene sulfonate (ANS) binding. The cytochrome c (cyt c) at pH 9 and 2 shows the loss of approximately 61% and 65% helical secondary structure. Addition of increasing concentrations of butanol (0-7.2 M) and sodium dodecyl sulphate (0-5 mM) led to an increase in ellipticity value at 208 and 222 nm, which is the characteristic of formation of alpha-helical structure. Cyt c is a heme protein in which the tryptophan fluorescence is quenched in the native state by resonance energy transfer to the heme group attached to cystines at positions 14 and 17. At alkaline and acidic pH protein shows enhancement in tryptophan fluorescence and quenched ANS fluorescence. Addition of increasing concentration of butanol and SDS to alkaline or acid unfolded state leads to decrease in tryptophan and increase in ANS fluorescence with a blue shift in lambda(max), respectively. In the presence of 7.2 M butanol and 5 mM SDS two different intermediate states I(1) and I(2) were obtained at alkaline and acidic pH, respectively. States I(1) and I(2) have native like secondary structure with disordered side chains (loss of tertiary structure) as predicted from tryptophan fluorescence and high ANS binding. These results altogether imply that the butanol and SDS induced intermediate states at alkaline and acid pH lies between the unfolded and native state. At pH 6, in the presence of 7.2 M butanol or 5 mM SDS leads to the loss of CD bands at 208 and 222 nm with the appearance of trough at 228 nm also with increase in tryptophan and ANS fluorescence in contrast to native protein. This partially unfolded intermediate state obtained represents the folding pathway from native to unfolded structure. To summarize; the 7.2 M butanol and 5 mM SDS stabilizes the intermediate state (I(1) and I(2)) obtained at low and alkaline pH. While the same destabilizes the native structure of protein at pH 6, suggesting a difference in the mechanism of conformational stability.

Effect of Colloidal Gold Size on the Conformational Changes of Adsorbed Cytochrome c: Probing by Circular Dichroism, UV−Visible, and Infrared Spectroscopy

The conformational changes of bovine heart cytochrome c (cyt c) induced by the adsorption on gold nanoparticles with different sizes have been investigated by electronic absorption, circular dichroism (CD), and Fourier transform infrared spectra. The combination of these techniques can give complementary information about adsorption-induced conformational changes. The results show that there are different conformational changes for cyt c adsorbed on gold nanoparticles with different sizes due to the different interaction forces between cyt c and gold nanoparticles. The colloidal gold concentration-dependent conformation distribution curves of cyt c obtained by analysis of CD spectra using the singular value decomposition least-squares method show that the coverage of cyt c on the gold nanoparticles surface also affects the conformational changes of the adsorbed cyt c.

Protonation of two adjacent tyrosine residues influences the reduction of cytochrome c by diphenylacetaldehyde: a possible mechanism to select the reducer agent of heme iron

We have shown that diphenlacetaldehyde (DPAA) is able to promote mitochondrial DeltaPsi disruption accompanied by damage in mitochondrial DNA, lipids, and proteins [Almeida, A. M.; Bechara, E. J. H.; Vercesi, A. E.; Nantes, I. L. Free Radic. Biol. Med. 27:744-747; 1999]. In this work, DPAA was used as a model of carbonyl reagent for cytochrome c. The results suggest that DPAA is a redox cytochrome c modifier. Conversion of Fe(III) to Fe(II) cytochrome c promoted by DPAA is pH dependent. The second-order rate determined for heme iron reduction (k2) is 698 M(-1) s(-1) and this process occurs with an activation energy of 8.5 +/- 0.8 kcal/mol. Analysis of the pH profile suggests the presence of two ionizable cytochrome c groups (pKa1 = 8.9 and pKa2 = 11.4) related to the electron transfer from DPAA to heme iron. The heats of ionization of the two prototropic groups, pKa1 (DeltaH(ion) = 6.5 kcal/mol, DeltaS(ion) = -29.0 cal/mol.K), and pKa2 (DeltaH(ion) = 5.0 kcal/mol, DeltaS(ion) = -24.0 cal/mol.K), suggest involvement of two tyrosine residues, probably Y67 and Y74, related to DPAA-promoted heme iron reduction. The cytochrome c chemical modification by iodination of tyrosine groups significantly decreased the reduction rate promoted by DPAA, and shifted the pH(opt) value from 10.0 to 9.25. The cytochrome c-promoted DPAA electron abstraction quickly produces the expected enol-derived radical, as indicated by 3,5-dibromo-4-nitrosobenzenesulfonate (DBNBS) spin trapping EPR measurements. This radical reacts with molecular oxygen, producing a peroxyl intermediate radical that, via a putative dioxetane intermediate, promotes formation of benzophenone as the main final product of this reaction, detected by high-performance liquid chromatography coupled with tandem mass spectrometry.

Cytochrome c and SDS: a molten globule protein with altered axial ligation

Saccharomices cerevisiae (yeast iso-1) cytochrome c has been investigated in the presence of 100 mM SDS in order to simulate the interaction of cytochrome c with membrane. Under these circumstances, a high spin species with detached methionine axial ligand is observed through NMR, in analogy to findings on the horse heart protein. However, at variance with the latter system, for the yeast protein also a low spin species is detected, which appears to be present with a concentration of about 40% with respect to that of the high spin species. The R(1), R(2), [1H]-15N NOE of backbone amides which are not affected by paramagnetism are homogeneous and allow a simultaneous analysis of the data for the two species. The result is that the rotational correlation time is larger than in water and larger than expected on the basis of viscosity of the SDS-containing solution. This finding suggests interactions of cytochrome c with SDS. Furthermore, it appears that there is subnanosecond backbone mobility, which also accounts for the decreased intensity of NOE cross-peaks and may be associated with equilibria between helical and random coil structure. The dynamic behavior appears to be a common feature of the high spin and low spin species and is consistent with the presence of a molten globule state. The molten globule nature of the protein could account for the presence of the different axial coordination of the heme iron. Such findings are meaningful with respect to the physiology of cytochrome c as electron transfer protein and as promoter of apoptosis.

Acid-induced unfolding of flounder hemoglobin: evidence for a molten globular state with enhanced pro-oxidative activity

The acid-induced unfolding of flounder oxyhemoglobin was investigated and the effect on pro-oxidative activity assessed. Hemoglobin exhibited multistep unfolding transitions as pH was lowered, with the major transition between pH 3.5 and 4 5. The protein was maximally acid-unfolded (but not fully unfolded) at approximately pH 2.5, and further titration with HCl led to a partially refolded protein due to a stabilizing effect of Cl(-) anions. At low pH, the protein retained a sizable amount of secondary structure and had increased ANS binding, suggesting a molten globular form at low pH. Dramatic changes in the heme environment occurred concurrently with the changes in protein conformation. These changes resulted in an enhancement in the pro-oxidative activity of the protein. The results show that an increase in flounder hemoglobin pro-oxidation was correlated with the extent of its unfolding, and they provide useful insight into what may occur with hemoglobin in processes where highly acidic conditions are employed.

Glycerol-induced formation of the molten globule from acid-denatured cytochrome c: implication for hierarchical folding

At high concentration (98% or higher, v/v), glycerol induces collapse of acid-denatured cytochrome c into a compact state, the G(U) state, showing a molten globule character. The G(U) state possesses a nativelike alpha-helix structure but a tertiary conformation less packed with respect to the native state. The spectroscopic properties of the G(U) state closely resemble those of the molten globule stabilized by the organic solvent from the native protein (called the G(N) state), indicating that glycerol can stabilize the molten globule of cytochrome c either from the native or the acid-denatured protein. The G(U) and the G(N) states show spectroscopic (and, thus, structural) properties and stabilities comparable to those of molten globules stabilized by different effectors, despite the fact that the mechanisms involved in the molten globule formation may significantly differ. This implies in cytochrome c a hierarchy for the rupture (native-to-molten globule) or the formation (unfolded-to-molten globule) of intramolecular interactions leading to the stabilization of the molten globule state of the protein, independently from the effector responsible for the structural transition, in accord with the sequential model proposed by Englander and collaborators.

Structural models of human apolipoprotein A-I: a critical analysis and review

Human apolipoprotein (apo) A-I has been the subject of intense investigation because of its well-documented anti-atherogenic properties. About 70% of the protein found in high density lipoprotein complexes is apo A-I, a molecule that contains a series of highly homologous amphipathic alpha-helices. A number of significant experimental observations have allowed increasing sophisticated structural models for both the lipid-bound and the lipid-free forms of the apo A-I molecule to be tested critically. It seems clear, for example, that interactions between amphipathic domains in apo A-I may be crucial to understanding the dynamic nature of the molecule and the pathways by which the lipid-free molecule binds to lipid, both in a discoidal and a spherical particle. The state of the art of these structural studies is discussed and placed in context with current models and concepts of the physiological role of apo A-I and high-density lipoprotein in atherosclerosis and lipid metabolism.