Cytochrome c reconstituted from two peptide fragments displays native-like redox properties

On the application of CZE to the study of protein denaturation

Experimental mobilities obtained from CZE are used to study protein denaturation through a model based on known physicochemical theories. This model is able to provide additional information concerning the folded and unfolded protein states from mobility data. Its use comprises first the evaluation of relevant parameters of the protein microstates like the electrostatic free energy, apart from the classical conformational free energy, and second the expression of raw experimental data concerning the folding-unfolding transition into more specific physicochemical parameters like protein hydrodynamic radius, net charge number, and hydration. Spurious effects that are intrinsic to the experimental evaluation of the mobility of protein states, like BGE viscosity, pH, and ionic strength variations accompanying the changes of the denaturant agent intensity are eliminated. In order to illustrate the proposal of this work, two case studies are considered here. The first one concerns thermal and urea denaturations of horse heart ferricytochrome c and the second one involves thermal denaturation of hen egg-white lysozyme. Thus, relevant theoretical thermodynamic considerations of the folded-unfolded protein transition are presented, where the electrostatic free energy is included explicitly in the effective free energy. It is found that this transition involves sharp increases of hydrodynamic radius and protein hydration.

Electron-transfer mechanism of the triplet state quenching of aluminium tetrasulfonated phthalocyanine by cytochrome c

The mechanism of electron-transfer from aluminium tetrasulfonated phthalocyanine triplet state to cytochrome c was investigated in this work. This reaction successfully quenches the dye triplet state due to the formation of complexes between the solute and the protein at the active site. The electron-transfer rate constant is around 3x10(7) s(-1), and is in accordance with previous results for the singlet excited state quenching [C.A.T. Laia, S.M.B. Costa, D. Phillips, A. Beeby. Electron-transfer kinetics in sulfonated aluminum phthalocyanines/cytochrome c complexes, J. Phys. Chem. B 108 (2004) 7506-7514.] in the framework of the Marcus theory, with a reorganization energy equal to 0.94 eV. The complex formation is diffusion controlled, but heterogeneities of the protein surface charge distribution lead to quenching rate constants smaller than predicted on a hard-spheres model with electrostatic interactions. Also the binding equilibrium constant is strongly affected by this phenomenon. Ionic strength plays an important role on the complex formation, but its effect on the unimolecular electron-transfer rate constant is negligible within experimental error.

A model for the misfolded bis-His intermediate of cytochrome c: the 1-56 N-fragment

We have characterized the ferric and ferrous forms of the heme-containing (1-56 residues) N-fragment of horse heart cytochrome c (cyt c) at different pH values and low ionic strength by UV-visible absorption and resonance Raman (RR) scattering. The results are compared with native cyt c in the same experimental conditions as this may provide a deeper insight into the cyt c unfolding-folding process. Folding of cyt c leads to a state having the heme iron coordinated to a histidine (His18) and a methionine (Met80) as axial ligands. At neutral pH the N-fragment (which lacks Met80) shows absorption and RR spectra that are consistent with the presence of a bis-His low spin heme, like several non-native forms of the parental protein. In particular, the optical spectra are identical to those of cyt c in the presence of a high concentration of denaturants; this renders the N-fragment a suitable model to study the heme pocket microenvironment of the misfolded (His-His) intermediate formed during folding of cyt c. Acid pH affects the ligation state in both cyt c and the N-fragment. Data obtained as a function of pH allow a correlation between the structural properties in the heme pocket of the N-fragment and those of non-native forms of cyt c. The results underline that the (57-104 residues) segment under native-like conditions imparts structural stability to the protein by impeding solvent access into the heme pocket.

ATP specifically drives refolding of non-native conformations of cytochrome c

An increasing body of evidence ascribes to misfolded forms of cytochrome c (cyt c) a role in pathophysiological events such as apoptosis and disease. Here, we examine the conformational changes induced by lipid binding to horse heart cyt c at pH 7 and study the ability of ATP (and other nucleotides) to refold several forms of unfolded cyt c such as oleic acid-bound cyt c, nicked cyt c, and acid denatured cyt c. The CD and fluorescence spectra demonstrate that cyt c unfolded by oleic acid has an intact secondary structure, and a disrupted tertiary structure and heme environment. Furthermore, evidence from the Soret CD, electronic absorption, and resonance Raman spectra indicates the presence of an equilibrium of at least two low-spin species having distinct heme-iron(III) coordination. As a whole, the data indicate that binding of cyt c to oleic acid leads to a partially unfolded conformation of the protein, resembling that typical of the molten globule state. Interestingly, the native conformation is almost fully recovered in the presence of ATP or dATP, while other nucleotides, such as GTP, are ineffective. Molecular modeling of ATP binding to cyt c and mutagenesis experiments show the interactions of phosphate groups with Lys88 and Arg91, with adenosine ring interaction with Glu62 explaining the unfavorable binding of GTP. The finding that ATP and dATP are unique among the nucleotides in being able to turn non-native states of cyt c back to native conformation is discussed in the light of cyt c involvement in cell apoptosis.

Detection of coexisting protein conformations in capillary zone electrophoresis subsequent to transient contact with sodium dodecyl sulfate solutions

Non-native conformations of proteins were generated by temporary contact with aqueous solutions of sodium dodecyl sulfate (SDS) and separated from the native state with capillary zone electrophoresis (CZE) in alkaline borate buffer deficient of SDS. Nine proteins at concentrations of 2.0 or 3.0 mg.L(-1) were compared in terms of their susceptibility to SDS. For superoxide dismutase and ferritin the tendency of unfolding was modest with < 25% of the protein being transformed to the non-native state at 10 mmol.L(-1) SDS. Highest susceptibility was observed for albumin, myoglobin (Mb), and hemoglobin with > 75% in the non-native state even at 2.0 mmol.L(-1) SDS. The influence of varying SDS concentrations on the conformational state of Mb was tested. Increasing the SDS concentration, circular dichroism revealed a reduction in alpha-helix, an increase in random coil, and an introduction of beta-sheet, which is absent in native structure. Modifications in the secondary structure were in agreement with distinct changes in the shape of the non-native Mb peak in CZE and make a gradual unfolding/refolding process with several coexisting molten globules instead of two-state transition of conformations most plausible for Mb. CZE was found to contribute to a further understanding of holo-Mb transformation towards a population of non-native conformations (i) by means of calculated peak area ratios of native to non-native states, which showed sigmoid transition, (ii) by detecting the release of the prosthetic heme group, and (iii) by changes in the effective electrophoretic mobility of the Mb-SDS peaks. Reconstituted holo-Mb forms differed in the Soret band around 410 nm, indicating diversity in the conformation of the heme pocket.

Effect of Axial Coordination on the Kinetics of Assembly and Folding of the Two Halves of Horse Heart Cytochrome c

The kinetics of the assembly of two complementary fragments of oxidized horse heart cytochrome c (cyt c), namely the heme-containing fragment-(1-56) and the fragment-(57-104), have been characterized at different pH values. At neutral pH the fragment-(1-56) is hexacoordinated and has two histidines axially ligated to the heme-Fe(III) (Santucci, R., Fiorucci, L., Sinibaldi, F., Polizio, F., Desideri, A., and Ascoli, F. (2000) Arch. Biochem. Biophys. 379, 331-336), thus mimicking what occurs in the folding intermediate of cyt c. The kinetics of the formation of the complex between the two fragments are characterized at pH 7.0 by a slow rate constant that is independent of the concentration of the reactants; conversely, at a low pH the kinetics are much faster and depend on the concentration of the fragments. This behavior suggests that the rate-limiting step observed in the recombination process of the fragments at neutral pH (that leads to the final coordination of Met-80) has to be ascribed to the detachment of the "misligated" histidine. Thus, the faster recombination rate at a low pH can be related to the fact that histidine is protonated and not able to coordinate to the metal. Furthermore, the independence of the rate constant on the concentration of the reactants observed at pH 7.0 can be accounted for by the occurrence of a conformational transition, which takes place immediately after the two fragments collapse together, likely simulating what induces the detachment of the misligated histidine during cytochrome folding.

The 40s ?-loop plays a critical role in the stability and the alkaline conformational transition of cytochrome c

The structural and redox properties of a non-covalent complex reconstituted upon mixing two non-contiguous fragments of horse cytochrome c, the residues 1-38 heme-containing N-fragment with the residues 57-104 C-fragment, have been investigated. With respect to native cyt c, the complex lacks a segment of 18 residues, corresponding, in the native protein, to an omega (Omega)-loop region. The fragment complex shows compact structure, native-like alpha-helix content but a less rigid atomic packing and reduced stability with respect to the native protein. Structural heterogeneity is observed at pH 7.0, involving formation of an axially misligated low-spin species and consequent partial displacement of Met80 from the sixth coordination position of the heme-iron. Spectroscopic data suggest that a lysine (located in the Met80-containing loop, namely Lys72, Lys73, or Lys79) replaces the methionine residue. The residues 1-38/57-104 fragment complex shows an unusual biphasic alkaline titration characterized by a low (p K(a1)=6.72) and a high p K(a)-associated state transition (p K(a2)=8.56); this behavior differs from that of native cyt c, which shows a monophasic alkaline transition (p K(a)=8.9). The data indicate that the 40s Omega-loop plays an important role in the stability of cyt c and in ensuring a correct alkaline conformational transition of the protein.

Redox and oxidant-mediated regulation of apoptosis signaling pathways: immuno-pharmaco-redox conception of oxidative siege versus cell death commitment

The mechanisms controlling apoptosis remain largely obscure. Because apoptosis is an integral part of the developmental program and is frequently the end-result of a temporal course of cellular events, it is referred to as programmed cell death. While there is considerable variation in the signals and requisite cellular metabolic events necessary to induce apoptosis in diverse cell types, the morphological features associated with apoptosis are highly conserved. Free radicals, particularly reactive oxygen species (ROS), have been proposed as common mediators for apoptosis. Many agents that induce apoptosis are either oxidants or stimulators of cellular oxidative metabolism. Conversely, many inhibitors of apoptosis have antioxidant activities or enhance cellular antioxidant defenses. Mammalian cells, therefore, exist in a state of oxidative siege in which survival requires an optimum balance of oxidants and antioxidants. The respiratory tract is subjected to a variety of environmental stresses, including oxidizing agents, particulates and airborne microorganisms that, together, may injure structural and functional lung components and thereby jeopardize the primary lung function of gas exchange. To cope with this challenge, the lung has developed elaborate defense mechanisms that include inflammatory-immune pathways as well as efficient antioxidant defense systems. In the absence of adequate antioxidant defenses, the damage produced is detected by the cell leading to the activation of genes responsible for the regulation of apoptosis, conceivably through stress-responsive transcription factors. Oxidative stress, in addition, may cause a shift in cellular redox state, which thereby modifies the nature of the stimulatory signal and which results in cell death as opposed to proliferation. ROS/redox modifications, therefore, may disrupt signal transduction pathways, can be perceived as abnormal and, under some conditions, may trigger apoptosis.

Rupture of the hydrogen bond linking two Omega-loops induces the molten globule state at neutral pH in cytochrome c

His26Tyr and His33Tyr mutants were obtained from the Cys102Thr variant of yeast iso-1-cytochrome c. Spectroscopic studies show that a mutation at position 26 at pH 7.0 enhances flexibility of the peptide, alters the heme pocket region and the axial coordination to heme-iron, and reduces protein stability. The His26Tyr mutant shows properties typical of the molten globule. Further, formation of an axially misligated minor low spin species occurs with partial displacement of Met80, the axial ligand of the heme-iron in the native protein. The pK(a) determined for the alkaline transition of this mutant is 7.48 (+/- 0.05), approximately 0.5 lower than that of the wild-type protein. Hence, the alkaline conformer is populated at pH 7.0, and the sixth ligand of the misligated species is proposed to be a lysine. Furthermore, a reduction in catalytic activity indicates that the functional properties are altered. The results suggest that the structural and functional changes observed in the His26Tyr mutant are because the mutation frees the two Omega-loops that, in the native protein, are linked by the hydrogen bond between His26 and Glu44. Hence, one may infer that the His26-Glu44 hydrogen bond is essential for the rigidity and stability of the native protein. In its absence, the heightened flexibility of the peptide fold results in conversion of the macromolecule to a molten globule state, even at neutral pH. Ligand exchange at the sixth coordination position of the heme-iron(III) observed as the minor species (i.e., the alkaline conformer) is therefore induced by a long-range effect. This result is of interest since mutations reported to date, which stabilize the alkaline conformer, all occur in the loop including Met80. By contrast, only very minor spectroscopic (and, thus, structural) changes are observed for the His33Tyr mutant. This suggests that His33 does not form intramolecular bonds considered important for the protein structure and stability, and is consistent with the high variability of residues at position 33 in cytochromes c.