Divergent Cytochrome c Maturation System in Kinetoplastid Protists.

In eukaryotes, heme attachment through two thioether bonds to mitochondrial cytochromes c and c1 is catalyzed by either multisubunit cytochrome c maturation system I or holocytochrome c synthetase (HCCS). The former was inherited from the alphaproteobacterial progenitor of mitochondria; the latter is a eukaryotic innovation for which prokaryotic ancestry is not evident. HCCS provides one of a few exemplars of de novo protein innovation in eukaryotes, but structure-function insight of HCCS is limited. Uniquely, euglenozoan protists, which include medically relevant kinetoplastids Trypanosoma and Leishmania parasites, attach heme to mitochondrial c-type cytochromes by a single thioether linkage. Yet the mechanism is unknown, as genes encoding proteins with detectable similarity to any proteins involved in cytochrome c maturation in other taxa are absent. Here, a bioinformatics search for proteins conserved in all hemoprotein-containing kinetoplastids identified kinetoplastid cytochrome c synthetase (KCCS), which we reveal as essential and mitochondrial and catalyzes heme attachment to trypanosome cytochrome c KCCS has no sequence identity to other proteins, apart from a slight resemblance within four short motifs suggesting relatedness to HCCS. Thus, KCCS provides a novel resource for studying eukaryotic cytochrome c maturation, possibly with wider relevance, since mutations in human HCCS leads to disease. Moreover, many examples of mitochondrial biochemistry are different in euglenozoans compared to many other eukaryotes; identification of KCCS thus provides another exemplar of extreme, unusual mitochondrial biochemistry in an evolutionarily divergent group of protists.IMPORTANCE Cytochromes c are essential proteins for respiratory and photosynthetic electron transfer. They are posttranslationally modified by covalent attachment of a heme cofactor. Kinetoplastids include important tropical disease-causing parasites; many aspects of their biology differ from other organisms, including their mammalian or plant hosts. Uniquely, kinetoplastids produce cytochromes c with a type of heme attachment not seen elsewhere in nature and were the only cytochrome c-bearing taxa without evidence of protein machinery to attach heme to the apocytochrome. Using bioinformatics, biochemistry, and molecular genetics, we report how kinetoplastids make their cytochromes c Unexpectedly, they use a highly diverged version of an enzyme used for heme-protein attachment in many eukaryotes. Mutations in the human enzyme lead to genetic disease. Identification of kinetoplastid cytochrome c synthetase, thus, solves an evolutionary unknown, provides a possible target for antiparasite drug development, and an unanticipated resource for studying the mechanistic basis of a human genetic disease.

Electrical unfolding of cytochrome c during translocation through a nanopore constriction.

Many small proteins move across cellular compartments through narrow pores. In order to thread a protein through a constriction, free energy must be overcome to either deform or completely unfold the protein. In principle, the diameter of the pore, along with the effective driving force for unfolding the protein, as well as its barrier to translocation, should be critical factors that govern whether the process proceeds via squeezing, unfolding/threading, or both. To probe this for a well-established protein system, we studied the electric-field-driven translocation behavior of cytochrome c (cyt c) through ultrathin silicon nitride (SiNx) solid-state nanopores of diameters ranging from 1.5 to 5.5 nm. For a 2.5-nm-diameter pore, we find that, in a threshold electric-field regime of ∼30 to 100 MV/m, cyt c is able to squeeze through the pore. As electric fields inside the pore are increased, the unfolded state of cyt c is thermodynamically stabilized, facilitating its translocation. In contrast, for 1.5- and 2.0-nm-diameter pores, translocation occurs only by threading of the fully unfolded protein after it transitions through a higher energy unfolding intermediate state at the mouth of the pore. The relative energies between the metastable, intermediate, and unfolded protein states are extracted using a simple thermodynamic model that is dictated by the relatively slow (∼ms) protein translocation times for passing through the nanopore. These experiments map the various modes of protein translocation through a constriction, which opens avenues for exploring protein folding structures, internal contacts, and electric-field-induced deformability.

[Role of CYCS in the cytogenesis and apoptosis of male germ cells and its clinical application].

In order to maintain the stability of intracellular environment, various cells die autonomously in vivo, which is called apoptosis. As a key factor of cell apoptosis in the mitochondrial pathway, cytochrome c, somatic (CYCS) plays an important role in the process of male spermatogenesis. Based on more than 40 recent studies on CYCS by Chinese and foreign scholars, this review focuses on the involvement of CYCS in the mechanisms of the apoptosis and anti-apoptosis of germ cells and in the regulation of their quality and activity and also outlines the role of CYCS in marking male reproductive performance as well as in male infertility and its underlying mechanisms.

Cryo-EM reveals the structural basis of long-range electron transport in a cytochrome-based bacterial nanowire.

Electrically conductive pili from Geobacter species, termed bacterial nanowires, are intensely studied for their biological significance and potential in the development of new materials. Using cryo-electron microscopy, we have characterized nanowires from conductive G. sulfurreducens pili preparations that are composed solely of head-to-tail stacked monomers of the six-heme C-type cytochrome OmcS. The unique fold of OmcS - closely wrapped around a continuous stack of hemes that can serve as an uninterrupted path for electron transport - generates a scaffold that supports the unbranched chain of hemes along the central axis of the filament. We present here, at 3.4 Å resolution, the structure of this cytochrome-based filament and discuss its possible role in long-range biological electron transport.

Key Role of Cytochrome C for Apoptosis Detection Using Raman Microimaging in an Animal Model of Brain Ischemia with Insulin Treatment.

Brain ischemia represents a leading cause of death and disability in industrialized countries. To date, therapeutic intervention is largely unsatisfactory and novel strategies are required for getting better protection of neurons injured by cerebral blood flow restriction. Recent evidence suggests that brain insulin leads to protection of neuronal population undergoing apoptotic cell death via modulation of oxidative stress and mitochondrial cytochrome c (CytC), an effect to be better clarified. In this work, we investigate on the effect of insulin given intracerebroventricular (ICV) before inducing a transient global ischemia by bilateral occlusion of the common carotid arteries (BCCO) in Mongolian gerbils (MG). The transient (3 min) global ischemia in MG is observed to produce neurodegenerative effect mainly into CA3 hippocampal region, 72 h after cerebral blood restriction. Intracerebroventricular microinfusion of insulin significantly prevents the apoptosis of CA3 hippocampal neurons. Histological observation, after hematoxylin and eosin staining, puts in evidence the neuroprotective role of insulin, but Raman microimaging provides a clearer insight in the CytC mechanism underlying the apoptotic process. Above all, CytC has been revealed to be an outstanding, innate Raman marker for monitoring the cells status, thanks to its resonant scattering at 530 nm of incident wavelength and to its crucial role in the early stages of cells apoptosis. These data support the hypothesis of an insulin-dependent neuroprotection and antiapoptotic mechanism occurring in the brain of MG undergoing transient brain ischemia. The observed effects occurred without any peripheral change on serum glucose levels, suggesting an alternative mechanism of insulin-induced neuroprotection.

Bif-1 Interacts with Prohibitin-2 to Regulate Mitochondrial Inner Membrane during Cell Stress and Apoptosis.

BACKGROUND: Mitochondria are dynamic organelles that undergo fission and fusion. During cell stress, mitochondrial dynamics shift to fission, leading to mitochondrial fragmentation, membrane leakage, and apoptosis. Mitochondrial fragmentation requires the cleavage of both outer and inner membranes, but the mechanism of inner membrane cleavage is unclear. Bif-1 and prohibitin-2 may regulate mitochondrial dynamics. METHODS: We used azide-induced ATP depletion to incite cell stress in mouse embryonic fibroblasts and renal proximal tubular cells, and renal ischemia-reperfusion to induce stress in mice. We also used knockout cells and mice to determine the role of Bif-1, and used multiple techniques to analyze the molecular interaction between Bif-1 and prohibitin-2. RESULTS: Upon cell stress, Bif-1 translocated to mitochondria to bind prohibitin-2, resulting in the disruption of prohibitin complex and proteolytic inactivation of the inner membrane fusion protein OPA1. Bif-1-deficiency inhibited prohibitin complex disruption, OPA1 proteolysis, mitochondrial fragmentation, and apoptosis. Domain deletion analysis indicated that Bif-1 interacted with prohibitin-2 via its C-terminus. Notably, mutation of Bif-1 at its C-terminal tryptophan-344 not only prevented Bif-1/prohibitin-2 interaction but also reduced prohibitin complex disruption, OPA1 proteolysis, mitochondrial fragmentation, and apoptosis, supporting a pathogenic role of Bif-1/prohibitin-2 interaction. In mice, Bif-1 bound prohibitin-2 during renal ischemia/reperfusion injury, and Bif-1-deficiency protected against OPA1 proteolysis, mitochondrial fragmentation, apoptosis and kidney injury. CONCLUSIONS: These findings suggest that during cell stress, Bif-1 regulates mitochondrial inner membrane by interacting with prohibitin-2 to disrupt prohibitin complexes and induce OPA1 proteolysis and inactivation.

Interplay between cytochrome c and gibberellins during Arabidopsis vegetative development.

We studied the effect of reducing the levels of the mitochondrial electron carrier cytochrome c (CYTc) in Arabidopsis thaliana. Plants with CYTc deficiency have delayed growth and development, and reach flowering several days later than the wild-type but with the same number of leaves. CYTc-deficient plants accumulate starch and glucose during the day, and contain lower levels of active gibberellins (GA) and higher levels of DELLA proteins, involved in GA signaling. GA treatment abolishes the developmental delay and reduces glucose accumulation in CYTc-deficient plants, which also show a lower raise in ATP levels in response to glucose. Treatment of wild-type plants with inhibitors of mitochondrial energy production limits plant growth and increases the levels of DELLA proteins, thus mimicking the effects of CYTc deficiency. In addition, an increase in the amount of CYTc decreases DELLA protein levels and expedites growth, and this depends on active GA synthesis. We conclude that CYTc levels impinge on the activity of the GA pathway, most likely through changes in mitochondrial energy production. In this way, hormone-dependent growth would be coupled to the activity of components of the mitochondrial respiratory chain.

Extracellular cytochrome c as an intercellular signaling molecule regulating microglial functions.

BACKGROUND: Cytochrome c is well known to be released from mitochondria into the cytosol where it can initiate apoptosis. Recent studies indicate that cytochrome c is also released into the extracellular space by both healthy and damaged cells, where its function is not well understood. We hypothesized that extracellular cytochrome c could function as an intercellular signaling molecule of the brain, which is recognized by brain microglia. These cells belong to the mononuclear phagocyte system and can be activated by endogenous substances associated with diverse pathologies including trauma, ischemic damage and neurodegenerative diseases. METHODS: Three different cell types were used to model microglia. Respiratory burst activity, nitric oxide production and cytotoxic secretions were measured following exposure of microglial cells to cytochrome c. RESULTS: We showed that extracellular cytochrome c primed the respiratory burst response of differentiated HL-60 cells, enhanced nitric oxide secretion by BV-2 cells, and augmented cytotoxicity of differentiated THP-1 cells. We demonstrated that the effects of cytochrome c on microglia-like cells were at least partially mediated by the toll-like receptor 4 (TLR4) and c-Jun N-terminal kinases (JNK) signaling pathway. CONCLUSIONS: Extracellular cytochrome c can interact with microglia TLR4 and modulate select functions of these brain immune cells. GENERAL SIGNIFICANCE: Our data identifies extracellular cytochrome c as a potential intercellular signaling molecule, which may be recognized by microglia causing or enhancing their immune activation. The data obtained support targeting TLR4 and JNK signaling as potential treatment strategies for brain diseases characterized by excessive cellular death and activation of microglia.

[The comeback of mitochondria in Drosophila apoptosis]. / Le come-back de la mitochondrie dans l'apoptose chez la drosophile.

The role of the mitochondrion in mammalian cell apoptosis has been established since the mid-1990s. However, the importance of this organelle in non-mammalian apoptosis has long been regarded as minor, notably because of the absence of a crucial role for cytochrome c in caspase activation. Recent results indicate that the control of caspase activation and apoptosis in Drosophila cell death occurs at the mitochondrial level. Numerous proteins that appear key for Drosophila apoptosis regulation constitutively or transiently bind to mitochondria. They participate in the cell death process at different levels such as degradation of an IAP caspase inhibitor, production of mitochondrial reactive oxygen species or stimulation of the mitochondrial fission machinery. The aim of this review is to take stock of these events that might have their counterpart in humans.

Structural and functional characterization of phosphomimetic mutants of cytochrome c at threonine 28 and serine 47.

Protein function is frequently modulated by post-translational modifications of specific residues. Cytochrome c, in particular, is phosphorylated in vivo at threonine 28 and serine 47. However, the effect of such modifications on the physiological functions of cytochrome c - namely, the transfer of electrons in the respiratory electron transport chain and the triggering of programmed cell death - is still unknown. Here we replace each of these two residues by aspartate, in order to mimic phosphorylation, and report the structural and functional changes in the resulting cytochrome c variants. We find that the T28D mutant causes a 30-mV decrease on the midpoint redox potential and lowers the affinity for the distal site of Arabidopsis thaliana cytochrome c1 in complex III. Both the T28D and S47D variants display a higher efficiency as electron donors for the cytochrome c oxidase activity of complex IV. In both protein mutants, the peroxidase activity is significantly higher, which is related to the ability of cytochrome c to leave the mitochondria and reach the cytoplasm. We also find that both mutations at serine 47 (S47D and S47A) impair the ability of cytoplasmic cytochrome c to activate the caspases cascade, which is essential for triggering programmed cell death.

Structure-function relationships in human cytochrome c: The role of tyrosine 67.

Spectroscopic and functional properties of human cytochrome c and its Tyr67 residue mutants (i.e., Tyr67His and Tyr67Arg) have been investigated. In the case of the Tyr67His mutant, we have observed only a very limited structural alteration of the heme pocket and of the Ω-loop involving, among others, the residue Met80 and its bond with the heme iron. Conversely, in the Tyr67Arg mutant the Fe-Met80 bond is cleaved; consequently, a much more extensive structural alteration of the Ω-loop can be envisaged. The structural, and thus the functional modifications, of the Tyr67Arg mutant are present in both the ferric [Fe(III)] and the ferrous [Fe(II)] forms, indicating that the structural changes are independent of the heme iron oxidation state, depending instead on the type of substituting residue. Furthermore, a significant peroxidase activity is evident for the Tyr67Arg mutant, highlighting the role of Arg as a basic, positively charged residue at pH7.0, located in the heme distal pocket, which may act as an acid to cleave the O-O bond in H2O2. As a whole, our results indicate that a delicate equilibrium is associated with the spatial arrangement of the Ω-loop. Clearly, Arg, but not His, is able to stabilize and polarize the negative charge on the Fe(III)-OOH complex during the formation of Compound I, with important consequences on cytochrome peroxidation activity and its role in the apoptotic process, which is somewhat different in yeast and mammals.

MiR-34a regulates blood-brain barrier permeability and mitochondrial function by targeting cytochrome c.

The blood-brain barrier is composed of cerebrovascular endothelial cells and tight junctions, and maintaining its integrity is crucial for the homeostasis of the neuronal environment. Recently, we discovered that mitochondria play a critical role in maintaining blood-brain barrier integrity. We report for the first time a novel mechanism underlying blood-brain barrier integrity: miR-34a mediated regulation of blood-brain barrier through a mitochondrial mechanism. Bioinformatics analysis suggests miR-34a targets several mitochondria-associated gene candidates. We demonstrated that miR-34a triggers the breakdown of blood-brain barrier in cerebrovascular endothelial cell monolayer in vitro, paralleled by reduction of mitochondrial oxidative phosphorylation and adenosine triphosphate production, and decreased cytochrome c levels.

Electromagnetic radiation at 900 MHz induces sperm apoptosis through bcl-2, bax and caspase-3 signaling pathways in rats.

BACKGROUND: The decreased reproductive capacity of men is an important factor contributing to infertility. Accumulating evidence has shown that Electromagnetic radiation potentially has negative effects on human health. However, whether radio frequency electromagnetic radiation (RF-EMR) affects the human reproductive system still requires further investigation. Therefore, The present study investigates whether RF-EMR at a frequency of 900 MHz can trigger sperm cell apoptosis and affect semen morphology, concentration, and microstructure. METHODS: Twenty four rats were exposed to 900 MHz electromagnetic radiation with a special absorption rate of 0.66 ± 0.01 W/kg for 2 h/d. After 50d, the sperm count, morphology, apoptosis, reactive oxygen species (ROS), and total antioxidant capacity (TAC), representing the sum of enzymatic and nonenzymatic antioxidants, were investigated. Western blotting and reverse transcriptase PCR were used to determine the expression levels of apoptosis-related proteins and genes, including bcl-2, bax, cytochrome c, and capase-3. RESULTS: In the present study, the percentage of apoptotic sperm cells in the exposure group was significantly increased by 91.42% compared with the control group. Moreover, the ROS concentration in exposure group was increased by 46.21%, while the TAC was decreased by 28.01%. Radiation also dramatically decreased the protein and mRNA expression of bcl-2 and increased that of bax, cytochrome c, and capase-3. CONCLUSION: RF-EMR increases the ROS level and decreases TAC in rat sperm. Excessive oxidative stress alters the expression levels of apoptosis-related genes and triggers sperm apoptosis through bcl-2, bax, cytochrome c and caspase-3 signaling pathways.

Microsecond protein folding events revealed by time-resolved fluorescence resonance energy transfer in a microfluidic mixer.

We demonstrate the combination of the time-resolved fluorescence resonance energy transfer (tr-FRET) measurement and the ultrarapid hydrodynamic focusing microfluidic mixer. The combined technique is capable of probing the intermolecular distance change with temporal resolution at microsecond level and structural resolution at Angstrom level, and the use of two-photon excitation enables a broader exploration of FRET with spectrum from near-ultraviolet to visible wavelength. As a proof of principle, we used the coupled microfluidic laminar flow and time-resolved two-photon excitation microscopy to investigate the early folding states of Cytochrome c (cyt c) by monitoring the distance between the tryptophan (Trp-59)-heme donor-acceptor (D-A) pair. The transformation of folding states of cyt c in the early 500 µs of refolding was revealed on the microsecond time scale. For the first time, we clearly resolved the early transient state of cyt c, which is populated within the dead time of the mixer (<10 µs) and has a characteristic Trp-59-heme distance of ∼31 Å. We believe this tool can find more applications in studying the early stages of biological processes with FRET as the probe.

Explorations of time and electrochemical potential: opportunities for fresh perspectives on signalling proteins.

Apoptosis is triggered by an accumulation of ROS (reactive oxygen species) produced by proteins of the mitochondrial respiratory chain. The levels of ROS are controlled by the activities of mitochondrial redox proteins such as glutaredoxin 2 that help to modulate the susceptibility of a cell to apoptosis. However, once downstream events have resulted in the release of cytochrome c to the cytosol, it is widely considered that cell death is inevitable. Cytochrome c may promote its own release from mitochondria through interactions with the mitochondrial phospholipid cardiolipin (diphosphatidylglycerol). In the present article, spectroelectrochemistry of the cardiolipin complex of cytochrome c and protein film electrochemistry of glutaredoxin 2 are reviewed to illustrate how electrochemical methods provide insight into the properties of signalling proteins.

Caspase-9, caspase-3 and caspase-7 have distinct roles during intrinsic apoptosis.

BACKGROUND: Apoptosis is a form of programmed cell death that is regulated by the Bcl-2 family and caspase family of proteins. The caspase cascade responsible for executing cell death following cytochrome c release is well described; however the distinct roles of caspases-9, -3 and -7 during this process are not completely defined. RESULTS: Here we demonstrate several unique functions for each of these caspases during cell death. Specific inhibition of caspase-9 allows for efficient release of cytochrome c, but blocks changes in mitochondrial morphology and ROS production. We show that caspase-9 can cleave Bid into tBid at amino acid 59 and that this cleavage of Bid is required for ROS production following serum withdrawal. We also demonstrate that caspase-3-deficient MEFs are less sensitive to intrinsic cell death stimulation, yet have higher ROS production. In contrast, caspase-7-deficient MEFs are not resistance to intrinsic cell death, but remain attached to the ECM. CONCLUSIONS: Taken together, these data suggest that caspase-9 is required for mitochondrial morphological changes and ROS production by cleaving and activating Bid into tBid. After activation by caspase-9, caspase-3 inhibits ROS production and is required for efficient execution of apoptosis, while effector caspase-7 is required for apoptotic cell detachment.

[Metabolism and biological function of cardiolipin].

Cardiolipin (CL) is a phospholipid, which is exclusively located in mitochondria, and has a unique structure that consists of 2 phosphate residues and 4 kinds of fatty acyl chains. Cardiolipin plays an important role in regulating various kinds of mitochondrial proteins such as electron transport complexes, carrier proteins and phosphate kinases, and is also essential for the organization of particular mitochondrial structures such as cristae and contact sites. Mitochondrial phospholipase D hydrolyzes CL to produce phosphatidic acid, which is required for mitochondrial fusion. Oxidative stress-induced peroxidation of CL occurs because CL is rich in polyunsaturated fatty acids, especially linoleic acid. Accumulation of CL hydroperoxide (CLOOH) triggers the initiation of apoptosis. Formation of CLOOH causes the release of proapoptotic factors such as cytochrome c from the inner mitochondrial membrane and triggers opening of the permeability transition pore. Levels of CL decrease in the heart following ischemia or disease. Apoptosis is enhanced in temperature-dependent mutant cells whose amounts of CL reduce to half when compared to that of wild type cells. Low levels of CL cause the accumulation of CLOOH and enhance sensitivity to apoptosis. Accumulation of CLOOH in mitochondria causes instability of the membrane, because swelling of mitochondria is induced by the presence of CLOOH in the membrane and is significantly enhanced in CLOOH-loaded mitochondria by the addition of inducer of swelling.

Propofol protects against nitrosative stress-induced apoptotic insults to cerebrovascular endothelial cells via an intrinsic mitochondrial mechanism.

BACKGROUND: Cerebrovascular endothelial cells (CECs), major component cells of the blood-brain barrier, can be injured by oxidative stress. Propofol can protect cells from oxidative injury. The aim of this study was to evaluate the effects of propofol on nitrosative stress-induced insults to CECs and its possible mechanisms. METHODS: Primary CECs isolated from mouse cerebral capillaries were exposed to2 nitric oxide (NO) donors: sodium nitroprusside (SNP) or S-nitrosoglutathione (GSNO). Cellular NO levels, cell morphologies, and cell viabilities were analyzed. DNA fragmentation and apoptotic cells were quantified using flow cytometry. Proapoptotic Bcl2-antagonist-killer (Bak) and cytochrome c were immunodetected. Bak translocation was analyzed using confocal microscopy. Caspases-9 and -3 activities were measured fluorometrically. Permeability of the CEC monolayer was assayed by measuring the transendothelial electrical resistance. RESULTS: Exposure of CECs to SNP increased cellular NO levels and simultaneously decreased cell viability (P < .01). Meanwhile, treatment of CECs with propofol at a therapeutic concentration (50 µM) decreased SNP-induced cell death (P < .01). SNP induced DNA fragmentation and cell apoptosis, but propofol decreased the cell injury (P < .01). Sequentially, propofol decreased SNP-enhanced Bak levels and translocation from the cytoplasm to mitochondria (P < .05). Exposure of CECs to propofol attenuated GSNO-induced cell death, apoptosis, and caspase-3 activation (P < .01). Additionally, propofol protected CECs against SNP-induced disruption of the CEC monolayer (P < .05). Consequently, SNP-enhanced cascade activation of caspases-9 and -3 was decreased by propofol (P < .01). CONCLUSION: This study suggested that propofol at a therapeutic concentration can protect against nitrosative stress-induced apoptosis of CECs due to downregulation of the intrinsic Bak-mitochondrion-cytochrome c-caspase protease pathway.

Is suppression of apoptosis a new therapeutic target in sepsis?

Sepsis remains as a leading cause of death in critically ill patients. Unfortunately, there have been very few successful specific therapeutic agents that can significantly reduce the attributable mortality and morbidity of sepsis. Developing novel therapeutic strategies to improve outcomes of sepsis remains an important focus of ongoing research in the field of critical care medicine. Apoptosis has recently been identified as an important mechanism of cell death and evidence suggests that prevention of cell apoptosis can improve survival in animal models of sepsis and endotoxaemia. In this review article, we summarise the critical role of apoptosis of the immune cells in the pathophysiology of sepsis and propose that blocking cell-signaling pathways leading to apoptosis may present a promising specific therapy for sepsis. Various methods to inhibit apoptosis including the cell surface Fas receptor pathway inhibitors, caspase inhibitors, over-expression of anti-apoptotic genes and small interfering ribonucleic acid therapy are discussed.

Role of Met80 and Tyr67 in the low-pH conformational equilibria of cytochrome c.

The low-pH conformational equilibria of ferric yeast iso-1 cytochrome c (ycc) and its M80A, M80A/Y67H, and M80A/Y67A variants were studied from pH 7 to 2 at low ionic strength through electronic absorption, magnetic circular dichroism, and resonance Raman spectroscopies. For wild-type ycc, the protein structure, axial heme ligands, and spin state of the iron atom convert from the native folded His/Met low-spin (LS) form to a molten globule His/H(2)O high-spin (HS) form and a totally unfolded bis-aquo HS state, in a single cooperative transition with an apparent pK(a) of ~3.0. An analogous cooperative transition occurs for the M80A and M80A/Y67H variants. This is preceded by protonation of heme propionate-7, with a pK(a) of ~4.2, and by an equilibrium between a His/OH(-)-ligated LS and a His/H(2)O-ligated HS conformer, with a pK(a) of ~5.9. In the M80A/Y67A variant, the cooperative low-pH transition is split into two distinct processes because of an increased stability of the molten globule state that is formed at higher pH values than the other species. These data show that removal of the axial methionine ligand does not significantly alter the mechanism of acidic unfolding and the ranges of stability of low-pH conformers. Instead, removal of a hydrogen bonding partner at position 67 increases the stability of the molten globule and renders cytochrome c more susceptible to acid unfolding. This underlines the key role played by Tyr67 in stabilizing the three-dimensional structure of cytochrome c by means of the hydrogen bonding network connecting the Ω loops formed by residues 71-85 and 40-57.