Extramitochondrial ATP/ADP-ratios regulate cytochrome c oxidase activity via binding to the cytosolic domain of subunit IV

The ratio of cytochrome c oxidase subunit 4 isoform 4I1 and 4I2 mRNA is changed in permanent atrial fibrillation

AIMS

The conditions of hypoxia are suggested to induce permanent atrial fibrillation (AF). The regulation of COX4I2 and COX4I1 depends on oxygen availability in tissues. A role of COX4I2 in the myocardium of AF patients is supposed for pathogenesis of AF and subsequent alterations in the electron transfer chain (ETC) under hypoxia.

METHODS AND RESULTS

In vitro, influence of hypoxia on HeLa 53 cells was studied and elevated parts of COX 4I2 were confirmed. Myocardial biopsies were taken ex vivo from the patients' Right Atria with SR (n = 31) and AF (n = 11), respectively. RT- PCR for mRNA expresson, mitochondrial respiration by polarography and the protein content of cytochrome c oxidase (CytOx) subunit 4I1 and CytOx subunit 4I2 by ELISA were studied. Clinical data were correlated to the findings of gene expressions in parallel. Patients with permanent AF had a change in isoform 4I2/4I1 expression along with a decrease of isoform COX 4I1 expression. The 4I2/4I1 ratio of mRNA expression was increased from 0.630 to 1.058 in comparison. However, the protein content of CytOx subunit 4 was much lower in the AF group, whereas the respiration/units enzyme activity in both groups remained the same.

CONCLUSIONS

This study describes a possible molecular correlate for the development of AF. Due to the known functional significance of COX 4I2, mitochondrial dysfunction can be assumed as a part of the pathogenesis of AF.

Mitochondrial Oxidative Phosphorylation in Viral Infections

Mitochondria have been identified as the "powerhouse" of the cell, generating the cellular energy, ATP, for almost seven decades. Research over time has uncovered a multifaceted role of the mitochondrion in processes such as cellular stress signaling, generating precursor molecules, immune response, and apoptosis to name a few. Dysfunctional mitochondria resulting from a departure in homeostasis results in cellular degeneration. Viruses hijack host cell machinery to facilitate their own replication in the absence of a bonafide replication machinery. Replication being an energy intensive process necessitates regulation of the host cell oxidative phosphorylation occurring at the electron transport chain in the mitochondria to generate energy. Mitochondria, therefore, can be an attractive therapeutic target by limiting energy for viral replication. In this review we focus on the physiology of oxidative phosphorylation and on the limited studies highlighting the regulatory effects viruses induce on the electron transport chain.

Calcium-Dependent Interaction of Nitric Oxide Synthase with Cytochrome c Oxidase: Implications for Brain Bioenergetics

Targeted nitric oxide production is relevant for maintaining cellular energy production, protecting against oxidative stress, regulating cell death, and promoting neuroprotection. This study aimed to characterize the putative interaction of nitric-oxide synthase with mitochondrial proteins. The primary finding of this study is that cytochrome c oxidase (CCO) subunit IV (CCOIV) is associated directly with NOS in brain mitochondria when calcium ions are present. The matrix side of CCOIV binds to the N-terminus of NOS, supported by the abrogation of the binding by antibodies towards the N-terminus of NOS. Evidence supporting the interaction between CCOIV and NOS was provided by the coimmunoprecipitation of NOS from detergent-solubilized whole rat brain mitochondria with antibodies to CCOIV and the coimmunoprecipitation of CCOIV from crude brain NOS preparations using antibodies to NOS. The CCOIV domain that interacts with NOS was identified using a series of overlapping peptides derived from the primary sequence of CCOIV. As calcium ions not only activate NOS, but also facilitate the docking of NOS to CCOIV, this study points to a dynamic mechanism of controlling the bioenergetics by calcium changes, thereby adapting bioenergetics to cellular demands.

Constitutive Expression of Hif2α Confers Acute O2 Sensitivity to Carotid Body Glomus Cells

Acute oxygen (O₂) sensing and adaptation to hypoxia are essential for physiological homeostasis. The prototypical acute O₂ sensing organ is the carotid body, which contains chemosensory glomus cells expressing O₂-sensitive K⁺ channels. Inhibition of these channels during hypoxia leads to cell depolarization, transmitter release, and activation of afferent sensory fibers terminating in the brain stem respiratory and autonomic centers. Focusing on recent data, here we discuss the special sensitivity of glomus cell mitochondria to changes in O₂ tension due to Hif2α-dependent expression of several atypical mitochondrial electron transport chain subunits and enzymes. These are responsible for an accelerated oxidative metabolism and the strict dependence of mitochondrial complex IV activity on O₂ availability. We report that ablation of Epas1 (the gene coding Hif2α) causes a selective downregulation of the atypical mitochondrial genes and a strong inhibition of glomus cell acute responsiveness to hypoxia. Our observations indicate that Hif2α expression is required for the characteristic metabolic profile of glomus cells and provide a mechanistic explanation for the acute O₂ regulation of breathing.

Cytochrome c Oxidase Inhibition by ATP Decreases Mitochondrial ROS Production

This study addresses the eventual consequence of cytochrome c oxidase (CytOx) inhibition by ATP at high ATP/ADP ratio in isolated rat heart mitochondria. Earlier, it has been demonstrated that the mechanism of allosteric ATP inhibition of CytOx is one of the key regulations of mitochondrial functions. It is relevant that aiming to maintain a high ATP/ADP ratio for the measurement of CytOx activity effectuating the enzymatic inhibition as well as mitochondrial respiration, optimal concentration of mitochondria is critically important. Likewise, only at this concentration, were the differences in ΔΨ_m and ROS concentrations measured under various conditions significant. Moreover, when CytOx activity was inhibited in the presence of ATP, mitochondrial respiration and ΔΨ_m both remained static, while the ROS production was markedly decreased. Consubstantial results were found when the electron transport chain was inhibited by antimycin A, letting only CytOx remain functional to support the energy production. This seems to corroborate that the decrease in mitochondrial ROS production is solely the effect of ATP binding to CytOx which results in static respiration as well as membrane potential.

Butyrate Ameliorates Mitochondrial Respiratory Capacity of The Motor-Neuron-like Cell Line NSC34-G93A, a Cellular Model for ALS

Mitochondrial defects in motor neurons are pathological hallmarks of ALS, a neuromuscular disease with no effective treatment. Studies have shown that butyrate, a natural gut-bacteria product, alleviates the disease progression of ALS mice overexpressing a human ALS-associated mutation, hSOD1^G93A. In the current study, we examined the potential molecular mechanisms underlying the effect of butyrate on mitochondrial function in cultured motor-neuron-like NSC34 with overexpression of hSOD1^G93A (NSC34-G93A). The live cell confocal imaging study demonstrated that 1mM butyrate in the culture medium improved the mitochondrial network with reduced fragmentation in NSC34-G93A cells. Seahorse analysis revealed that NSC34-G93A cells treated with butyrate showed an increase of ~5-fold in mitochondrial Spare Respiratory Capacity with elevated Maximal Respiration. The time-dependent changes in the mRNA level of PGC1α, a master regulator of mitochondrial biogenesis, revealed a burst induction with an early increase (~5-fold) at 4 h, a peak at 24 h (~19-fold), and maintenance at 48 h (8-fold) post-treatment. In line with the transcriptional induction of PGC1α, both the mRNA and protein levels of the key molecules (MTCO1, MTCO2, and COX4) related to the mitochondrial electron transport chain were increased following the butyrate treatment. Our data indicate that activation of the PGC1α signaling axis could be one of the molecular mechanisms underlying the beneficial effects of butyrate treatment in improving mitochondrial bioenergetics in NSC34-G93A cells.

Mitochondrial acute oxygen sensing and signaling

Oxygen (O₂) is essential for life and therefore the supply of sufficient O₂ to the tissues is a major physiological challenge. In mammals, a deficit of O₂ (hypoxia) triggers rapid cardiorespiratory reflexes (e.g. hyperventilation and increased heart output) that within a few seconds increase the uptake of O₂ by the lungs and its distribution throughout the body. The prototypical acute O₂-sensing organ is the carotid body (CB), which contains sensory glomus cells expressing O₂-regulated ion channels. In response to hypoxia, glomus cells depolarize and release transmitters which activate afferent fibers terminating at the brainstem respiratory and autonomic centers. In this review, we summarize the basic properties of CB chemoreceptor cells and the essential role played by their specialized mitochondria in acute O₂ sensing and signaling. We focus on recent data supporting a "mitochondria-to-membrane signaling" model of CB chemosensory transduction. The possibility that the differential expression of specific subunit isoforms and enzymes could allow mitochondria to play a generalized adaptive O₂-sensing and signaling role in a wide variety of cells is also discussed.

Mitochondrial respiration is controlled by Allostery, Subunit Composition and Phosphorylation Sites of Cytochrome c Oxidase: A trailblazer's tale - Bernhard Kadenbach

In memoriam of Bernhard Kadenbach: Although the main focus of his research was the structure, function, and regulation of mitochondrial cytochrome c oxidase (CytOx), he earlier studied the mitochondrial phosphate carrier and found an essential role of cardiolipin. Later, he discovered tissue-specific and developmental-specific protein isoforms of CytOx. Defective activity of CytOx is found with increasing age in human muscle and neuronal cells resulting in mitochondrial diseases. Kadenbach proposed a theory on the cause of oxidative stress, aging, and associated diseases stating that allosteric feedback inhibition of CytOx at high mitochondrial ATP/ADP ratios is essential for healthy living while stress-induced reversible dephosphorylation of CytOx results in the formation of excessive reactive oxygen species that trigger degenerative diseases. This article summarizes the main discoveries of Kadenbach related to mammalian CytOx and discusses their implications for human disease.

Structure and Mechanism of Respiratory III-IV Supercomplexes in Bioenergetic Membranes

In the final steps of energy conservation in aerobic organisms, free energy from electron transfer through the respiratory chain is transduced into a proton electrochemical gradient across a membrane. In mitochondria and many bacteria, reduction of the dioxygen electron acceptor is catalyzed by cytochrome c oxidase (complex IV), which receives electrons from cytochrome bc1 (complex III), via membrane-bound or water-soluble cytochrome c. These complexes function independently, but in many organisms they associate to form supercomplexes. Here, we review the structural features and the functional significance of the nonobligate III2IV1/2 Saccharomyces cerevisiae mitochondrial supercomplex as well as the obligate III2IV2 supercomplex from actinobacteria. The analysis is centered around the Q-cycle of complex III, proton uptake by CytcO, as well as mechanistic and structural solutions to the electronic link between complexes III and IV.

Cholate Disrupts Regulatory Functions of Cytochrome c Oxidase

Cytochrome c oxidase (CytOx), the oxygen-accepting and rate-limiting enzyme of mitochondrial respiration, binds with 10 molecules of ADP, 7 of which are exchanged by ATP at high ATP/ADP-ratios. These bound ATP and ADP can be exchanged by cholate, which is generally used for the purification of CytOx. Many crystal structures of isolated CytOx were performed with the enzyme isolated from mitochondria using sodium cholate as a detergent. Cholate, however, dimerizes the enzyme isolated in non-ionic detergents and induces a structural change as evident from a spectral change. Consequently, it turns off the "allosteric ATP-inhibition of CytOx", which is reversibly switched on under relaxed conditions via cAMP-dependent phosphorylation and keeps the membrane potential and ROS formation in mitochondria at low levels. This cholate effect gives an insight into the structural-functional relationship of the enzyme with respect to ATP inhibition and its role in mitochondrial respiration and energy production.

Cytochrome c Oxidase at Full Thrust: Regulation and Biological Consequences to Flying Insects

Flight dispersal represents a key aspect of the evolutionary and ecological success of insects, allowing escape from predators, mating, and colonization of new niches. The huge energy demand posed by flight activity is essentially met by oxidative phosphorylation (OXPHOS) in flight muscle mitochondria. In insects, mitochondrial ATP supply and oxidant production are regulated by several factors, including the energy demand exerted by changes in adenylate balance. Indeed, adenylate directly regulates OXPHOS by targeting both chemiosmotic ATP production and the activities of specific mitochondrial enzymes. In several organisms, cytochrome c oxidase (COX) is regulated at transcriptional, post-translational, and allosteric levels, impacting mitochondrial energy metabolism, and redox balance. This review will present the concepts on how COX function contributes to flying insect biology, focusing on the existing examples in the literature where its structure and activity are regulated not only by physiological and environmental factors but also how changes in its activity impacts insect biology. We also performed in silico sequence analyses and determined the structure models of three COX subunits (IV, VIa, and VIc) from different insect species to compare with mammalian orthologs. We observed that the sequences and structure models of COXIV, COXVIa, and COXVIc were quite similar to their mammalian counterparts. Remarkably, specific substitutions to phosphomimetic amino acids at critical phosphorylation sites emerge as hallmarks on insect COX sequences, suggesting a new regulatory mechanism of COX activity. Therefore, by providing a physiological and bioenergetic framework of COX regulation in such metabolically extreme models, we hope to expand the knowledge of this critical enzyme complex and the potential consequences for insect dispersal.

COXIV and SIRT2-mediated G6PD deacetylation modulate ROS homeostasis to extend pupal lifespan

Previous studies have shown that high physiological levels of reactive oxygen species (ROS) in the brain promote pupal diapause, which extends the pupal lifespan. However, the molecular mechanisms of ROS generation are unclear. In this paper, we found that mitochondrial ROS (mtROS) levels in the brains of Helicoverpa armigera diapause-destined pupae (DP) were higher and that the expression of cytochrome oxidase subunit IV (COXIV) was lower than in NP. In addition, downregulating COXIV caused mitochondrial dysfunction which elevated mtROS levels. Protein kinase A (PKA) was downregulated in DP, which led to the downregulated expression of the mitochondrial transcription factor TFAM. Low TFAM activity failed to promote COXIV expression and resulted in the high ROS levels that induced diapause. In addition, low sirtuin 2 expression suppressed glucose-6-phosphate dehydrogenase (G6PD) deacetylation at K382, which led to reduced G6PD activity and low NADPH levels, thereby maintaining high levels of ROS. Two proteins, COXIV and G6PD, thus play key roles in the elevated accumulation of ROS that induce diapause and extend the pupal lifespan.

Complex IV - The regulatory center of mitochondrial oxidative phosphorylation

ATP, the universal energy currency in all living cells, is mainly synthesized in mitochondria by oxidative phosphorylation (OXPHOS). The final and rate limiting step of the respiratory chain is cytochrome c oxidase (COX) which represents the regulatory center of OXPHOS. COX is regulated through binding of various effectors to its "supernumerary" subunits, by reversible phosphorylation, and by expression of subunit isoforms. Of particular interest is its feedback inhibition by ATP, the final product of OXPHOS. This "allosteric ATP-inhibition" of phosphorylated and dimeric COX maintains a low and healthy mitochondrial membrane potential (relaxed state), and prevents the formation of ROS (reactive oxygen species) which are known to cause numerous diseases. Excessive work and stress abolish this feedback inhibition of COX by Ca²⁺-activated dephosphorylation which leads to monomerization and movement of NDUFA4 from complex I to COX with higher rates of COX activity and ATP synthesis (active state) but increased ROS formation and decreased efficiency.

Regulation of cytochrome c oxidase contributes to health and optimal life

The generation of cellular energy in the form of ATP occurs mainly in mitochondria by oxidative phosphorylation. Cytochrome c oxidase (CytOx), the oxygen accepting and rate-limiting step of the respiratory chain, regulates the supply of variable ATP demands in cells by “allosteric ATP-inhibition of CytOx.” This mechanism is based on inhibition of oxygen uptake of CytOx at high ATP/ADP ratios and low ferrocytochrome c concentrations in the mitochondrial matrix via cooperative interaction of the two substrate binding sites in dimeric CytOx. The mechanism keeps mitochondrial membrane potential ΔΨ_m and reactive oxygen species (ROS) formation at low healthy values. Stress signals increase cytosolic calcium leading to Ca²⁺-dependent dephosphorylation of CytOx subunit I at the cytosolic side accompanied by switching off the allosteric ATP-inhibition and monomerization of CytOx. This is followed by increase of ΔΨ_m and formation of ROS. A hypothesis is presented suggesting a dynamic change of binding of NDUFA4, originally identified as a subunit of complex I, between monomeric CytOx (active state with high ΔΨ_m, high ROS and low efficiency) and complex I (resting state with low ΔΨ_m, low ROS and high efficiency).

Cytochrome C Oxidase Subunit 4 (COX4): A Potential Therapeutic Target for the Treatment of Medullary Thyroid Cancer

The nuclear-encoded subunit 4 of cytochrome c oxidase (COX4) plays a role in regulation of oxidative phosphorylation and contributes to cancer progression. We sought to determine the role of COX4 in differentiated (DTC) and medullary (MTC) thyroid cancers. We examined the expression of COX4 in human thyroid tumors by immunostaining and used shRNA-mediated knockdown of COX4 to evaluate its functional contributions in thyroid cancer cell lines. In human thyroid tissue, the expression of COX4 was higher in cancers than in either normal thyroid (p = 0.0001) or adenomas (p = 0.001). The level of COX4 expression correlated with tumor size (p = 0.04) and lymph-node metastases (p = 0.024) in patients with MTCs. COX4 silencing had no effects on cell signaling activation and mitochondrial respiration in DTC cell lines (FTC133 and BCPAP). In MTC-derived TT cells, COX4 silencing inhibited p70S6K/pS6 and p-ERK signaling, and was associated with decreased oxygen consumption and ATP production. Treatment with potassium cyanide had minimal effects on FTC133 and BCPAP, but inhibited mitochondrial respiration and induced apoptosis in MTC-derived TT cells. Our data demonstrated that metastatic MTCs are characterized by increased expression of COX4, and MTC-derived TT cells are vulnerable to COX4 silencing. These data suggest that COX4 can be considered as a novel molecular target for the treatment of MTC.

Stress-mediated generation of deleterious ROS in healthy individuals - role of cytochrome c oxidase

Psychosocial stress is known to cause an increased incidence of coronary heart disease. In addition, multiple other diseases like cancer and diabetes mellitus have been related to stress and are mainly based on excessive formation of reactive oxygen species (ROS) in mitochondria. The molecular interactions between stress and ROS, however, are still unknown. Here we describe the missing molecular link between stress and an increased cellular ROS, based on the regulation of cytochrome c oxidase (COX). In normal healthy cells, the "allosteric ATP inhibition of COX" decreases the oxygen uptake of mitochondria at high ATP/ADP ratios and keeps the mitochondrial membrane potential (ΔΨm) low. Above ΔΨm values of 140 mV, the production of ROS in mitochondria increases exponentially. Stress signals like hypoxia, stress hormones, and high glutamate or glucose in neurons increase the cytosolic Ca2+ concentration which activates a mitochondrial phosphatase that dephosphorylates COX. This dephosphorylated COX exhibits no allosteric ATP inhibition; consequently, an increase of ΔΨm and ROS formation takes place. The excess production of mitochondrial ROS causes apoptosis or multiple diseases.

The biology of Lonp1: More than a mitochondrial protease

Initially discovered as a protease responsible for degradation of misfolded or damaged proteins, the mitochondrial Lon protease (Lonp1) turned out to be a multifaceted enzyme, that displays at least three different functions (proteolysis, chaperone activity, binding of mtDNA) and that finely regulates several cellular processes, within and without mitochondria. Indeed, LONP1 in humans is ubiquitously expressed, and is involved in regulation of response to oxidative stress and, heat shock, in the maintenance of mtDNA, in the regulation of mitophagy. Furthermore, its proteolytic activity can regulate several biochemical pathways occurring totally or partially within mitochondria, such as TCA cycle, oxidative phosphorylation, steroid and heme biosynthesis and glutamine production. Because of these multiple activities, Lon protease is highly conserved throughout evolution, and mutations occurring in its gene determines severe diseases in humans, including a rare syndrome characterized by Cerebral, Ocular, Dental, Auricular and Skeletal anomalies (CODAS). Finally, alterations of LONP1 regulation in humans can favor tumor progression and aggressiveness, further highlighting the crucial role of this enzyme in mitochondrial and cellular homeostasis.

Mitochondrial glycerol phosphate oxidation is modulated by adenylates through allosteric regulation of cytochrome c oxidase activity in mosquito flight muscle

The huge energy demand posed by insect flight activity is met by an efficient oxidative phosphorylation process that takes place within flight muscle mitochondria. In the major arbovirus vector Aedes aegypti, mitochondrial oxidation of pyruvate, proline and glycerol 3-phosphate (G3P) represent the major energy sources of ATP to sustain flight muscle energy demand. Although adenylates exert critical regulatory effects on several mitochondrial enzyme activities, the potential consequences of altered adenylate levels to G3P oxidation remains to be determined. Here, we report that mitochondrial G3P oxidation is controlled by adenylates through allosteric regulation of cytochrome c oxidase (COX) activity in A. aegypti flight muscle. We observed that ADP significantly activated respiratory rates linked to G3P oxidation, in a protonmotive force-independent manner. Kinetic analyses revealed that ADP activates respiration through a slightly cooperative mechanism. Despite adenylates caused no effects on G3P-cytochrome c oxidoreductase activity, COX activity was allosterically activated by ADP. Conversely, ATP exerted powerful inhibitory effects on respiratory rates linked to G3P oxidation and on COX activity. We also observed that high energy phosphate recycling mechanisms did not contribute to the regulatory effects of adenylates on COX activity or G3P oxidation. We conclude that mitochondrial G3P oxidation in A. aegypti flight muscle is regulated by adenylates through the allosteric modulation of COX activity, underscoring the bioenergetic relevance of this novel mechanism and the potential consequences for mosquito dispersal.

Reversible dimerization of cytochrome c oxidase regulates mitochondrial respiration

Almost all energy consumed by higher organisms, either in the form of ATP or heat, is produced in mitochondria by respiration and oxidative phosphorylation through five protein complexes in the inner membrane. High-resolution x-ray analysis of crystallized cytochrome c oxidase (CytOx), the final oxygen-accepting complex of the respiratory chain, isolated by using cholate as detergent, revealed a dimeric structure with 13 subunits in each monomer. In contrast, CytOx isolated with non-ionic detergents appeared to be monomeric. Our data indicate in vivo a continuous transition between CytOx monomers and dimers via reversible phosphorylation. Increased intracellular calcium, as a consequence of stress, dephosphorylates and monomerises CytOx, whereas cAMP rephosphorylates and dimerises it. Only dimeric CytOx exhibits an "allosteric ATP-inhibition" which inhibits respiration at high cellular ATP/ADP-ratios and could prevent oxygen radical formation and the generation of diseases.

Myocardial insufficiency is related to reduced subunit 4 content of cytochrome c oxidase

BACKGROUND

Treatment of heart failure remains one of the most challenging task for intensive care medicine, cardiology and cardiac surgery. New options and better indicators are always required. Understanding the basic mechanisms underlying heart failure promote the development of adjusted therapy e.g. assist devices and monitoring of recovery. If cardiac failure is related to compromised cellular respiration of the heart, remains unclear. Myocardial respiration depends on Cytochrome c- Oxidase (CytOx) activity representing the rate limiting step for the mitochondrial respiratory chain. The enzymatic activity as well as mRNA expression of enzyme's mitochondrial encoded catalytic subunit 2, nuclear encoded regulatory subunit 4 and protein contents were studied in biopsies of cardiac patients suffering from myocardial insufficiency and dilated cardiomyopathy (DCM).

METHODS

Fifty-four patients were enrolled in the study and underwent coronary angiography. Thirty male patients (mean age: 45 +/- 15 yrs.) had a reduced ejection fraction (EF) 35 ± 12% below 45% and a left ventricular end diastolic diameter (LVEDD) of 71 ± 10 mm bigger than 56 mm. They were diagnosed as having idiopathic dilated cardiomyopathy (DCM) without coronary heart disease and NYHA-class 3 and 4. Additionally, 24 male patients (mean age: 52 +/- 11 yrs.) after exclusion of secondary cardiomyopathies, coronary artery or valve disease, served as control (EF: 68 ± 7, LVEDD: 51 ± 7 mm). Total RNA was extracted from two biopsies of each person. Real-time PCR analysis was performed with specific primers followed by a melt curve analysis. Corresponding protein expression in the tissue was studied with immune-histochemistry while enzymatic activity was evaluated by spectroscopy.

RESULTS

Gene and protein expression analysis of patients showed a significant decrease of subunit 4 (1.1 vs. 0.6, p < 0.001; 7.7 ± 3.1% vs. 2.8 ± 1.4%, p < 0.0001) but no differences in subunit 2. Correlations were found between reduced subunit 2 expression, low EF (r = 0.766, p < 0.00045) and increased LVEDD (r = 0.492, p < 0.0068). In case of DCM less subunit 4 expression and reduced shortening fraction (r = 0.524, p < 0.017) was found, but enzymatic activity was higher (0.08 ± 0.06 vs. 0.26 ± 0.08 U/mg, p < 0.001) although myocardial oxygen consumption continued to the same extent.

CONCLUSION

In case of myocardial insufficiency and DCM, decreased expression of COX 4 results in an impaired CytOx activity. Higher enzymatic activity but equal oxygen consumption contribute to the pathophysiology of the myocardial insufficiency and appears as an indicator of oxidative stress. This kind of dysregulation should be in the focus for the development of diagnostic and therapy procedures.

HIF-1α regulates COXIV subunits, a potential mechanism of self-protective response to microwave induced mitochondrial damages in neurons

Anxiety and speculation about potential health hazards of microwaves exposure are spreading in the past decades. Hypoxia-inducible factor-1α (HIF-1α), which can be activated by reactive oxygen species (ROS), played pivotal roles in protective responses against microwave in neuron-like cells. In this study, we established 30 mW/cm² microwave exposed animal model, which could result in revisable injuries of neuronal mitochondria, including ultrastructure and functions, such as ROS generation and cytochrome c oxidase (COX) activity. We found that the ratio of COXIV-1/COXIV-2, two isoforms of COXIV, decreased at 1 d and increased from 3 d to 14 d. Similar expression changes of HIF-1α suggested that COXIV-1 and COXIV-2 might be regulated by HIF-1α. In neuron-like cells, 30 mW/cm² microwave down-regulated COX activity from 30 min to 6 h, and then started to recover. And, both HIF-1α transcriptional activity and COXIV-1/COXIV-2 ratio were up-regulated at 6 h and 9 h after exposure. Moreover, HIF-1α inhibition down-regulated COXIV-1 expression, promoted ROS generation, impaired mitochondrial membrane potentials (MMP), as well as abolished microwave induced ATP production. In conclusion, microwave induced mitochondrial ROS production activated HIF-1α and regulated COXIV-1 expression to restore mitochondria functions. Therefore, HIF-1α might be a potential target to impair microwave induced injuries.

Comparative biochemistry of cytochrome c oxidase in animals

Cytochrome c oxidase (COX), the terminal enzyme of the electron transport system, is central to aerobic metabolism of animals. Many aspects of its structure and function are highly conserved, yet, paradoxically, it is also an important model for studying the evolution of the metabolic phenotype. In this review, part of a special issue honouring Peter Hochachka, we consider the biology of COX from the perspective of comparative and evolutionary biochemistry. The approach is to consider what is known about the enzyme in the context of conventional biochemistry, but focus on how evolutionary researchers have used this background to explore the role of the enzyme in biochemical adaptation of animals. In synthesizing the conventional and evolutionary biochemistry, we hope to identify synergies and future research opportunities. COX represents a rare opportunity for researchers to design studies that span the breadth of biology: molecular genetics, protein biochemistry, enzymology, metabolic physiology, organismal performance, evolutionary biology, and phylogeography.

Mitochondrial cytochrome c oxidase is inhibited by ATP only at very high ATP/ADP ratios

In the past, divergent results have been reported based on different methods and conditions used for enzymatic activity measurements of cytochrome c oxidase (CytOx). Here, we analyze in detail and show comparable and reproducible polarographic activity measurements of ATP-dependent inhibition of CytOx kinetics in intact and non-intact rat heart mitochondria and mitoplasts. We found that this mechanism is always present in isolated rat heart mitochondria and mitoplasts; however, it is measurable only at high ATP/ADP ratios using optimal protein concentrations. In the kinetics assay, measurement of this mechanism is independent of presence or absence of Tween-20 and the composition of measuring buffer. Furthermore, the effect of atractyloside on intact rat heart mitochondria confirms that (i) ATP inhibition occurs under uncoupled conditions [in the presence of carbonly cyanide m-chlorophenyl hydrazone (CCCP)] when the classical respiratory control is absent and (ii) high ATP/ADP ratios in the matrix as well as in the cytosolic space are required for full ATP inhibition of CytOx. Additionally, ATP inhibition measured in intact mitochondria extends in the presence of oligomycin, thus indicating further that the problem to measure the inhibitory effect of ATP on CytOx is apparently due to the lack of very high ATP/ADP ratios in isolated mitochondria.

Tissue- and Condition-Specific Isoforms of Mammalian Cytochrome c Oxidase Subunits: From Function to Human Disease

Cytochrome c oxidase (COX) is the terminal enzyme of the electron transport chain and catalyzes the transfer of electrons from cytochrome c to oxygen. COX consists of 14 subunits, three and eleven encoded, respectively, by the mitochondrial and nuclear DNA. Tissue- and condition-specific isoforms have only been reported for COX but not for the other oxidative phosphorylation complexes, suggesting a fundamental requirement to fine-tune and regulate the essentially irreversible reaction catalyzed by COX. This article briefly discusses the assembly of COX in mammals and then reviews the functions of the six nuclear-encoded COX subunits that are expressed as isoforms in specialized tissues including those of the liver, heart and skeletal muscle, lung, and testes: COX IV-1, COX IV-2, NDUFA4, NDUFA4L2, COX VIaL, COX VIaH, COX VIb-1, COX VIb-2, COX VIIaH, COX VIIaL, COX VIIaR, COX VIIIH/L, and COX VIII-3. We propose a model in which the isoforms mediate the interconnected regulation of COX by (1) adjusting basal enzyme activity to mitochondrial capacity of a given tissue; (2) allosteric regulation to adjust energy production to need; (3) altering proton pumping efficiency under certain conditions, contributing to thermogenesis; (4) providing a platform for tissue-specific signaling; (5) stabilizing the COX dimer; and (6) modulating supercomplex formation.

Subfunctionalization of COX4 paralogs in fish

Cytochrome c oxidase (COX) subunit 4 has two paralogs in most vertebrates. The mammalian COX4-2 gene is hypoxia responsive, and the protein has a disrupted ATP-binding site that confers kinetic properties on COX that distinguish it from COX4-1. The structure-function of COX4-2 orthologs in other vertebrates remains uncertain. Phylogenetic analyses suggest the two paralogs arose in basal vertebrates, but COX4-2 orthologs diverged faster than COX4-1 orthologs. COX4-1/4-2 protein levels in tilapia tracked mRNA levels across tissues, and did not change in hypoxia, arguing against a role for differential post-translational regulation of paralogs. The heart, and to a lesser extent the brain, showed a size-dependent shift from COX4-1 to COX4-2 (transcript and protein). ATP allosterically inhibited both velocity and affinity for oxygen in COX assayed from both muscle (predominantly COX4-2) and gill (predominantly COX4-1). We saw some evidence of cellular and subcellular discrimination of COX4 paralogs in heart. In cardiac ventricle, some non-cardiomyocyte cells were COX positive but lacked detectible COX4-2. Within heart, the two proteins partitioned to different mitochondrial subpopulations. Cardiac subsarcolemmal mitochondria had mostly COX4-1 and intermyofibrillar mitochondria had mostly COX4-2. Collectively, these data argue that, despite common evolutionary origins, COX4-2 orthologs of fish show unique patterns of subfunctionalization with respect to transcriptional and posttranslation regulation relative to the rodents and primates that have been studied to date.

Revisiting Kadenbach: Electron flux rate through cytochrome c-oxidase determines the ATP-inhibitory effect and subsequent production of ROS

Mitochondrial respiration is the predominant source of ATP. Excessive rates of electron transport cause a higher production of harmful reactive oxygen species (ROS). There are two regulatory mechanisms known. The first, according to Mitchel, is dependent on the mitochondrial membrane potential that drives ATP synthase for ATP production, and the second, the Kadenbach mechanism, is focussed on the binding of ATP to Cytochrome c Oxidase (CytOx) at high ATP/ADP ratios, which results in an allosteric conformational change to CytOx, causing inhibition. In times of stress, ATP-dependent inhibition is switched off and the activity of CytOx is exclusively determined by the membrane potential, leading to an increase in ROS production. The second mechanism for respiratory control depends on the quantity of electron transfer to the Heme aa3 of CytOx. When ATP is bound to CytOx the enzyme is inhibited, and ROS formation is decreased, although the mitochondrial membrane potential is increased.

Hypoxia Inducible Factor Pathway and Physiological Adaptation: A Cell Survival Pathway?

Oxygen homeostasis reflects the constant body requirement to generate energy. Hypoxia (0.1–1% O₂), physioxia or physoxia (∼1–13%), and normoxia (∼20%) are terms used to define oxygen concentration in the cellular environment. A decrease in oxygen (hypoxia) or excess oxygen (hyperoxia) could be deleterious for cellular adaptation and survival. Hypoxia can occur under both physiological (e.g., exercise, embryonic development, underwater diving, or high altitude) and pathological conditions (e.g., inflammation, solid tumor formation, lung disease, or myocardial infarction). Hypoxia plays a key role in the pathophysiology of heart disease, cancers, stroke, and other causes of mortality. Hypoxia inducible factor(s) (HIFs) are key oxygen sensors that mediate the ability of the cell to cope with decreased oxygen tension. These transcription factors regulate cellular adaptation to hypoxia and protect cells by responding acutely and inducing production of endogenous metabolites and proteins to promptly regulate metabolic pathways. Here, we review the role of the HIF pathway as a metabolic adaptation pathway and how this pathway plays a role in cell survival. We emphasize the roles of the HIF pathway in physiological adaptation, cell death, pH regulation, and adaptation during exercise.

The subunit composition and function of mammalian cytochrome c oxidase

Cytochrome c oxidase (COX) from mammals and birds is composed of 13 subunits. The three catalytic subunits I-III are encoded by mitochondrial DNA, the ten nuclear-coded subunits (IV, Va, Vb, VIa, VIb, VIc, VIIa, VIIb, VIIc, VIII) by nuclear DNA. The nuclear-coded subunits are essentially involved in the regulation of oxygen consumption and proton translocation by COX, since their removal or modification changes the activity and their mutation causes mitochondrial diseases. Respiration, the basis for ATP synthesis in mitochondria, is differently regulated in organs and species by expression of tissue-, developmental-, and species-specific isoforms for COX subunits IV, VIa, VIb, VIIa, VIIb, and VIII, but the holoenzyme in mammals is always composed of 13 subunits. Various proteins and enzymes were shown, e.g., by co-immunoprecipitation, to bind to specific COX subunits and modify its activity, but these interactions are reversible, in contrast to the tightly bound 13 subunits. In addition, the formation of supercomplexes with other oxidative phosphorylation complexes has been shown to be largely variable. The regulatory complexity of COX is increased by protein phosphorylation. Up to now 18 phosphorylation sites have been identified under in vivo conditions in mammals. However, only for a few phosphorylation sites and four nuclear-coded subunits could a specific function be identified. Research on the signaling pathways leading to specific COX phosphorylations remains a great challenge for understanding the regulation of respiration and ATP synthesis in mammalian organisms. This article reviews the function of the individual COX subunits and their isoforms, as well as proteins and small molecules interacting and regulating the enzyme.

The deafness gene DFNA5 induces programmed cell death through mitochondria and MAPK-related pathways

Cell death exists in many different forms. Some are accidental, but most of them have some kind of regulation and are called programmed cell death. Programmed cell death (PCD) is a very diverse and complex mechanism and must be tightly regulated. This study investigated PCD induced by DFNA5, a gene responsible for autosomal dominant hearing loss (HL) and a tumor suppressor gene (TSG) involved in frequent forms of cancer. Mutations in DFNA5 lead to exon 8 skipping and result in HL in several families. Expression of mutant DFNA5, a cDNA construct where exon 8 is deleted, was linked to PCD both in human cell lines and in Saccharomyces cerevisiae. To further investigate the cell death mechanism induced by mutant DFNA5, we performed a microarray study in both models. We used wild-type DFNA5, which does not induce cell death, as a reference. Our data showed that the yeast pathways related to mitochondrial ATP-coupled electron transport chain, oxidative phosphorylation and energy metabolism were up-regulated, while in human cell lines, MAP kinase-related activity was up-regulated. Inhibition of this pathway was able to partially attenuate the resulting cell death induced by mutant DFNA5 in human cell lines. In yeast, the association with mitochondria was demonstrated by up-regulation of several cytochrome c oxidase (COX) genes involved in the cellular oxidative stress production. Both models show a down-regulation of protein sorting- and folding-related mechanisms suggesting an additional role for the endoplasmic reticulum (ER). The exact relationship between ER and mitochondria in DFNA5-induced cell death remains unknown at this moment, but these results suggest a potential link between the two.

Comparisons of subunit 5A and 5B isoenzymes of yeast cytochrome c oxidase

Subunit 5 of Saccharomyces cerevisiae cytochrome c oxidase (CcO) is essential for assembly and has two isoforms, 5A and 5B. 5A is expressed under normoxic conditions, whereas 5B is expressed at very low oxygen tensions. As a consequence, COX5A-deleted strains (Δcox5A) have no or only low levels of CcO under normoxic conditions rendering them respiratory deficient. Previous studies have reported that respiratory growth could be restored by combining Δcox5A with mutations of ROX1 that encodes a repressor of COX5B expression. In these mutants, 5B isoenzyme expression level was 30–50% of wild-type (5A isoenzyme) and exhibited a maximum catalytic activity up to 3-fold faster than that of 5A isoenzyme. To investigate the origin of this effect, we constructed a mutant strain in which COX5B replaced COX5A downstream of the COX5A promoter. This strain expressed wild-type levels of the 5B isoenzyme, without the complication of additional effects caused by mutation of ROX1. When produced this way, the isoenzymes displayed no significant differences in their maximum catalytic activities or in their affinities for oxygen or cytochrome c. Hence the elevated activity of the 5B isoenzyme in the rox1 mutant is not caused simply by exchange of isoforms and must arise from an additional effect that remains to be resolved.

Yeast cytochrome c oxidase isoenzymes with subunits 5A or 5B can exhibit different kinetic properties. We demonstrate that these differences are not induced by simple 5A/5B replacement and that the effect additionally requires expression level changes and/or undefined post-translational effects.

Evolution of the oxygen sensitivity of cytochrome c oxidase subunit 4

Vertebrates possess two paralogs of cytochrome c oxidase (COX) subunit 4: a ubiquitous COX4-1 and a hypoxia-linked COX4-2. Mammalian COX4-2 is thought to have a role in relation to fine-tuning metabolism in low oxygen levels, conferred through both structural differences in the subunit protein structure and regulatory differences in the gene. We sought to elucidate the pervasiveness of this feature across vertebrates. The ratio of COX4-2/4-1 mRNA is generally low in mammals, but this ratio was higher in fish and reptiles, particularly turtles. The COX4-2 gene appeared unresponsive to low oxygen in nonmammalian models (zebrafish, goldfish, tilapia, anoles, and turtles) and fish cell lines. Reporter genes constructed from the amphibian and reptile homologues of the mammalian oxygen-responsive elements and hypoxia-responsive elements did not respond to low oxygen. Unlike the rodent ortholog, the promoter of goldfish COX4-2 did not respond to hypoxia or anoxia. The protein sequences of the COX4-2 peptide showed that the disulfide bridge seen in human and rodent orthologs would be precluded in other mammalian lineages and lower vertebrates, all of which lack the requisite pair of cysteines. The coordinating ligands of the ATP-binding site are largely conserved across mammals and reptiles, but in Xenopus and fish, sequence variations may disrupt the ability of the protein to bind ATP at this site. Collectively, these results suggest that many of the genetic and structural features of COX4-2 that impart responsiveness and benefits in hypoxia may be restricted to the Euarchontoglires lineage that includes primates, lagomorphs, and rodents.

Control mechanisms in mitochondrial oxidative phosphorylation

Distribution and activity of mitochondria are key factors in neuronal development, synaptic plasticity and axogenesis. The majority of energy sources, necessary for cellular functions, originate from oxidative phosphorylation located in the inner mitochondrial membrane. The adenosine-5’- triphosphate production is regulated by many control mechanism–firstly by oxygen, substrate level, adenosine-5’-diphosphate level, mitochondrial membrane potential, and rate of coupling and proton leak. Recently, these mechanisms have been implemented by “second control mechanisms,” such as reversible phosphorylation of the tricarboxylic acid cycle enzymes and electron transport chain complexes, allosteric inhibition of cytochrome c oxidase, thyroid hormones, effects of fatty acids and uncoupling proteins. Impaired function of mitochondria is implicated in many diseases ranging from mitochondrial myopathies to bipolar disorder and schizophrenia. Mitochondrial dysfunctions are usually related to the ability of mitochondria to generate adenosine-5’-triphosphate in response to energy demands. Large amounts of reactive oxygen species are released by defective mitochondria, similarly, decline of antioxidative enzyme activities (e.g. in the elderly) enhances reactive oxygen species production. We reviewed data concerning neuroplasticity, physiology, and control of mitochondrial oxidative phosphorylation and reactive oxygen species production.

Energy metabolism and cytochrome oxidase activity: linking metabolism to gene expression

Modification of mitochondrial content demands the synthesis of hundreds of proteins encoded by nuclear and mitochondrial genomes. The responsibility for coordination of this process falls to nuclear-encoded master regulators of transcription. DNA-binding proteins and coactivators integrate information from energy-sensing pathways and hormones to alter mitochondrial gene expression. In mammals, the signaling cascade for mitochondrial biogenesis can be described as follows: hormonal signals and energetic information are sensed by protein-modifying enzymes that in turn regulate the post-translational modification of transcription factors. Once activated, transcription-factor complexes form on promoter elements of many of the nuclear-encoded mitochondrial genes, recruiting proteins that remodel chromatin and initiate transcription. One master regulator in mammals, PGC-1α, is well studied because of its role in determining the metabolic phenotype of muscles, but also due to its importance in mitochondria-related metabolic diseases. However, relatively little is known about the role of this pathway in other vertebrates. These uncertainties raise broader questions about the evolutionary origins of the pathway and its role in generating the diversity in muscle metabolic phenotypes seen in nature.

Decrease in cytochrome c oxidase reserve capacity diminishes robustness of Drosophila melanogaster and shortens lifespan

The phenotypic effects of under- and over-expression of CcO (cytochrome c oxidase) regulatory subunits IV and Vb were examined in Drosophila melanogaster in order to test further the hypothesis that suppression of the activities of mitochondrial ETC (electron-transport chain) oxidoreductases retards the aging process and extends lifespan. Underexpression of both CcO subunits, induced by RNAi, resulted in decreases in the respective mRNA and protein levels, CcO holoenzyme activity, rate of mitochondrial respiration, walking speed and the lifespan of fruitflies. Overexpression of CcO IV or Vb in young fruitflies increased the amount of mRNA, but had no effect on the protein level or CcO catalytic activity. On the other hand, in older fruitflies, overexpression of CcO Vb, but not CcO IV, elevated the mRNA and protein amounts as well as the CcO holoenzyme activity, thereby preventing the typical age-related decline in CcO activity. Nevertheless, lifespans of the fruitflies overexpressing CcO IV or Vb were neither extended nor shortened. Our results demonstrate that: (i) the suppression of CcO function exerts deleterious rather than benign effects on fitness and survival, and (ii) the structure/function of CcO, an ETC oxidoreductase, can be 're-engineered' in vivo.

Prenatal Hypoxia Reduces Mitochondrial Protein Levels and Cytochrome c Oxidase Activity in Offspring Guinea Pig Hearts

Prenatal hypoxia (HPX) reduces mitochondrial cytochrome c oxidase (CCO and COX) activity in fetal guinea pig (GP) hearts. The aim of this study was to quantify the lasting effects of chronic prenatal HPX on cardiac mitochondrial enzyme activity and protein expression in offspring hearts. Pregnant GPs were exposed to either normoxia (NMX) or HPX (10.5%O₂) during the last 14 days of pregnancy. Both NMX and HPX fetuses, delivered vaginally, were housed under NMX conditions until 90 days of age. Total RNA and mitochondrial fractions were isolated from hearts of anesthetized NMX and HPX offspring and showed decreased levels of CCO but not medium-chain acyl dehydrogenase activity, protein levels of nuclear- and mitochondrial-encoded COX4 and COX1, respectively, and messenger RNA expression of peroxisome proliferator-activated receptor gamma coactivator 1-alpha, COX5b, and 4.1 compared to NMX controls. Prenatal HPX may alter mitochondrial function in the offspring by disrupting protein expression associated with the respiratory chain.

Regulation of cellular gas exchange, oxygen sensing, and metabolic control

Cells must continuously monitor and couple their metabolic requirements for ATP utilization with their ability to take up O2 for mitochondrial respiration. When O2 uptake and delivery move out of homeostasis, cells have elaborate and diverse sensing and response systems to compensate. In this review, we explore the biophysics of O2 and gas diffusion in the cell, how intracellular O2 is regulated, how intracellular O2 levels are sensed and how sensing systems impact mitochondrial respiration and shifts in metabolic pathways. Particular attention is paid to how O2 affects the redox state of the cell, as well as the NO, H2S, and CO concentrations. We also explore how these agents can affect various aspects of gas exchange and activate acute signaling pathways that promote survival. Two kinds of challenges to gas exchange are also discussed in detail: when insufficient O2 is available for respiration (hypoxia) and when metabolic requirements test the limits of gas exchange (exercising skeletal muscle). This review also focuses on responses to acute hypoxia in the context of the original "unifying theory of hypoxia tolerance" as expressed by Hochachka and colleagues. It includes discourse on the regulation of mitochondrial electron transport, metabolic suppression, shifts in metabolic pathways, and recruitment of cell survival pathways preventing collapse of membrane potential and nuclear apoptosis. Regarding exercise, the issues discussed relate to the O2 sensitivity of metabolic rate, O2 kinetics in exercise, and influences of available O2 on glycolysis and lactate production.

Impaired mitochondrial function in chronically ischemic human heart

Chronic ischemic heart disease is associated with myocardial hypoperfusion. The resulting hypoxia potentially inflicts damage upon the mitochondria, leading to a compromised energetic state. Furthermore, ischemic damage may cause excessive production of reactive oxygen species (ROS), producing mitochondrial damage, hereby reinforcing a vicious circle. Ischemic preconditioning has been proven protective in acute ischemia, but the subject of chronic ischemic preconditioning has not been explored in humans. We hypothesized that mitochondrial respiratory capacity would be diminished in chronic ischemic regions of human myocardium but that these mitochondria would be more resistant to ex vivo ischemia and, second, that ROS generation would be higher in ischemic myocardium. The aim of this study was to test mitochondrial respiratory capacity during hyperoxia and hypoxia, to investigate ROS production, and finally to assess myocardial antioxidant levels. Mitochondrial respiration in biopsies from ischemic and nonischemic regions from the left ventricle of the same heart was compared in nine human subjects. Maximal oxidative phosphorylation capacity in fresh muscle fibers was lower in ischemic compared with nonischemic myocardium (P < 0.05), but the degree of coupling (respiratory control ratio) did not differ (P > 0.05). The presence of ex vivo hypoxia did not reveal any chronic ischemic preconditioning of the ischemic myocardial regions (P > 0.05). ROS production was higher in ischemic myocardium (P < 0.05), and the levels of antioxidant protein expression was lower. Diminished mitochondrial respiration capacity and excessive ROS production demonstrate an impaired mitochondrial function in ischemic human heart muscle. No chronic ischemic preconditioning effect was found.

Oxygen-dependent expression of cytochrome c oxidase subunit 4-2 gene expression is mediated by transcription factors RBPJ, CXXC5 and CHCHD2

Cytochrome c oxidase (COX) is the terminal enzyme of the electron transport chain, made up of 13 subunits encoded by both mitochondrial and nuclear DNA. Subunit 4 (COX4), a key regulatory subunit, exists as two isoforms, the ubiquitous isoform 1 and the tissue-specific (predominantly lung) isoform 2 (COX4I2). COX4I2 renders lung COX about 2-fold more active compared with liver COX, which lacks COX4I2. We previously identified a highly conserved 13-bp sequence in the proximal promoter of COX4I2 that functions as an oxygen responsive element (ORE), maximally active at a 4% oxygen concentration. Here, we have identified three transcription factors that bind this conserved ORE, namely recombination signal sequence–binding protein Jκ (RBPJ), coiled-coil-helix-coiled-coil-helix domain 2 (CHCHD2) and CXXC finger protein 5 (CXXC5). We demonstrate that RBPJ and CHCHD2 function towards activating the ORE at 4% oxygen, whereas CXXC5 functions as an inhibitor. To validate results derived from cultured cells, we show using RNA interference a similar effect of these transcription factors in the gene regulation of COX4I2 in primary pulmonary arterial smooth muscle cells. Depending on the oxygen tension, a concerted action of the three transcription factors regulates the expression of COX4I2 that, as we discuss, could augment both COX activity and its ability to cope with altered cellular energy requirements.

High efficiency versus maximal performance--the cause of oxidative stress in eukaryotes: a hypothesis

Degenerative diseases are in part based on elevated production of ROS (reactive oxygen species) in mitochondria, mainly during stress and excessive work under stress (strenuous exercise). The production of ROS increases with increasing mitochondrial membrane potential (ΔΨ(m)). A mechanism is described which is suggested to keep ΔΨ(m) at low values under normal conditions thus preventing ROS formation, but is switched off under stress and excessive work to maximize the rate of ATP synthesis, accompanied by decreased efficiency. Low ΔΨ(m) and low ROS production are suggested to occur by inhibition of respiration at high [ATP]/[ADP] ratios. The nucleotides interact with phosphorylated cytochrome c oxidase (COX), representing the step with the highest flux-control coefficient of mitochondrial respiration. At stress and excessive work neural signals are suggested to dephosphorylate the enzyme and abolish the control of COX activity (respiration) by the [ATP]/[ADP] ratio with consequent increase of ΔΨ(m) and ROS production. The control of COX by the [ATP]/[ADP] ratio, in addition, is proposed to increase the efficiency of ATP production via a third proton pumping pathway, identified in eukaryotic but not in prokaryotic COX. We conclude that 'oxidative stress' occurs when the control of COX activity by the [ATP]/[ADP] ratio is switched off via neural signals.

Cytochrome c oxidase dysfunction in oxidative stress

Cytochrome c oxidase (CcO) is the terminal oxidase of the mitochondrial electron transport chain. This bigenomic enzyme in mammals contains 13 subunits of which the 3 catalytic subunits are encoded by the mitochondrial genes. The remaining 10 subunits with suspected roles in the regulation, and/or assembly, are coded by the nuclear genome. The enzyme contains two heme groups (heme a and a3) and two Cu(2+) centers (Cu(2+) A and Cu(2+) B) as catalytic centers and handles more than 90% of molecular O(2) respired by the mammalian cells and tissues. CcO is a highly regulated enzyme which is believed to be the pacesetter for mitochondrial oxidative metabolism and ATP synthesis. The structure and function of the enzyme are affected in a wide variety of diseases including cancer, neurodegenerative diseases, myocardial ischemia/reperfusion, bone and skeletal diseases, and diabetes. Despite handling a high O(2) load the role of CcO in the production of reactive oxygen species still remains a subject of debate. However, a volume of evidence suggests that CcO dysfunction is invariably associated with increased mitochondrial reactive oxygen species production and cellular toxicity. In this paper we review the literature on mechanisms of multimodal regulation of CcO activity by a wide spectrum of physiological and pathological factors. We also review an array of literature on the direct or indirect roles of CcO in reactive oxygen species production.

Cytochrome c oxidase and its role in neurodegeneration and neuroprotection

A hallmark of neurodegenerative diseases, such as Alzheimer's and Parkinson's diseases, and stroke is a malfunction of mitochondria including cytochrome c oxidase (COX), the terminal enzyme complex of the respiratory chain. COX is ascribed a key role based on mainly two regulatory mechanisms. These are the expression of isoforms and the binding of specific allosteric factors to nucleus--encoded subunits. These characteristics represent a unique feature of COX compared with the other respiratory chain complexes. Additional regulatory mechanisms, such as posttranslational modification, substrate availability, and allosteric feedback inhibition by products of the COX reaction, control the enzyme activity in a complex way. In many tissues and cell types, COX represents the rate-limiting enzyme of the respiratory chain which further emphasizes the impact of the regulation of COX as a central site for regulating energy metabolism and oxidative stress. Two of the best-analyzed regulatory mechanisms of COX to date are the allosteric feedback inhibition of the enzyme by its indirect product ATP and the expression of COX subunit IV isoforms. This ATP feedback inhibition of COX requires the expression of COX isoform IV-1. At high ATP/ADP ratios, ADP is exchanged for ATP at the matrix side of COX IV-1 leading to an inhibition of COX activity, thus enabling COX to sense the energy level and to adjust ATP synthesis to energy demand. However, under hypoxic, toxic, and degenerative conditions, COX isoform IV-2 expression is up-regulated and exchanged for COX IV-1 in the enzyme complex. This COX IV isoform switch causes an abolition of the allosteric ATP feedback inhibition of COX and consequently the loss of sensing the energy level. Thus, COX activity is increased leading to higher levels of ATP in neural cells independently of the cellular energy level. Concomitantly, ROS production is increased. Thus, under pathological conditions, neural cells are provided with ATP to meet the energy demand, but at the expense of elevated oxidative stress. This mechanism explains the functional relevance of COX subunit IV isoform expression for cellular energy sensing, ATP production, and oxidative stress levels. This, in turn, affects neural cell function, signaling, and -survival. Thus, COX is a crucial factor in etiology, progression, and prevalence of numerous human neurodegenerative diseases and represents an important target for developing diagnostic and therapeutic tools against those diseases.

Individual biochemical behaviour versus biological robustness: spotlight on the regulation of cytochrome c oxidase

During evolution from prokaryotes to eukaryotes, the main function of cytochrome c oxidase (COX), i.e., the coupling of oxygen reduction to proton translocation without the production of ROS (reactive oxygen species) remained unchanged demonstrating its robustness. A new regulation of respiration by the ATP/ADP ratio was introduced in eukaryotes based on nucleotide interaction with the added COX subunit IV. This allosteric ATP-inhibition was proposed to keep the mitochondrial membrane potential (ΔΨ(m)) at low healthy values and thus prevents the formation of ROS at complexes I and III. ROS have been implicated in various degenerative diseases. The allosteric ATP-inhibition of COX is reversibly switched on and off by phosphorylation of COX at a serine or threonine. In more than 100 individual preparations of rat heart and liver mitochondria, prepared under identical conditions, the extent of allosteric ATP-inhibition varied. This variability correlates with the variable inhibition of uncoupled respiration in intact isolated mitochondria by ATP. It is concluded that in higher organisms the allosteric ATP-inhibition is continually switched on and off by neuronal signalling in order to change oxidative phosphorylation from optimal efficiency with lower rate of ATP synthesis under resting conditions (low ΔΨ(m) and ROS production) to maximal rate of ATP synthesis under active (working, stress) conditions (elevated ΔΨ(m) and ROS production).

Introduction to mitochondrial oxidative phosphorylation

The basic mechanism of ATP synthesis in the mitochondria by oxidative phosphorylation (OxPhos) was revealed in the second half of the twentieth century. The OxPhos complexes I-V have been analyzed concerning their subunit composition, genes, and X-ray structures. This book presents new developments regarding the morphology, biogenesis, gene evolution, heat, and reactive oxygen species (ROS) generation in mitochondria, as well as the structure and supercomplex formation of OxPhos complexes. In addition, multiple mitochondrial diseases based on mutations of nuclear-encoded genes have been identified. Little is known, however, of the regulation of OxPhos according to the variable cellular demands of ATP. In particular, the functions of the supernumerary (nuclear-encoded) subunits of mitochondrial OxPhos complexes, which are mostly absent in bacteria, remain largely unknown, although the corresponding and conserved core subunits exhibit the same catalytic activity. Identification of regulatory pathways modulating OxPhos activity, by subunit isoform expression, by allosteric interaction with ATP/ADP, by reversible phosphorylation of protein subunits, or by supercomplex formation, will help to understand the role of mitochondria in the many degenerative diseases, mostly based on ROS formation in mitochondria and/or insufficient energy production.

Protein phosphorylation and prevention of cytochrome oxidase inhibition by ATP: coupled mechanisms of energy metabolism regulation

Rapid regulation of oxidative phosphorylation is crucial for mitochondrial adaptation to swift changes in fuels availability and energy demands. An intramitochondrial signaling pathway regulates cytochrome oxidase (COX), the terminal enzyme of the respiratory chain, through reversible phosphorylation. We find that PKA-mediated phosphorylation of a COX subunit dictates mammalian mitochondrial energy fluxes and identify the specific residue (S58) of COX subunit IV-1 (COXIV-1) that is involved in this mechanism of metabolic regulation. Using protein mutagenesis, molecular dynamics simulations, and induced fit docking, we show that mitochondrial energy metabolism regulation by phosphorylation of COXIV-1 is coupled with prevention of COX allosteric inhibition by ATP. This regulatory mechanism is essential for efficient oxidative metabolism and cell survival. We propose that S58 COXIV-1 phosphorylation has evolved as a metabolic switch that allows mammalian mitochondria to rapidly toggle between energy utilization and energy storage.

The power of life--cytochrome c oxidase takes center stage in metabolic control, cell signalling and survival

Mitochondrial dysfunction is increasingly recognized as a major factor in the etiology and progression of numerous human diseases, such as (neuro-)degeneration, ischemia reperfusion injury, cancer, and diabetes. Cytochrome c oxidase (COX) represents the rate-limiting enzyme of the mitochondrial respiratory chain and is thus predestined for being a central site of regulation of oxidative phosphorylation, proton pumping efficiency, ATP and reactive oxygen species production, which in turn affect cell signaling and survival. A unique feature of COX is its regulation by various factors and mechanisms interacting with the nucleus-encoded subunits, whose actual functions we are only beginning to understand.

Variation of flux control coefficient of cytochrome c oxidase and of the other respiratory chain complexes at different values of protonmotive force occurs by a threshold mechanism

The metabolic control analysis was applied to digitonin-permeabilized HepG2 cell line to assess the flux control exerted by cytochrome c oxidase on the mitochondrial respiration. Experimental conditions eliciting different energy/respiratory states in mitochondria were settled. The results obtained show that the mitochondrial electrochemical potential accompanies a depressing effect on the control coefficient exhibited by the cytochrome c oxidase. Both the components of the protonmotive force, i.e. the voltage (ΔΨ(m)) and the proton (ΔpH(m)) gradient, displayed a similar effect. Quantitative estimation of the ΔΨ(m) unveiled that the voltage-dependent effect on the control coefficient of cytochrome c oxidase takes place sharply in a narrow range of membrane potential from 170-180 to 200-210mV consistent with the physiologic transition from state 3 to state 4 of respiration. Extension of the metabolic flux control analysis to the NADH dehydrogenase and bc(1) complexes of the mitochondrial respiratory chain resulted in a similar effect. A mechanistic model is put forward whereby the respiratory chain complexes are proposed to exist in a voltage-mediated threshold-controlled dynamic equilibrium between supercomplexed and isolated states.

The effect of 5-aminolevulinic acid on cytochrome c oxidase activity in mouse liver

Background

5-Aminolevulinic acid (ALA) is a precursor of heme that is fundamentally important in aerobic energy metabolism. Among the enzymes involved in aerobic energy metabolism, cytochrome c oxidase (COX) is crucial. In this study, the effect of ALA on cytochrome c oxidase activity was measured.

Findings

c57BL/6N species of mice were administered ALA orally for 15 weeks. After ALA administration, mice were sacrificed and livers were obtained. COX activity in mitochondria from ALA-administered mouse livers was 1.5-fold higher than that in mitochondria from PBS-administered mouse livers (P < 0.05). Furthermore, ATP levels in ALA-administered mouse livers were much higher than those in PBS-administered mouse livers. These data suggest that oral administration of ALA promotes aerobic energy metabolism, especially COX activity.

Conclusions

This is the first report of a drug that functions in aerobic energy metabolism directly. Since COX activity is decreased in various diseases and aging, the pharmacological effects of ALA will be expanding.