Interaction of Carbon Monoxide with the Apoptosis-Inducing Cytochrome c−Cardiolipin Complex

Nitric Oxide Binding Geometry in Heme-Proteins: Relevance for Signal Transduction

Nitric oxide (NO) synthesis, signaling, and scavenging is associated to relevant physiological and pathological events. In all tissues and organs, NO levels and related functions are regulated at different levels, with heme proteins playing pivotal roles. Here, we focus on the structural changes related to the different binding modes of NO to heme-Fe(II), as well as the modulatory effects of this diatomic messenger on heme-protein functions. Specifically, the ability of heme proteins to bind NO at either the distal or proximal side of the heme and the transient interchanging of the binding site is reported. This sheds light on the regulation of O2 supply to tissues with high metabolic activity, such as the retina, where a precise regulation of blood flow is necessary to meet the demand of nutrients.

Insights into the function of cytoglobin

Since its discovery in 2001, the function of cytoglobin has remained elusive. Through extensive in vitro and in vivo research, a range of potential physiological and pathological mechanisms has emerged for this multifunctional member of the hemoglobin family. Currently, over 200 research publications have examined different aspects of cytoglobin structure, redox chemistry and potential roles in cell signalling pathways. This research is wide ranging, but common themes have emerged throughout the research. This review examines the current structural, biochemical and in vivo knowledge of cytoglobin published over the past two decades. Radical scavenging, nitric oxide homeostasis, lipid binding and oxidation and the role of an intramolecular disulfide bond on the redox chemistry are examined, together with aspects and roles for Cygb in cancer progression and liver fibrosis.

Prolonged hypothermic storage of oocytes of the European common frog Rana temporaria in a gas mixture of oxygen and carbon monoxide

The maximum hypothermic storage time of amphibian oocytes is several hours, which is due to the peculiarities of the structure of the cell envelope. The authors of this paper have already demonstrated the possibility of increasing the storage period of unfertilized oocytes of the common frog (Rana temporaria) up to 5-7 days. The aim of the current study was to determine the possibility of using a 6.5 atm gaseous mixture of carbon monoxide and oxygen, for prolonged hypothermic preservation of unfertilized oocytes for 4 to 12 days. After four days, oocytes stored under CO+O2 conditions exhibited fertilization and hatching rates that were 1.6 and 2.2-fold higher than control, respectively. While no oocytes in the control group survived to day twelve, oocytes held under CO +O2 gas exhibited a 39±14% (38 out of 99 oocytes in total) fertilization rate, however only 1±2% (1/99) of those hatched. This approach is promising for the storage of genetic material from female amphibians, particularly in respect to managing and restoring endangered species, but may also be applicable to oocytes of other classes of vertebrates.

Nitrite Reductase Activity of Ferrous Nitrobindins: A Comparative Study

Nitrobindins (Nbs) are all-β-barrel heme proteins spanning from bacteria to Homo sapiens. They inactivate reactive nitrogen species by sequestering NO, converting NO to HNO2, and promoting peroxynitrite isomerization to NO3−. Here, the nitrite reductase activity of Nb(II) from Mycobacterium tuberculosis (Mt-Nb(II)), Arabidopsis thaliana (At-Nb(II)), Danio rerio (Dr-Nb(II)), and Homo sapiens (Hs-Nb(II)) is reported. This activity is crucial for the in vivo production of NO, and thus for the regulation of blood pressure, being of the utmost importance for the blood supply to poorly oxygenated tissues, such as the eye retina. At pH 7.3 and 20.0 °C, the values of the second-order rate constants (i.e., kon) for the reduction of NO2− to NO and the concomitant formation of nitrosylated Mt-Nb(II), At-Nb(II), Dr-Nb(II), and Hs-Nb(II) (Nb(II)-NO) were 7.6 M−1 s−1, 9.3 M−1 s−1, 1.4 × 101 M−1 s−1, and 5.8 M−1 s−1, respectively. The values of kon increased linearly with decreasing pH, thus indicating that the NO2−-based conversion of Nb(II) to Nb(II)-NO requires the involvement of one proton. These results represent the first evidence for the NO2 reductase activity of Nbs(II), strongly supporting the view that Nbs are involved in NO metabolism. Interestingly, the nitrite reductase reactivity of all-β-barrel Nbs and of all-α-helical globins (e.g., myoglobin) was very similar despite the very different three-dimensional fold; however, differences between all-α-helical globins and all-β-barrel Nbs suggest that nitrite reductase activity appears to be controlled by distal steric barriers, even though a more complex regulatory mechanism can be also envisaged.

Resonance Raman Studies on Heme Ligand Stretching Modes in Methionine80-Depleted Cytochrome c: Fe-His, Fe-O2, and O-O Stretching Modes

The peroxidase activity of cytochrome (cyt) c increases when Met80 dissociates from the heme iron, which is related to the initial cyt c membrane permeation step of apoptosis. Met80-dissociated cyt c can form an oxygenated species. Herein, resonance Raman spectra of Met80-depleted horse cyt c (M80A cyt c) were analyzed to elucidate the heme ligand properties of Met80-dissociated cyt c. The Fe-His stretching (ν_Fe-His) mode of ferrous M80A cyt c was observed at 236 cm^-1, and this frequency decreased by 1.5 cm^-1 for the ¹⁵N-labeled protein. The higher ν_Fe-His frequency of M80A cyt c than of other His-ligated heme proteins indicates strong heme coordination and the imidazolate character of His18. Peaks attributed to the Fe-O₂ stretching (ν_Fe-O2) and O-O stretching (ν_O-O) modes of the oxygenated species of M80A cyt c were observed at 576 and 1148 cm^-1, respectively, under an ¹⁶O₂ atmosphere, whereas the frequencies decreased to 544 and 1077 cm^-1, respectively, under an ¹⁸O₂ atmosphere. The ν_Fe-O2 mode of Hydrogenobacter thermophilus (HT) M59A cyt c₅₅₂ was observed at 580 cm^-1 under an ¹⁶O₂ atmosphere, whereas the frequency decreased to 553 cm^-1 under an ¹⁸O₂ atmosphere, indicating that relatively high ν_Fe-O2 frequencies are characteristic of c-type cyt proteins. By comparison of the simultaneously observed ν_Fe-O2 and ν_O-O frequencies of oxygenated cyt c and other oxygenated His-ligated heme proteins, the frequencies tend to have a positive linear relationship; the ν_Fe-O2 frequency increases when the ν_O-O frequency increases. The imidazolate character of the heme-coordinated His and strong Fe-O and O-O bonds are characteristic of cyt c and apparently related to the peroxidase activity when Met80 dissociates from the heme iron.

Carbon Monoxide Signaling: Examining Its Engagement with Various Molecular Targets in the Context of Binding Affinity, Concentration, and Biologic Response

Carbon monoxide (CO) has been firmly established as an endogenous signaling molecule with a variety of pathophysiological and pharmacological functions, including immunomodulation, organ protection, and circadian clock regulation, among many others. In terms of its molecular mechanism(s) of action, CO is known to bind to a large number of hemoproteins with at least 25 identified targets, including hemoglobin, myoglobin, neuroglobin, cytochrome c oxidase, cytochrome P450, soluble guanylyl cyclase, myeloperoxidase, and some ion channels with dissociation constant values spanning the range of sub-nM to high μM. Although CO's binding affinity with a large number of targets has been extensively studied and firmly established, there is a pressing need to incorporate such binding information into the analysis of CO's biologic response in the context of affinity and dosage. Especially important is to understand the reservoir role of hemoglobin in CO storage, transport, distribution, and transfer. We critically review the literature and inject a sense of quantitative assessment into our analyses of the various relationships among binding affinity, CO concentration, target occupancy level, and anticipated pharmacological actions. We hope that this review presents a picture of the overall landscape of CO's engagement with various targets, stimulates additional research, and helps to move the CO field in the direction of examining individual targets in the context of all of the targets and the concentration of available CO. We believe that such work will help the further understanding of the relationship of CO concentration and its pathophysiological functions and the eventual development of CO-based therapeutics. SIGNIFICANCE STATEMENT: The further development of carbon monoxide (CO) as a therapeutic agent will significantly rely on the understanding of CO's engagement with therapeutically relevant targets of varying affinity. This review critically examines the literature by quantitatively analyzing the intricate relationships among targets, target affinity for CO, CO level, and the affinity state of carboxyhemoglobin and provide a holistic approach to examining the molecular mechanism(s) of action for CO.

Modulating the pro-apoptotic activity of cytochrome c at a biomimetic electrified interface

Programmed cell death via apoptosis is a natural defence against excessive cell division, crucial for fetal development to maintenance of homeostasis and elimination of precancerous and senescent cells. Here, we demonstrate an electrified liquid biointerface that replicates the molecular machinery of the inner mitochondrial membrane at the onset of apoptosis. By mimicking in vivo cytochrome c (Cyt c) interactions with cell membranes, our platform allows us to modulate the conformational plasticity of the protein by simply varying the electrochemical environment at an aqueous-organic interface. We observe interfacial electron transfer between an organic electron donor decamethylferrocene and O₂, electrocatalyzed by Cyt c. This interfacial reaction requires partial Cyt c unfolding, mimicking Cyt c in vivo peroxidase activity. As proof of concept, we use our electrified liquid biointerface to identify drug molecules, such as bifonazole, that can potentially down-regulate Cyt c and protect against uncontrolled neuronal cell death in neurodegenerative disorders.

Wheel and Deal in the Mitochondrial Inner Membranes: The Tale of Cytochrome c and Cardiolipin

Cardiolipin oxidation and degradation by different factors under severe cell stress serve as a trigger for genetically encoded cell death programs. In this context, the interplay between cardiolipin and another mitochondrial factor—cytochrome c—is a key process in the early stages of apoptosis, and it is a matter of intense research. Cytochrome c interacts with lipid membranes by electrostatic interactions, hydrogen bonds, and hydrophobic effects. Experimental conditions (including pH, lipid composition, and post-translational modifications) determine which specific amino acid residues are involved in the interaction and influence the heme iron coordination state. In fact, up to four binding sites (A, C, N, and L), driven by different interactions, have been reported. Nevertheless, key aspects of the mechanism for cardiolipin oxidation by the hemeprotein are well established. First, cytochrome c acts as a pseudoperoxidase, a process orchestrated by tyrosine residues which are crucial for peroxygenase activity and sensitivity towards oxidation caused by protein self-degradation. Second, flexibility of two weakest folding units of the hemeprotein correlates with its peroxidase activity and the stability of the iron coordination sphere. Third, the diversity of the mode of interaction parallels a broad diversity in the specific reaction pathway. Thus, current knowledge has already enabled the design of novel drugs designed to successfully inhibit cardiolipin oxidation.

Ultrafast Dynamics and Vibrational Relaxation in Six-Coordinate Heme Proteins Revealed by Femtosecond Stimulated Raman Spectroscopy

Identifying the structural rearrangements during photoinduced reactions is a fundamental challenge for understanding from a microscopic perspective the dynamics underlying the functional mechanisms of heme proteins. Here, femtosecond stimulated Raman spectroscopy is applied to follow the ultrafast evolution of two different proteins, each bearing a six-coordinate heme with two amino acid axial ligands. By exploiting the sensitivity of Raman spectra to the structural configuration, we investigate the effects of photolysis and the binding of amino acid residues in cytochrome c and neuroglobin. By comparing the system response for different time delays and Raman pump resonances, we show how detailed properties of atomic motions and energy redistribution can be unveiled. In particular, we demonstrate substantially faster energy flow from the dissociated heme to the protein moiety in cytochrome c, which we assign to the presence of covalent heme–protein bonds.

Met80 and Tyr67 affect the chemical unfolding of yeast cytochromec: comparing the solutionvs.immobilized state

The motional regime affects the unfolding propensity and axial heme coordination of the Met80Ala and Met80Ala/Tyr67Ala variants of yeast iso-1 cytochromec.

Redox-State-Mediated Regulation of Cytochrome c Release in Apoptosis Revealed by Surface-Enhanced Raman Scattering on Nickel Substrates

The interaction of cytochrome c (Cyt c) with cardiolipin (CL) is believed to play an important role in the initial events of apoptosis. Herein, we investigate the structural changes of CL-bound Fe²⁺ Cyt c and the correlation with Cyt c release through surface-enhanced Raman spectroscopy (SERS) on nickel substrates. The SERS results together with molecular dynamics simulation reveal that Fe²⁺ Cyt c undergoes autoxidation and a relatively larger conformational alteration after binding with CL, inducing higher peroxidase activity of Cyt c and higher permeability of the CL membrane compared with those induced by the Fe³⁺ Cyt c. The proapoptotic activity and SERS effect of the Ni nanostructures allow the in situ study of the redox-state-dependent Cyt c release from isolated mitochondria, which reveals for the first time that the ferrous state of Cyt c most likely plays a more important role in triggering apoptosis.

Neurotoxicity of anesthetics: Mechanisms and meaning from mouse intervention studies

Volatile anesthetics are widely used in human medicine and generally considered to be safe in healthy individuals. In recent years, the safety of volatile anesthesia in pediatric patients has been questioned following reports of anesthetic induced neurotoxicity in pre-clinical studies. These studies in mice, rats, and primates have demonstrated that exposure to anesthetic agents during early post-natal periods can cause acute neurotoxicity, as well as later-life cognitive defects including deficits in learning and memory. In recent years, the focus of many pre-clinical studies has been on identifying candidate pathways or potential therapeutic targets through intervention trials. These reports have shed light on the mechanisms underlying anesthesia induced neurotoxicity as well as highlighting the challenges of pre-clinical modeling of anesthesia induced neurotoxicity in mice. Here, we summarize the data derived from intervention studies in neonatal mouse models of anesthetic exposure and provide an overview of mechanisms proposed to mediate anesthesia induced neurotoxicity in mice based on these reports. The majority of these studies implicate one of three mechanisms: reactive oxygen species (ROS) mediated stress and signaling, growth/nutrient signaling, or direct neuronal modulation.

Peroxidase Activity of Human Hemoproteins: Keeping the Fire under Control

The heme in the active center of peroxidases reacts with hydrogen peroxide to form highly reactive intermediates, which then oxidize simple substances called peroxidase substrates. Human peroxidases can be divided into two groups: (1) True peroxidases are enzymes whose main function is to generate free radicals in the peroxidase cycle and (pseudo)hypohalous acids in the halogenation cycle. The major true peroxidases are myeloperoxidase, eosinophil peroxidase and lactoperoxidase. (2) Pseudo-peroxidases perform various important functions in the body, but under the influence of external conditions they can display peroxidase-like activity. As oxidative intermediates, these peroxidases produce not only active heme compounds, but also protein-based tyrosyl radicals. Hemoglobin, myoglobin, cytochrome c/cardiolipin complexes and cytoglobin are considered as pseudo-peroxidases. Рeroxidases play an important role in innate immunity and in a number of physiologically important processes like apoptosis and cell signaling. Unfavorable excessive peroxidase activity is implicated in oxidative damage of cells and tissues, thereby initiating the variety of human diseases. Hence, regulation of peroxidase activity is of considerable importance. Since peroxidases differ in structure, properties and location, the mechanisms controlling peroxidase activity and the biological effects of peroxidase products are specific for each hemoprotein. This review summarizes the knowledge about the properties, activities, regulations and biological effects of true and pseudo-peroxidases in order to better understand the mechanisms underlying beneficial and adverse effects of this class of enzymes.

An Overview of the Potential Therapeutic Applications of CO-Releasing Molecules

Carbon monoxide (CO) has long been known as the “silent killer” owing to its ability to form carboxyhemoglobin—the main cause of CO poisoning in humans. Its role as an endogenous neurotransmitter, however, was suggested in the early 1990s. Since then, the biological activity of CO has been widely examined via both the direct administration of CO and in the form of so-called “carbon monoxide releasing molecules (CORMs).” This overview will explore the general physiological effects and potential therapeutic applications of CO when delivered in the form of CORMs.

Relating the multi-functionality of cytochrome c to membrane binding and structural conversion

Cytochrome c is known as an electron-carrying protein in the respiratory chain of mitochondria. Over the last 20 years, however, alternative functions of this very versatile protein have become the focus of research interests. Upon binding to anionic lipids such as cardiolipin, the protein acquires peroxidase activity. Multiple lines of evidence suggest that this requires a conformational change of the protein which involves partial unfolding of its tertiary structure. This review summarizes the current state of knowledge of how cytochrome c interacts with cardiolipin-containing surfaces and how this affects its structure and function. In this context, we delineate partially conflicting results regarding the affinity of cytochrome c binding to cardiolipin-containing liposomes of different size and its influence on the structure of the protein and the morphology of the membrane.

Anesthesia-Related Carbon Monoxide Exposure: Toxicity and Potential Therapy

Exposure to carbon monoxide (CO) during general anesthesia can result from volatile anesthetic degradation by carbon dioxide absorbents and rebreathing of endogenously produced CO. Although adherence to the Anesthesia Patient Safety Foundation guidelines reduces the risk of CO poisoning, patients may still experience subtoxic CO exposure during low-flow anesthesia. The consequences of such exposures are relatively unknown. In contrast to the widely recognized toxicity of high CO concentrations, the biologic activity of low concentration CO has recently been shown to be cytoprotective. As such, low-dose CO is being explored as a novel treatment for a variety of different diseases. Here, we review the concept of anesthesia-related CO exposure, identify the sources of production, detail the mechanisms of overt CO toxicity, highlight the cellular effects of low-dose CO, and discuss the potential therapeutic role for CO as part of routine anesthetic management.

Reductive nitrosylation of ferric carboxymethylated-cytochrome c

Horse heart carboxymethylated-cyt[Formula: see text] (CM-cyt[Formula: see text] displays myoglobin-like properties due to the cleavage of the heme-Fe-Met80 axial bond. Here, reductive nitrosylation of CM-cyt[Formula: see text](III) between pH 8.5 and 9.5, at [Formula: see text] 20.0 C, is reported. Under anaerobic conditions, the addition of NO to CM-cyt[Formula: see text](III) leads to the transient formation of CM-cyt[Formula: see text](III)-NO in equilibrium with CM-cyt[Formula: see text](II)-NO[Formula: see text]. In turn, CM-cyt[Formula: see text](II)-NO[Formula: see text] is converted to CM-cyt[Formula: see text](II) by OH[Formula: see text]-based catalysis. Then, CM-cyt[Formula: see text](II) binds NO very rapidly leading to CM-cyt[Formula: see text](II)-NO. Kinetics of NO binding to CM-cyt[Formula: see text](III) is independent of the ligand concentration, [Formula: see text] values ranging between 3.6 ± 0.4 s[Formula: see text] and 7.1 ± 0.7 s[Formula: see text]. This indicates that the formation of the CM-cytc(III)-NO complex is rate-limited by the cleavage of the weak heme-Fe(III) distal bond (likely Lys79). The conversion of CM-cyt[Formula: see text](III)-NO to CM-cyt[Formula: see text](II)-NO is rate-limited by the OH[Formula: see text]-mediated reduction of CM-cyt[Formula: see text](II)-NO[Formula: see text] ([Formula: see text] (1.2 ± 0.1) × 103 M[Formula: see text].s[Formula: see text]. Lastly, the very fast nitrosylation of CM-cyt[Formula: see text](II) takes place, values of [Formula: see text] ranging between[Formula: see text]5.3 × 106 M[Formula: see text].s[Formula: see text] and 1.4 × 107 M[Formula: see text].s[Formula: see text]. These results indicate that CM-cyt[Formula: see text] behaves as the cardiolipin-cyt[Formula: see text] complex highlighting the role of the sixth axial ligand of the heme-Fe atom in the modulation of the metal-based reactivity.

Autoxidation of Reduced Horse Heart Cytochrome c Catalyzed by Cardiolipin-Containing Membranes

Visible circular dichroism, absorption, and fluorescence spectroscopy were used to probe the binding of horse heart ferrocytochrome c to anionic cardiolipin (CL) head groups on the surface of 1,1',2,2'-tetraoleoyl cardiolipin (TOCL)/1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) (20%:80%) liposomes in an aerobic environment. We found that ferrocytochrome c undergoes a conformational transition upon binding that leads to complete oxidation of the protein at intermediate and high CL concentrations. At low lipid concentrations, the protein maintains a structure that is only slightly different from its native one, whereas an ensemble of misligated predominantly hexacoordinated low-spin states become increasingly populated at high lipid concentrations. A minor fraction of conformations with either high- or quantum-mixed-spin states were detected at a CL to protein ratio of 200 (the largest one investigated). The population of the non-native state is less pronounced than that found for cytochrome c-CL interactions initiated with oxidized cytochrome c. Under anaerobic conditions, the protein maintains its reduced state but still undergoes some conformational change upon binding to CL head groups on the liposome surface. Our data suggest that CL-containing liposomes function as catalysts by reducing the activation barrier for a Fe²⁺ → O₂ electron transfer. Adding NaCl to the existing cytochrome-liposome mixtures under aerobic conditions inhibits protein autoxidation of ferrocytochrome c and stabilizes the reduced state of the membrane-bound protein.

Carbon monoxide and anesthesia-induced neurotoxicity

The majority of commonly used anesthetic agents induce widespread neuronal degeneration in the developing mammalian brain. Downstream, the process appears to involve activation of the oxidative stress-associated mitochondrial apoptosis pathway. Targeting this pathway could result in prevention of anesthetic toxicity in the immature brain. Carbon monoxide (CO) is a gas that exerts biological activity in the developing brain and low dose exposures have the potential to provide neuroprotection. In recent work, low concentration CO exposures limited isoflurane-induced neuronal apoptosis in a dose-dependent manner in newborn mice and modulated oxidative stress within forebrain mitochondria. Because infants and children are routinely exposed to low levels of CO during low-flow general endotracheal anesthesia, such anti-oxidant and pro-survival cellular effects are clinically relevant. Here we provide an overview of anesthesia-related CO exposure, discuss the biological activity of low concentration CO, detail the effects of CO in the brain during development, and provide evidence for CO-mediated inhibition of anesthesia-induced neurotoxicity.

Self-oxidation of cytochrome c at methionine80 with molecular oxygen induced by cleavage of the Met-heme iron bond

Met80 of cytochrome c (cyt c) has been shown to dissociate from its heme iron when cyt c interacts with cardiolipin (CL), which triggers the release of cyt c into the cytosol initiating apoptosis. We found that the mass of human cyt c increases by 16 Da in the Met80-Lys86 region by reaction with molecular oxygen in the presence of CL-containing liposomes and dithiothreitol (DTT). To investigate the effect of Met80 dissociation on the reaction of cyt c with molecular oxygen without affecting its secondary structures, a human cyt c mutant (Δ8384 cyt c) was constructed by removing two amino acids (Val83 and Gly84) from the loop containing Met80. According to MALDI-TOF-MS and tandem mass measurements, Met80 of Δ8384 cyt c was modified site-specifically to methionine sulfoxide when purified in the presence of molecular oxygen, whereas Met80 was not modified in the absence of molecular oxygen. A red-shift of the Soret band from 406 to 412 nm and absorption increase at ∼536 and ∼568 nm were observed for Δ8384 cyt c when it reacted with DTT and molecular oxygen, followed by a further red-shift of the Soret band to 416 nm and absorption increase at ∼620 and ∼650 nm. These results indicate that Met80 of cyt c is oxidized site-specifically by formation of the oxy and subsequent compound I-like species when Met80 dissociates from the heme iron, where the Met80 modification may affect its peroxidase activity related to apoptosis.

Cardiolipin-cytochrome c complex: Switching cytochrome c from an electron-transfer shuttle to a myoglobin- and a peroxidase-like heme-protein

Cytochrome c (cytc) is a small heme-protein located in the space between the inner and the outer membrane of the mitochondrion that transfers electrons from cytc-reductase to cytc-oxidase. The hexa-coordinated heme-Fe atom of cytc displays a very low reactivity toward ligands and does not exhibit significant catalytic properties. However, upon cardiolipin (CL) binding, cytc achieves ligand binding and catalytic properties reminiscent of those of myoglobin and peroxidase. In particular, the peroxidase activity of the cardiolipin-cytochrome c complex (CL-cytc) is critical for the redistribution of CL from the inner to the outer mitochondrial membranes and is essential for the execution and completion of the apoptotic program. On the other hand, the capability of CL-cytc to bind NO and CO and the heme-Fe-based scavenging of reactive nitrogen and oxygen species may affect apoptosis. Here, the ligand binding and catalytic properties of CL-cytc are analyzed in parallel with those of CL-free cytc, myoglobin, and peroxidase to dissect the potential mechanisms of CL in modulating the pro- and anti-apoptotic actions of cytc.

Carbon monoxide pollution and neurodevelopment: A public health concern

Although an association between air pollution and adverse systemic health effects has been known for years, the effect of pollutants on neurodevelopment has been underappreciated. Recent evidence suggests a possible link between air pollution and neurocognitive impairment and behavioral disorders in children, however, the exact nature of this relationship remains poorly understood. Infants and children are uniquely vulnerable due to the potential for exposure in both the fetal and postnatal environments during critical periods in development. Carbon monoxide (CO), a common component of indoor and outdoor air pollution, can cross the placenta to gain access to the fetal circulation and the developing brain. Thus, CO is of particular interest as a known neurotoxin and a potential public health threat. Here we review overt CO toxicity and the policies regulating CO exposure, detail the evidence suggesting a potential link between CO-associated ambient air pollution, tobacco smoke, and learning and behavioral abnormalities in children, describe the effects of subclinical CO exposure on the brain during development, and provide mechanistic insight into a potential connection between CO exposure and neurodevelopmental outcome. CO can disrupt a number of critical processes in the developing brain, providing a better understanding of how this specific neurotoxin may impair neurodevelopment. However, further investigation is needed to better define the effects of perinatal CO exposure on the immature brain. Current policies regarding CO standards were established based on evidence of cardiovascular risk in adults with pre-existing comorbidities. Thus, recent and emerging data highlighted in this review regarding CO exposure in the fetus and developing child may be important to consider when the standards and guidelines are evaluated and revised in the future.

Immobilized cytochrome c bound to cardiolipin exhibits peculiar oxidation state-dependent axial heme ligation and catalytically reduces dioxygen

Mitochondrial cytochrome c (cytc) plays an important role in programmed cell death upon binding to cardiolipin (CL), a negatively charged phospholipid of the inner mitochondrial membrane (IMM). Although this binding has been thoroughly investigated in solution, little is known on the nature and reactivity of the adduct (cytc-CL) immobilized at IMM. In this work, we have studied electrochemically cytc-CL immobilized on a hydrophobic self-assembled monolayer (SAM) of decane-1-thiol. This construct would reproduce the motional restriction and the nonpolar environment experienced by cytc-CL at IMM. Surface-enhanced resonance Raman (SERR) studies allowed the axial heme iron ligands to be identified, which were found to be oxidation state dependent and differ from those of cytc-CL in solution. In particular, immobilized cytc-CL experiences an equilibrium between a low-spin (LS) 6c His/His and a high-spin (HS) 5c His/- coordination states. The former prevails in the oxidized and the latter in the reduced form. Axial coordination of the ferric heme thus differs from the (LS) 6c His/Lys and (LS) 6c His/OH(-) states observed in solution. Moreover, a relevant finding is that the immobilized ferrous cytc-CL is able to catalytically reduce dioxygen, likely to superoxide ion. These findings indicate that restriction of motional freedom due to interaction with the membrane is an additional factor playing in the mechanism of cytc unfolding and cytc-mediated peroxidation functional to the apoptosis cascade.

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.

Dynamics of the heme-binding bacterial gas-sensing dissimilative nitrate respiration regulator (DNR) and activation barriers for ligand binding and escape

DNR (dissimilative nitrate respiration regulator) is a heme-binding transcription factor that is involved in the regulation of denitrification in Pseudomonas aeruginosa. In the ferrous deoxy state, the heme is 6-coordinate; external NO and CO can replace an internal ligand. Using fluorescence anisotropy, we show that high-affinity sequence-specific DNA binding occurs only when the heme is nitrosylated, consistent with the proposed function of DNR as NO sensor and transcriptional activator. This role is moreover supported by the NO "trapping" properties revealed by ultrafast spectroscopy that are similar to those of other heme-based NO sensor proteins. Dissociated CO-heme pairs rebind in an essentially barrierless way. This process competes with migration out of the heme pocket. The latter process is thermally activated (Ea ∼ 7 kJ/mol). This result is compared with other heme proteins, including the homologous CO sensor/transcription factor CooA, variants of the 5-coordinate mycobacterial sensor DosT and the electron transfer protein cytochrome c. This comparison indicates that thermal activation of ligand escape from the heme pocket is specific for systems where an external ligand replaces an internal one. The origin of this finding and possible implications are discussed.

Subclinical carbon monoxide limits apoptosis in the developing brain after isoflurane exposure

BACKGROUND

Volatile anesthetics cause widespread apoptosis in the developing brain. Carbon monoxide (CO) has antiapoptotic properties, and exhaled endogenous CO is commonly rebreathed during low-flow anesthesia in infants and children, resulting in subclinical CO exposure. Thus, we aimed to determine whether CO could limit isoflurane-induced apoptosis in the developing brain.

METHODS

Seven-day-old male CD-1 mouse pups underwent 1-hour exposure to 0 (air), 5, or 100 ppm CO in air with or without isoflurane (2%). We assessed carboxyhemoglobin levels, cytochrome c peroxidase activity, and cytochrome c release from forebrain mitochondria after exposure and quantified the number of activated caspase-3 positive cells and TUNEL positive nuclei in neocortex, hippocampus, and hypothalamus/thalamus.

RESULTS

Carboxyhemoglobin levels approximated those expected in humans after a similar time-weighted CO exposure. Isoflurane significantly increased cytochrome c peroxidase activity, cytochrome c release, the number of activated caspase-3 cells, and TUNEL positive nuclei in the forebrain of air-exposed mice. CO, however, abrogated isoflurane-induced cytochrome c peroxidase activation and cytochrome c release from forebrain mitochondria and decreased the number of activated caspase-3 positive cells and TUNEL positive nuclei after simultaneous exposure with isoflurane.

CONCLUSIONS

Taken together, the data indicate that CO can limit apoptosis after isoflurane exposure via inhibition of cytochrome c peroxidase depending on concentration. Although it is unknown whether CO directly inhibited isoflurane-induced apoptosis, it is possible that low-flow anesthesia designed to target rebreathing of specific concentrations of CO may be a desired strategy to develop in the future in an effort to prevent anesthesia-induced neurotoxicity in infants and children.

Structure of a mitochondrial cytochrome c conformer competent for peroxidase activity

At the onset of apoptosis, the peroxidation of cardiolipin at the inner mitochondrial membrane by cytochrome c requires an open coordination site on the heme. We report a 1.45-Å resolution structure of yeast iso-1-cytochrome c with the Met80 heme ligand swung out of the heme crevice and replaced by a water molecule. This conformational change requires modest adjustments to the main chain of the heme crevice loop and is facilitated by a trimethyllysine 72-to-alanine mutation. This mutation also enhances the peroxidase activity of iso-1-cytochrome c. The structure shows a buried water channel capable of facilitating peroxide access to the active site and of moving protons produced during peroxidase activity to the protein surface. Alternate positions of the side chain of Arg38 appear to mediate opening and closing of the buried water channel. In addition, two buried water molecules can adopt alternate positions that change the network of hydrogen bonds in the buried water channel. Taken together, these observations suggest that low and high proton conductivity states may mediate peroxidase function. Comparison of yeast and mammalian cytochrome c sequences, in the context of the steric factors that permit opening of the heme crevice, suggests that higher organisms have evolved to inhibit peroxidase activity, providing a more stringent barrier to the onset of apoptosis.

NO binding to the proapoptotic cytochrome c-cardiolipin complex

Cytochrome c is a heme protein that is localized in the compartment between the inner and outer mitochondrial membranes where it functions to transfer electrons between complex III and complex IV of the respiratory chain. It can also form an intimate association with the mitochondrion-specific phospholipid cardiolipin that induces a conformational change in the protein enabling it to act as a peroxidase catalyzing the oxidation of cardiolipin and thereby instigating a chain of events that leads to apoptosis. Unlike the native protein, cytochrome c within the complex binds ligands rapidly; in particular, NO can coordinate to either the ferric or ferrous iron of the heme. Remarkably, in the ferrous form, NO binds preferentially to the proximal side of the heme and thus behaves in a way similar to cytochrome c'-type proteins and to guanylate cyclase. The implications of NO binding to the proapoptotic cytochrome c/cardiolipin complex are discussed in terms of modulating the apoptotic response and buffering NO concentrations. Insights into the structure of the complex are provided by comparison with cytochrome c' for which X-ray structures are available.

Structural transformations of cytochrome c upon interaction with cardiolipin

Interactions of cytochrome c (cyt c) with cardiolipin (CL) play a critical role in early stages of apoptosis. Upon binding to CL, cyt c undergoes changes in secondary and tertiary structure that lead to a dramatic increase in its peroxidase activity. Insertion of the protein into membranes, insertion of CL acyl chains into the protein interior, and extensive unfolding of cyt c after adsorption to the membrane have been proposed as possible modes for interaction of cyt c with CL. Dissociation of Met80 is accompanied by opening of the heme crevice and binding of another heme ligand. Fluorescence studies have revealed conformational heterogeneity of the lipid-bound protein ensemble with distinct polypeptide conformations that vary in the degree of protein unfolding. We correlate these recent findings to other biophysical observations and rationalize the role of experimental conditions in defining conformational properties and peroxidase activity of the cyt c ensemble. Latest time-resolved studies propose the trigger and the sequence of cardiolipin-induced structural transitions of cyt c.

Dynamics of the His79-heme alkaline transition of yeast iso-1-cytochrome c probed by conformationally gated electron transfer with Co(II)bis(terpyridine)

Alkaline conformers of cytochrome c may be involved in both its electron transport and apoptotic functions. We use cobalt(II)bis(terpyridine), Co(terpy)2(2+), as a reagent for conformationally gated electron-transfer (gated ET) experiments to study the alkaline conformational transition of K79H variants of yeast iso-1-cytochrome c expressed in Escherichia coli , WT*K79H, with alanine at position 72 and Saccharomyces cerevisiae , yK79H, with trimethyllysine (Tml) at position 72. Co(terpy)2(2+) is well-suited to the 100 ms to 1 s time scale of the His79-mediated alkaline conformational transition of these variants. Reduction of the His79-heme alkaline conformer by Co(terpy)2(2+) occurs primarily by gated ET, which involves conversion to the native state followed by reduction, with a small fraction of the His79-heme alkaline conformer directly reduced by Co(terpy)2(2+). The gated ET experiments show that the mechanism of formation of the His79-heme alkaline conformer involves only two ionizable groups. In previous work, we showed that the mechanism of the His73-mediated alkaline conformational transition requires three ionizable groups. Thus, the mechanism of heme crevice opening depends upon the position of the ligand mediating the process. The microscopic rate constants provided by gated ET studies show that mutation of Tml72 (yK79H variant) in the heme crevice loop to Ala72 (WT*K79H variant) affects the dynamics of heme crevice opening through a small destabilization of both the native conformer and the transition state relative to the His79-heme alkaline conformer. Previous pH jump data had indicated that the Tml72→Ala mutation primarily stabilized the transition state for the His79-mediated alkaline conformational transition.

Dynamics of geminate rebinding of CO to cytochrome c in guanidine HCl probed by femtosecond vibrational spectroscopy

Femtosecond vibrational spectroscopy was used to probe the rebinding dynamics of CO to cytochrome c (Cytc) in 1.8 and 7 M guanidine HCl (GdnHCl) after photodeligation of the corresponding CO-bound protein in D2O buffer (pD = 7.4) at 283 K. Geminate rebinding (GR) dynamics of CO to the folded Cytc in 1.8 M GdnHCl (nCytc) is similar to that to chemically modified cytochrome c (cCytc), suggesting that the overall conformations of nCytcCO and cCytcCO are similar. About 86% of the dissociated CO molecules were geminately rebound to nCytc nonexponentially within 1 ns. The efficient GR of CO to the folded Cytc can be attributed to the organized protein matrix near the active site of nCytc that provides an efficient trap for the diffusing CO ligand after photodissociation. Although the concentration of nCytc did not affect its GR yield of CO, GR yield of CO to the unfolded Cytc in 7 M GdnHCl (uCytc) increased from 5 to 30% as the protein concentration increased from 0.3 to 9 mM. Time-resolved spectra of the (13)CO dissociated from both 9 mM nCytc(13)CO and 9 mM uCytc(13)CO showed a growing band with a peak at 2090 cm(-1) on the picosecond time scale, which was assigned to (13)CO in D2O solvent. At 1 ns, the fraction of the CO band in the solvent was about 10% of the nascent photodeligated protein in nCytc and more than 50% in the concentrated uCytc. Whereas a small opening in the active site of nCytc is responsible for the ultrafast escape of CO to solution in the folded protein, a large fraction of the CO escape to the solvent in uCytc results from the denatured structure of the active site in the unfolded protein. The spectrum of the CO dissociated from the concentrated uCytcCO contained a band that decayed as efficiently as that for the folded protein, suggesting that some fraction of uCytcCO might form aggregates even in 7 M denaturant, such that the aggregate acts as an efficient trap for the diffusing CO after deligation. No hint of precipitate in the concentrated uCytcCO and protein refolding upon dilution of the GdnHCl indicate that the aggregate does not grow continuously but remains as a soluble oligomer. The delayed appearance of the solvated CO and the inefficient GR of CO in uCytcCO suggest that the monomeric unfolded CytcCO so loosely arranged that the protein matrix cannot trap CO efficiently but the bound CO is still buried within hydrophobic residues even under the harsh denaturation condition.

Cytochrome c causes pore formation in cardiolipin-containing membranes

The release of cytochrome c from mitochondria is a key signaling mechanism in apoptosis. Although extramitochondrial proteins are thought to initiate this release, the exact mechanisms remain unclear. Cytochrome c (cyt c) binds to and penetrates lipid structures containing the inner mitochondrial membrane lipid cardiolipin (CL), leading to protein conformational changes and increased peroxidase activity. We describe here a direct visualization of a fluorescent cyt c crossing synthetic, CL-containing membranes in the absence of other proteins. We observed strong binding of cyt c to CL in phospholipid vesicles and bursts of cyt c leakage across the membrane. Passive fluorescent markers such as carboxyfluorescein and a 10-kDa dextran polymer crossed the membrane simultaneously with cyt c, although larger dextrans did not. The data show that these bursts result from the opening of lipid pores formed by the cyt c-CL conjugate. Pore formation and cyt c leakage were significantly reduced in the presence of ATP. We suggest a model, consistent with these findings, in which the formation of toroidal lipid pores is driven by initial cyt c-induced negative spontaneous membrane curvature and subsequent protein unfolding interactions. Our results suggest that the CL-cyt c interaction may be sufficient to allow cyt c permeation of mitochondrial membranes and that cyt c may contribute to its own escape from mitochondria during apoptosis.

Mutation of trimethyllysine 72 to alanine enhances His79-heme-mediated dynamics of iso-1-cytochrome c

Trimethyllysine 72 (Tml72) of yeast iso-1-cytochrome c lies across the surface of the heme crevice loop (Ω-loop D, residues 70-85) like a brace. Lys72 is oriented similarly in horse cytochrome c (Cytc). To determine whether this residue affects the dynamics of opening the heme crevice loop, we have studied the effect of a Tml72 to Ala substitution on the formation of the His79-heme alkaline conformer near neutral pH using a variant of iso-1-Cytc including K72A and K79H mutations. Guanidine hydrochloride denaturation shows that the Tml72 to Ala substitution within error does not affect the global stability of the protein. The effect of the Tml72 to Ala substitution on the thermodynamics of the His79-heme alkaline transition is also small. However, pH-jump kinetic studies of the His79-heme alkaline transition show that both the forward and backward rates of conformational change are increased by the Tml72 to Ala substitution. The barrier for opening the heme crevice is reduced by 0.5 kcal/mol and for closing the heme crevice by 0.3 kcal/mol. The ability of Tml72 to modulate the heme crevice dynamics may indicate a crucial role in regulating function, such as in the peroxidase activity seen in the early stages of apoptosis.

Elucidating the mechanism of ferrocytochrome c heme disruption by peroxidized cardiolipin

The interaction of peroxidized cardiolipin with ferrocytochrome c induces two kinetically and chemically distinct processes. The first is a rapid oxidation of ferrocytochrome c, followed by a slower, irreversible disruption of heme c. The oxidation of ferrocytochrome c by peroxidized cardiolipin is explained by a Fenton-type reaction. Heme scission is a consequence of the radical-mediated reactions initiated by the interaction of ferric heme iron with peroxidized cardiolipin. Simultaneously with the heme c disruption, generation of hydroxyl radical is detected by EPR spectroscopy using the spin trapping technique. The resulting apocytochrome c sediments as a heterogeneous mixture of high aggregates, as judged by sedimentation analysis. Both the oxidative process and the destructive process were suppressed by nonionic detergents and/or high ionic strength. The mechanism for generating radicals and heme rupture is presented.

Nitric oxide binding to the cardiolipin complex of ferric cytochrome C

Cardiolipin, a phospholipid specific to the mitochondrion, interacts with the small electron transfer heme protein cytochrome c through both electrostatic and hydrophobic interactions. Once in a complex with cardiolipin, cytochrome c has been shown to undergo a conformational change that leads to the rupture of the bond between the heme iron and the intrinsic sulfur ligand of a methionine residue and to enhance the peroxidatic properties of the protein considered important to its apoptotic activity. Here we report that the ferric cytochrome c/cardiolipin complex binds nitric oxide tightly through a multistep process in which the first step is the relatively slow displacement (5 s(-1)) from heme coordination of an intrinsic ligand that replaces methionine in the complex. Nanosecond photolysis of the nitrosyl adduct demonstrated that a fraction of the nitric oxide escapes from the heme pocket and subsequently recombines to the heme in second-order processes (k = 1.8 × 10(6) and 5.5 × 10(5) M(-1) s(-1)) that, under these conditions, were much faster than recombination of the intrinsic ligand with which they compete. Ultrafast (femtosecond) laser photolysis showed that the geminate recombination of nitric oxide to the heme occurred with time constants (τ = 22 and 72 ps) and that ~23% of the photolyzed nitric oxide escaped into the bulk phase. This high value for the escape fraction relative to other heme proteins indicates the open nature of the heme pocket in this complex. These results are summarized in a scheme and are discussed in terms of the possible modulation of the apoptotic activity of cytochrome c by nitric oxide.