Zinc ions as cytochrome C oxidase inhibitors: two sites of action

Investigation of the Mechanism of Membrane Potential Generation by Heme-Copper Respiratory Oxidases in a Real Time Mode

Heme-copper respiratory oxidases are highly efficient molecular machines. These membrane enzymes catalyze the final step of cellular respiration in eukaryotes and many prokaryotes: the transfer of electrons from cytochromes or quinols to molecular oxygen and oxygen reduction to water. The free energy released in this redox reaction is converted by heme-copper respiratory oxidases into the transmembrane gradient of the electrochemical potential of hydrogen ions H+). Heme-copper respiratory oxidases have a unique mechanism for generating H+, namely, a redox-coupled proton pump. A combination of direct electrometric method for measuring the kinetics of membrane potential generation with the methods of prestationary kinetics and site-directed mutagenesis in the studies of heme-copper oxidases allows to obtain a unique information on the translocation of protons inside the proteins in real time. The review summarizes the data of studies employing time-resolved electrometry to decipher the mechanisms of functioning of these important bioenergetic enzymes.

Regulating Photosensitizer Metabolism with DNAzyme-Loaded Nanoparticles for Amplified Mitochondria-Targeting Photodynamic Immunotherapy

Mitochondria-specific photosensitizer accumulation is highly recommended for photodynamic therapy and mitochondrial DNA (mtDNA) oxidative damage-based innate immunotherapy but remains challenging. 5-Aminolevulinic acid (ALA), precursor of photosensitizer protoporphyrin IX (PpIX), can induce the exclusive biosynthesis of PpIX in mitochondria. Nevertheless, its photodynamic effect is limited by the intracellular biotransformation of ALA in tumors. Here, we report a photosensitizer metabolism-regulating strategy using ALA/DNAzyme-co-loaded nanoparticles (ALA&Dz@ZIF-PEG) for mitochondria-targeting photodynamic immunotherapy. The zeolitic imidazolate framework (ZIF-8) nanoparticles can be disassembled and release large amounts of zinc ions (Zn2+) within tumor cells. Notably, Zn2+ can relieve tumor hypoxia for promoting the conversion of ALA to PpIX. Moreover, Zn2+ acts as a cofactor of rationally designed DNAzyme for silencing excessive ferrochelatase (FECH; which catalyzes PpIX into photoinactive Heme), cooperatively promoting the exclusive accumulation of PpIX in mitochondria via the "open source and reduced expenditure" manner. Subsequently, the photodynamic effects derived from PpIX lead to the damage and release of mtDNA and activate the innate immune response. In addition, the released Zn2+ further enhances the mtDNA/cGAS-STING pathway mediated innate immunity. The ALA&Dz@ZIF-PEG system induced 3 times more PpIX accumulation than ALA-loaded liposome, significantly enhancing tumor regression in xenograft tumor models.

Zinc antagonizes iron-regulation of tyrosine hydroxylase activity and dopamine production in Drosophila melanogaster

Background

Dopamine (DA) is a neurotransmitter that plays roles in movement, cognition, attention, and reward responses, and deficient DA signaling is associated with the progression of a number of neurological diseases, such as Parkinson’s disease. Due to its critical functions, DA expression levels in the brain are tightly controlled, with one important and rate-limiting step in its biosynthetic pathway being catalyzed by tyrosine hydroxylase (TH), an enzyme that uses iron ion (Fe²⁺) as a cofactor. A role for metal ions has additionally been associated with the etiology of Parkinson’s disease. However, the way dopamine synthesis is regulated in vivo or whether regulation of metal ion levels is a component of DA synthesis is not fully understood. Here, we analyze the role of Catsup, the Drosophila ortholog of the mammalian zinc transporter SLC39A7 (ZIP7), in regulating dopamine levels.

Results

We found that Catsup is a functional zinc transporter that regulates intracellular zinc distribution between the ER/Golgi and the cytosol. Loss-of-function of Catsup leads to increased DA levels, and we showed that the increased dopamine production is due to a reduction in zinc levels in the cytosol. Zinc ion (Zn²⁺) negatively regulates dopamine synthesis through direct inhibition of TH activity, by antagonizing Fe²⁺ binding to TH, thus rendering the enzyme ineffective or non-functional.

Conclusions

Our findings uncovered a previously unknown mechanism underlying the control of cellular dopamine expression, with normal levels of dopamine synthesis being maintained through a balance between Fe²⁺ and Zn²⁺ ions. The findings also provide support for metal modulation as a possible therapeutic strategy in the treatment of Parkinson’s disease and other dopamine-related diseases.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12915-021-01168-0.

Proton Pumping and Non-Pumping Terminal Respiratory Oxidases: Active Sites Intermediates of These Molecular Machines and Their Derivatives

Terminal respiratory oxidases are highly efficient molecular machines. These most important bioenergetic membrane enzymes transform the energy of chemical bonds released during the transfer of electrons along the respiratory chains of eukaryotes and prokaryotes from cytochromes or quinols to molecular oxygen into a transmembrane proton gradient. They participate in regulatory cascades and physiological anti-stress reactions in multicellular organisms. They also allow microorganisms to adapt to low-oxygen conditions, survive in chemically aggressive environments and acquire antibiotic resistance. To date, three-dimensional structures with atomic resolution of members of all major groups of terminal respiratory oxidases, heme-copper oxidases, and bd-type cytochromes, have been obtained. These groups of enzymes have different origins and a wide range of functional significance in cells. At the same time, all of them are united by a catalytic reaction of four-electron reduction in oxygen into water which proceeds without the formation and release of potentially dangerous ROS from active sites. The review analyzes recent structural and functional studies of oxygen reduction intermediates in the active sites of terminal respiratory oxidases, the features of catalytic cycles, and the properties of the active sites of these enzymes.

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.

Mechanisms of Energy Transduction by Charge Translocating Membrane Proteins

Life relies on the constant exchange of different forms of energy, i.e., on energy transduction. Therefore, organisms have evolved in a way to be able to harvest the energy made available by external sources (such as light or chemical compounds) and convert these into biological useable energy forms, such as the transmembrane difference of electrochemical potential (Δμ̃). Membrane proteins contribute to the establishment of Δμ̃ by coupling exergonic catalytic reactions to the translocation of charges (electrons/ions) across the membrane. Irrespectively of the energy source and consequent type of reaction, all charge-translocating proteins follow two molecular coupling mechanisms: direct- or indirect-coupling, depending on whether the translocated charge is involved in the driving reaction. In this review, we explore these two coupling mechanisms by thoroughly examining the different types of charge-translocating membrane proteins. For each protein, we analyze the respective reaction thermodynamics, electron transfer/catalytic processes, charge-translocating pathways, and ion/substrate stoichiometries.

Non-redox cycling mechanisms of oxidative stress induced by PM metals

Metallic compounds contribute to the oxidative stress of ambient particulate matter (PM) exposure. The toxicity of redox inert ions of cadmium, mercury, lead and zinc, as well as redox-active ions of vanadium and chromium is underlain by dysregulation of mitochondrial function and loss of signaling quiescence. Central to the initiation of these effects is the interaction of metal ions with cysteinyl thiols on glutathione and key regulatory proteins, which leads to impaired mitochondrial electron transport and persistent pan-activation of signal transduction pathways. The mitochondrial and signaling effects are linked by the production of H₂O₂, generated from mitochondrial superoxide anion or through the activation of NADPH oxidase, which extends the range and amplifies the magnitude of the oxidative effects of the metals. This oxidative burden can be further potentiated by inhibitory effects of the metals on the enzymes of the glutathione and thioredoxin systems. Along with the better-known Fenton-based mechanisms, the non-redox cycling mechanisms of oxidative stress induced by metals constitute significant pathways for cellular injury induced by PM inhalation.

Critical Role of Zinc as Either an Antioxidant or a Prooxidant in Cellular Systems

Zinc is recognized as an essential trace metal required for human health; its deficiency is strongly associated with neuronal and immune system defects. Although zinc is a redox-inert metal, it functions as an antioxidant through the catalytic action of copper/zinc-superoxide dismutase, stabilization of membrane structure, protection of the protein sulfhydryl groups, and upregulation of the expression of metallothionein, which possesses a metal-binding capacity and also exhibits antioxidant functions. In addition, zinc suppresses anti-inflammatory responses that would otherwise augment oxidative stress. The actions of zinc are not straightforward owing to its numerous roles in biological systems. It has been shown that zinc deficiency and zinc excess cause cellular oxidative stress. To gain insights into the dual action of zinc, as either an antioxidant or a prooxidant, and the conditions under which each role is performed, the oxidative stresses that occur in zinc deficiency and zinc overload in conjunction with the intracellular regulation of free zinc are summarized. Additionally, the regulatory role of zinc in mitochondrial homeostasis and its impact on oxidative stress are briefly addressed.

Time-resolved generation of membrane potential by ba3 cytochrome c oxidase from Thermus thermophilus coupled to single electron injection into the O and OH states

Two electrogenic phases with characteristic times of ~14μs and ~290μs are resolved in the kinetics of membrane potential generation coupled to single-electron reduction of the oxidized "relaxed" O state of ba₃ oxidase from T. thermophilus (O→E transition). The rapid phase reflects electron redistribution between Cu_A and heme b. The slow phase includes electron redistribution from both Cu_A and heme b to heme a₃, and electrogenic proton transfer coupled to reduction of heme a₃. The distance of proton translocation corresponds to uptake of a proton from the inner water phase into the binuclear center where heme a₃ is reduced, but there is no proton pumping and no reduction of Cu_B. Single-electron reduction of the oxidized "unrelaxed" state (O_H→E_H transition) is accompanied by electrogenic reduction of the heme b/heme a₃ pair by Cu_A in a "fast" phase (~22μs) and transfer of protons in "middle" and "slow" electrogenic phases (~0.185ms and ~0.78ms) coupled to electron redistribution from the heme b/heme a₃ pair to the Cu_B site. The "middle" and "slow" electrogenic phases seem to be associated with transfer of protons to the proton-loading site (PLS) of the proton pump, but when all injected electrons reach Cu_B the electronic charge appears to be compensated by back-leakage of the protons from the PLS into the binuclear site. Thus proton pumping occurs only to the extent of ~0.1 H⁺/e^-, probably due to the formed membrane potential in the experiment.

Intracellular Zn(II) Intoxication Leads to Dysregulation of the PerR Regulon Resulting in Heme Toxicity in Bacillus subtilis

Transition metal ions (Zn(II), Cu(II)/(I), Fe(III)/(II), Mn(II)) are essential for life and participate in a wide range of biological functions. Cellular Zn(II) levels must be high enough to ensure that it can perform its essential roles. Yet, since Zn(II) binds to ligands with high avidity, excess Zn(II) can lead to protein mismetallation. The major targets of mismetallation, and the underlying causes of Zn(II) intoxication, are not well understood. Here, we use a forward genetic selection to identify targets of Zn(II) toxicity. In wild-type cells, in which Zn(II) efflux prevents intoxication of the cytoplasm, extracellular Zn(II) inhibits the electron transport chain due to the inactivation of the major aerobic cytochrome oxidase. This toxicity can be ameliorated by depression of an alternate oxidase or by mutations that restrict access of Zn(II) to the cell surface. Conversely, efflux deficient cells are sensitive to low levels of Zn(II) that do not inhibit the respiratory chain. Under these conditions, intracellular Zn(II) accumulates and leads to heme toxicity. Heme accumulation results from dysregulation of the regulon controlled by PerR, a metal-dependent repressor of peroxide stress genes. When metallated with Fe(II) or Mn(II), PerR represses both heme biosynthesis (hemAXCDBL operon) and the abundant heme protein catalase (katA). Metallation of PerR with Zn(II) disrupts this coordination, resulting in depression of heme biosynthesis but continued repression of catalase. Our results support a model in which excess heme partitions to the membrane and undergoes redox cycling catalyzed by reduced menaquinone thereby resulting in oxidative stress.

Interplay Between Cytosolic Free Zn2+ and Mitochondrion Morphological Changes in Rat Ventricular Cardiomyocytes

The Zn²⁺ in cardiomyocytes is buffered by structures near T-tubulus and/or sarcoplasmic/endoplasmic reticulum (S(E)R) while playing roles as either an antioxidant or a toxic agent, depending on the concentration. Therefore, we aimed first to examine a direct effect of ZnPO₄ (extracellular exposure) or Zn²⁺ pyrithione (ZnPT) (intracellular exposure) application on the structure of the mitochondrion in ventricular cardiomyocytes by using histological investigations. The light microscopy data demonstrated that Zn²⁺ exposure induced marked increases on cellular surface area, an indication of hypertrophy, in a concentration-dependent manner. Furthermore, a whole-cell patch-clamp measurement of cell capacitance also supported the hypertrophy in the cells. We observed marked increases in mitochondrial matrix/cristae area and matrix volume together with increased lysosome numbers in ZnPO₄- or ZnPT-incubated cells by using transmission electron microscopy, again in a concentration-dependent manner. Furthermore, we observed notable clustering and vacuolated mitochondrion, markedly disrupted and damaged myofibrils, and electron-dense small granules in Zn²⁺-exposed cells together with some implications of fission-fusion defects in the mitochondria. Moreover, we observed marked depolarization in mitochondrial membrane potential during 1-μM ZnPT minute applications by using confocal microscopy. We also showed that 1-μM ZnPT incubation induced significant increases in the phosphorylation levels of GSK3β (Ser21 and Ser9), Akt (Ser473), and NFκB (Ser276 and Thr254) together with increased expression levels in ER stress proteins such as GRP78 and calregulin. Furthermore, a new key player at ER-mitochondria sites, promyelocytic leukemia protein (PML) level, was markedly increased in ZnPT-incubated cells. As a summary, our present data suggest that increased cytosolic free Zn²⁺ can induce marked alterations in mitochondrion morphology as well as depolarization in mitochondrion membrane potential and changes in some cytosolic signaling proteins as well as a defect in ER-mitochondria cross talk.

Developing a Gene Biomarker at the Tipping Point of Adaptive and Adverse Responses in Human Bronchial Epithelial Cells

Determining mechanism-based biomarkers that distinguish adaptive and adverse cellular processes is critical to understanding the health effects of environmental exposures. Shifting from in vivo, low-throughput toxicity studies to high-throughput screening (HTS) paradigms and risk assessment based on in vitro and in silico testing requires utilizing toxicity pathway information to distinguish adverse outcomes from recoverable adaptive events. Little work has focused on oxidative stresses in human airway for the purposes of predicting adverse responses. We hypothesize that early gene expression-mediated molecular changes could be used to delineate adaptive and adverse responses to environmentally-based perturbations. Here, we examined cellular responses of the tracheobronchial airway to zinc (Zn) exposure, a model oxidant. Airway derived BEAS-2B cells exposed to 2–10 μM Zn²⁺ elicited concentration- and time-dependent cytotoxicity. Normal, adaptive, and cytotoxic Zn²⁺ exposure conditions were determined with traditional apical endpoints, and differences in global gene expression around the tipping point of the responses were used to delineate underlying molecular mechanisms. Bioinformatic analyses of differentially expressed genes indicate early enrichment of stress signaling pathways, including those mediated by the transcription factors p53 and NRF2. After 4 h, 154 genes were differentially expressed (p < 0.01) between the adaptive and cytotoxic Zn²⁺ concentrations. Nearly 40% of the biomarker genes were related to the p53 signaling pathway with 30 genes identified as likely direct targets using a database of p53 ChIP-seq studies. Despite similar p53 activation profiles, these data revealed widespread dampening of p53 and NRF2-related genes as early as 4 h after exposure at higher, unrecoverable Zn²⁺ exposures. Thus, in our model early increased activation of stress response pathways indicated a recoverable adaptive event. Overall, this study highlights the importance of characterizing molecular mechanisms around the tipping point of adverse responses to better inform HTS paradigms.

Membrane-Anchored Cyclic Peptides as Effectors of Mitochondrial Oxidative Phosphorylation

The echinocandins are membrane-anchored, cyclic lipopeptides (CLPs) with antifungal activity due to their ability to inhibit a glucan synthase located in the plasma membrane of fungi such as Candida albicans. A hydrophobic tail of an echinocandin CLP inserts into a membrane, placing a six-amino acid cyclic peptide near the membrane surface. Because processes critical for the function of the electron transfer complexes of mitochondria, such as proton uptake and release, take place near the surface of the membrane, we have tested the ability of two echinocandin CLPs, caspofungin and micafungin, to affect the activity of electron transfer complexes in isolated mammalian mitochondria. Indeed, caspofungin and micafungin both inhibit whole chain electron transfer in isolated mitochondria at low micromolar concentrations. The effects of the CLPs are fully reversible, in some cases simply via the addition of bovine serum albumin to bind the CLPs via their hydrophobic tails. Each CLP affects more than one complex, but they still exhibit specificity of action. Only caspofungin inhibits complex I, and the CLP inhibits liver but not heart complex I. Both CLPs inhibit heart and liver complex III. Caspofungin inhibits complex IV activity, while, remarkably, micafungin stimulates complex IV activity nearly 3-fold. Using a variety of assays, we have developed initial hypotheses for the mechanisms by which caspofungin and micafungin alter the activities of complexes IV and III. The dication caspofungin partially inhibits cytochrome c binding at the low-affinity binding site of complex IV, while it also appears to inhibit the release of protons from the outer surface of the complex, similar to Zn(2+). Anionic micafungin appears to stimulate complex IV activity by enhancing the transfer of protons to the O2 reduction site. For complex III, we hypothesize that each CLP binds to the cytochrome b subunit and the Fe-S subunit to inhibit the required rotational movement of the latter.

Electrochemistry suggests proton access from the exit site to the binuclear center in Paracoccus denitrificans cytochrome c oxidase pathway variants

Two different pathways through which protons access cytochrome c oxidase operate during oxygen reduction from the mitochondrial matrix, or the bacterial cytoplasm. Here, we use electrocatalytic current measurements to follow oxygen reduction coupled to proton uptake in cytochrome c oxidase isolated from Paracoccus denitrificans. Wild type enzyme and site-specific variants with defects in both proton uptake pathways (K354M, D124N and K354M/D124N) were immobilized on gold nanoparticles, and oxygen reduction was probed electrochemically in the presence of varying concentrations of Zn(2+) ions, which are known to inhibit both the entry and the exit proton pathways in the enzyme. Our data suggest that under these conditions substrate protons gain access to the oxygen reduction site via the exit pathway.

Effects of zinc on particulate methane monooxygenase activity and structure

Particulate methane monooxygenase (pMMO) is a membrane-bound metalloenzyme that oxidizes methane to methanol in methanotrophic bacteria. Zinc is a known inhibitor of pMMO, but the details of zinc binding and the mechanism of inhibition are not understood. Metal binding and activity assays on membrane-bound pMMO from Methylococcus capsulatus (Bath) reveal that zinc inhibits pMMO at two sites that are distinct from the copper active site. The 2.6 Å resolution crystal structure of Methylocystis species strain Rockwell pMMO reveals two previously undetected bound lipids, and metal soaking experiments identify likely locations for the two zinc inhibition sites. The first is the crystallographic zinc site in the pmoC subunit, and zinc binding here leads to the ordering of 10 previously unobserved residues. A second zinc site is present on the cytoplasmic side of the pmoC subunit. Parallels between these results and zinc inhibition studies of several respiratory complexes suggest that zinc might inhibit proton transfer in pMMO.

Zinc toxicity and ATP production in Pseudomonas fluorescens

AIMS

To identify the molecular networks in Pseudomonas fluorescens that convey resistance to toxic concentrations of Zn, a common pollutant and hazard to biological systems.

METHODS AND RESULTS

Pseudomonas fluorescens strain ATCC 13525 was cultured in growth medium with millimolar concentrations of Zn. Enzymatic activities and metabolite levels were monitored with the aid of in-gel activity assays and high-performance liquid chromatography, respectively. As oxidative phosphorylation was rendered ineffective, the assimilation of citric acid mediated sequentially by citrate lyase (CL), phosphoenolpyruvate carboxylase (PEPC) and pyruvate phosphate dikinase (PPDK) appeared to play a key role in ATP synthesis via substrate-level phosphorylation (SLP). Enzymes generating the antioxidant, reduced nicotinamide adenine dinucleotide phosphate (NADPH) were enhanced, while metabolic modules mediating the formation of the pro-oxidant, reduced nicotinamide adenine dinucleotide (NADH) were downregulated.

CONCLUSIONS

Pseudomonas fluorescens reengineers its metabolic networks to generate ATP via SLP, a stratagem that allows the microbe to compensate for an ineffective electron transport chain provoked by excess Zn.

SIGNIFICANCE AND IMPACT OF THE STUDY

The molecular insights described here are critical in devising strategies to bioremediate Zn-polluted environments.

Cation binding site of cytochrome c oxidase: progress report

Cytochrome c oxidase from bovine heart binds Ca(2+) reversibly at a specific Cation Binding Site located near the outer face of the mitochondrial membrane. Ca(2+) shifts the absorption spectrum of heme a, which allowed earlier the determination of the kinetic and equilibrium characteristics of the binding, and, as shown recently, the binding of calcium to the site inhibits cytochrome oxidase activity at low turnover rates of the enzyme [Vygodina, Т., Kirichenko, A., Konstantinov, A.A (2013). Direct Regulation of Cytochrome c Oxidase by Calcium Ions. PloS ONE 8, e74436]. This paper summarizes further progress in the studies of the Cation Binding Site in this group presenting the results to be reported at 18th EBEC Meeting in Lisbon, 2014. The paper revises specificity of the bovine oxidase Cation Binding Site for different cations, describes dependence of the Ca(2+)-induced inhibition on turnover rate of the enzyme and reports very high affinity binding of calcium with the "slow" form of cytochrome oxidase. This article is part of a Special Issue entitled: 18th European Bioenergetic Conference. Guest Editors: Manuela Pereira and Miguel Teixeira.

Direct regulation of cytochrome c oxidase by calcium ions

Cytochrome c oxidase from bovine heart binds Ca²⁺ reversibly at a specific Cation Binding Site located near the outer face of the mitochondrial membrane. Ca²⁺ shifts the absorption spectrum of heme a, which allowed previously to determine the kinetics and equilibrium characteristics of the binding. However, no effect of Ca²⁺ on the functional characteristics of cytochrome oxidase was revealed earlier. Here we report that Ca²⁺ inhibits cytochrome oxidase activity of isolated bovine heart enzyme by 50–60% with K_i of ∼1 µM, close to K_d of calcium binding with the oxidase determined spectrophotometrically. The inhibition is observed only at low, but physiologically relevant, turnover rates of the enzyme (∼10 s⁻¹ or less). No inhibitory effect of Ca²⁺ is observed under conventional conditions of cytochrome c oxidase activity assays (turnover number >100 s⁻¹ at pH 8), which may explain why the effect was not noticed earlier. The inhibition is specific for Ca²⁺ and is reversed by EGTA. Na⁺ ions that compete with Ca²⁺ for binding with the Cation Binding Site, do not affect significantly activity of the enzyme but counteract the inhibitory effect of Ca²⁺. The Ca²⁺-induced inhibition of cytochrome c oxidase is observed also with the uncoupled mitochondria from several rat tissues. At the same time, calcium ions do not inhibit activity of the homologous bacterial cytochrome oxidases. Possible mechanisms of the inhibition are discussed as well as potential physiological role of Ca²⁺ binding with cytochrome oxidase. Ca²⁺- binding at the Cation Binding Site is proposed to inhibit proton-transfer through the exit part of the proton conducting pathway H in the mammalian oxidases.

Role of aspartate 132 at the orifice of a proton pathway in cytochrome c oxidase

Proton transfer across biological membranes underpins central processes in biological systems, such as energy conservation and transport of ions and molecules. In the membrane proteins involved in these processes, proton transfer takes place through specific pathways connecting the two sides of the membrane via control elements within the protein. It is commonly believed that acidic residues are required near the orifice of such proton pathways to facilitate proton uptake. In cytochrome c oxidase, one such pathway starts near a conserved Asp-132 residue. Results from earlier studies have shown that replacement of Asp-132 by, e.g., Asn, slows proton uptake by a factor of ∼5,000. Here, we show that proton uptake at full speed (∼10(4) s(-1)) can be restored in the Asp-132-Asn oxidase upon introduction of a second structural modification further inside the pathway (Asn-139-Thr) without compensating for the loss of the negative charge. This proton-uptake rate was insensitive to Zn(2+) addition, which in the wild-type cytochrome c oxidase slows the reaction, indicating that Asp-132 is required for Zn(2+) binding. Furthermore, in the absence of Asp-132 and with Thr at position 139, at high pH (>9), proton uptake was significantly accelerated. Thus, the data indicate that Asp-132 is not strictly required for maintaining rapid proton uptake. Furthermore, despite the rapid proton uptake in the Asn-139-Thr/Asp-132-Asn mutant cytochrome c oxidase, proton pumping was impaired, which indicates that the segment around these residues is functionally linked to pumping.

[The change in the structural and functional organization of the Guerin's carcinoma cytochrome part of respiratory chain in tumor carriers in the conditions of preliminary low-level irradiation]

The effect of low-level irradiation of tumor-bearing rats on the structural and functional organization of the cytochrome part of respiratory chain of mitochondria isolated from Guerin's carcinoma has been investigated. The maximal reduction in the mitochondrial cytochromes a, b and c content was observed at the terminal stage of Guerin's carcinoma. A low-level irradiation during initial stages of oncogenesis produced opposite changes in the mitochondrial cytochrome content. The possible mechanism of mitochondrial haem-containing cytochromes content reduction may be attributed to impairment in their formation caused by inhibition of the key enzyme of haem synthesis, 5-aminolevulinate synthase. The determined changes of the mitochondrial cytochromes quantitative content were accompanied by decreased activity of cytochrome oxidase. The preliminary low-level irradiation of the tumor-bearing animals produced further reduction in the cytochrome oxidase activity observed in all experimental periods.

Transmembrane electric potential difference in the protein-pigment complex of photosystem 2

The protein-pigment complex of photosystem 2 (PS2) localized in the thylakoid membranes of higher plants, algae, and cyanobacteria is the main source of oxygen on Earth. The light-induced functioning of PS2 is directly linked to electron and proton transfer across the membrane, which results in the formation of transmembrane electric potential difference (ΔΨ). The major contribution to ΔΨ of the PS2 reaction center is due to charge separation between the primary chlorophyll donor P(680) and the quinone acceptor Q(A), accompanied by re-reduction of P(680)(+) by the redox-active tyrosine residue Y(Z). The processes associated with the uptake and release of protons on the acceptor and donor sides of the enzyme, respectively, are also coupled with ΔΨ generation. The objective of this work was to describe the mechanisms of ΔΨ generation associated with the S-state transitions of the water-oxidizing complex in intact PS2 complex and in PS2 preparation depleted of Mn(4)Ca cluster in the presence of artificial electron donors. The findings elucidate the mechanisms of electrogenic reactions on the PS2 donor side and may be a basis for development of an effective solar energy conversion system.

Acquired cytochrome C oxidase impairment in apheresis platelets during storage: a possible mechanism for depletion of metabolic adenosine triphosphate

BACKGROUND

Intracellular adenosine triphosphate (ATP) levels decline significantly during storage of platelet (PLT) products, in part due to PLT degranulation. However, metabolic ATP stores also become depleted during storage through an unclear mechanism. Since both anaerobic glycolysis and oxidative phosphorylation are important for PLT ATP production, it is possible that the reduction in metabolic ATP reflects impaired oxidative phosphorylation. To assess this, we evaluated the kinetic activity and protein expression of cytochrome C oxidase (CcOX) in stored apheresis PLTs.

STUDY DESIGN AND METHODS

Apheresis PLTs were collected and stored with agitation at 22 ± 2°C for 7 days. In vitro measurements of PLT metabolic state, function, and activation were performed on Days 0, 2, 4, and 7 of storage. Total PLT ATP content, steady-state CcOX kinetic activity, and protein immunoblotting for CcOX Subunits I and IV were also performed using isolated PLT mitochondria from simultaneously collected samples.

RESULTS

Intra-PLT ATP and steady-state PLT CcOX activity declined significantly and in a progressive manner throughout storage while steady-state levels of CcOX I and IV protein remained unchanged. Time-dependent decline in CcOX activity correlated with progressive ATP depletion over time.

CONCLUSION

During storage of apheresis PLTs for 7 days, the parallel decline in CcOX function and intra-PLT ATP suggests development of an acquired impairment in PLT oxidative phosphorylation associated with perturbed ATP homeostasis in stored PLTs.

Effects of heavy metal cations on the mitochondrial ornithine/citrulline transporter reconstituted in liposomes

The effect of heavy metal cations on the mitochondrial ornithine/citrulline transporter was tested in proteoliposomes reconstituted with the protein purified from rat liver. The transport activity was measured as [(3)H]ornithine uptake in proteoliposomes containing internal ornithine (ornithine/ornithine antiport mode) or as [(3)H]ornithine efflux in the absence of external substrate (ornithine/H(+) transport mode). 0.1 mM Cu(2+), Pb(2+), Hg(2+), Cd(2+) and Zn(2+) strongly inhibited (more than 85%) the antiport; whereas Mn(2+), Co(2+) and Ni(2+) inhibited less efficiently (25, 47 and 69%, respectively). The IC(50) values of the transporter for the different metal ions ranged from 0.71 to 350 μM. Co(2+) and Ni(2+) also inhibited the [(3)H]ornithine efflux whereas Cu(2+), Pb(2+), Hg(2+), Cd(2+) and Zn(2+) stimulated the [(3)H]ornithine efflux. The stimulation of the [(3)H]ornithine efflux by Cu(2+) and Cd(2+) (as well as by Pb(2+), Hg(2+) and Zn(2+)) was not prevented by NEM and was reversed by DTE. These features indicated that the inhibition of the antiport was due to the interaction of the Cu(2+), Pb(2+), Hg(2+), Cd(2+) and Zn(2+) with a population of SH groups, of the transporter, responsible for the inhibition of the physiological function; whereas the stimulation of [(3)H]ornithine efflux was due to the induction of a pore-like function of the transporter caused by interaction of cations with a different population of SH groups. Differently, the inhibition of the ornithine transporter by Ni(2+), Co(2+) or Mn(2+) was caused by interaction with the substrate binding site, as indicated by the competitive or mixed inhibition.

Differential regulation of vacuolar H+-ATPase and H+-PPase in Cucumis sativus roots by zinc and nickel

Zinc and nickel, as micronutrients, are essential for all organisms. We investigated the effect of 10 and 100 μM Zn and Ni on two tonoplast proton pumps, vacuolar H+-ATPase (V-ATPase) (EC 3.6.3.14) and vacuolar H+-pyrophosphatase (V-PPase) (EC 3.6.1.1), in cucumber roots. ATP-dependent proton transport as well as ATP hydrolysis, catalyzed by V-ATPase, decreased in roots after exposure of plants to both Zn and Ni under all the examined conditions. In contrast, V-PPase activities, measured as PP(i) hydrolysis and PP(i)-driven H+ transport, were stimulated by lower concentration of metals. However, at higher metal concentration, hydrolytic activity of V-PPase remained unchanged, while PP(i)-dependent proton pumping into the tonoplast vesicles was reduced. When heavy metals were introduced into the enzyme reaction medium, both V-ATPase and V-PPase activities were lowered by Zn and Ni in a similar manner. As the gene expression and immunoblot analyses depicted, observed changes in the activity of both tonoplast proton pumps in response to zinc and nickel were not due to the modification in the expression of the CsVHA-A, CsVHA-c and CsVP genes encoding V-ATPase subunit A and c, and V-PPase, respectively, in cucumber roots or in amounts of enzyme proteins. Moreover, Zn as well as Ni ions did not enhance the lipid peroxidation in the root tonoplast fractions. Comparison of ATP and pyrophosphate contents in the control roots and roots treated with heavy metals revealed that Zn and Ni do not affect the ATP amount but reduce the PP(i) level.

Inhibition of purified bovine liver glutathione reductase with some metal ions

Glutathione reductase (GR; E.C. 1.6.4.2) is a flavoprotein that catalyzes the NADPH-dependent reduction of oxidized glutathione (GSSG). In this study we tested the effects of Al3+, Ba2+, Ca2+, Li+, Mn2+, Mo6+, Cd2+, Ni2+, and Zn2+ on purified bovine liver GR. In a range of 10 microM-10 mM concentrations, Al3+, Ba2+, Li+, Mn2+, and Mo6+, and Ca2+ at 5 microM-1.25 mM, had no effect on bovine liver GR. Cadmium (Cd2+), nickel (Ni2+), and zinc (Zn2+) showed inhibitory effects on this enzyme. The obtained IC50 values of Cd2+, Ni2+, and Zn2+ were 0.08, 0.8, and 1 mM, respectively. Cd2+ inhibition was non-competitive with respect to both GSSG (Ki(GSSG) 0.221 +/- 0.02 mM) and NADPH (Ki(NADPH) 0.113 +/- 0.008 mM). Ni2+ inhibition was non-competitive with respect to GSSG (Ki(GSSG) 0.313 +/- 0.01 mM) and uncompetitive with respect to NADPH (Ki(NADPH) 0.932 +/- 0.03 mM). The effect of Zn2+ on GR activity was consistent with a non-competitive inhibition pattern when the varied substrates were GSSG (Ki(GSSG) 0.320 +/- 0.018 mM) and NADPH (Ki(NADPH) 0.761 +/- 0.04 mM), respectively.

Divalent metal ions in plant mitochondria and their role in interactions with proteins and oxidative stress-induced damage to respiratory function

Understanding the metal ion content of plant mitochondria and metal ion interactions with the proteome are vital for insights into both normal respiratory function and the process of protein damage during oxidative stress. We have analyzed the metal content of isolated Arabidopsis (Arabidopsis thaliana) mitochondria, revealing a 26:8:6:1 molar ratio for iron:zinc:copper:manganese and trace amounts of cobalt and molybdenum. We show that selective changes occur in mitochondrial copper and iron content following in vivo and in vitro oxidative stresses. Immobilized metal affinity chromatography charged with Cu(2+), Zn(2+), and Co(2+) was used to identify over 100 mitochondrial proteins with metal-binding properties. There were strong correlations between the sets of immobilized metal affinity chromatography-interacting proteins, proteins predicted to contain metal-binding motifs, and protein sets known to be oxidized or degraded during abiotic stress. Mitochondrial respiratory chain pathways and matrix enzymes varied widely in their susceptibility to metal-induced loss of function, showing the selectivity of the process. A detailed study of oxidized residues and predicted metal interaction sites in the tricarboxylic acid cycle enzyme aconitase identified selective oxidation of residues in the active site and showed an approach for broader screening of functionally significant oxidation events in the mitochondrial proteome.

A comparison of Zn2+- and Ca2+-triggered depolarization of liver mitochondria reveals no evidence of Zn2+-induced permeability transition

Intracellular Zn(2+) toxicity is associated with mitochondrial dysfunction. Zn(2+) depolarizes mitochondria in assays using isolated organelles as well as cultured cells. Some reports suggest that Zn(2+)-induced depolarization results from the opening of the mitochondrial permeability transition pore (mPTP). For a more detailed analysis of this relationship, we compared Zn(2+)-induced depolarization with the effects of Ca(2+) in single isolated rat liver mitochondria monitored with the potentiometric probe rhodamine 123. Consistent with previous work, we found that relatively low levels of Ca(2+) caused rapid, complete and irreversible loss of mitochondrial membrane potential, an effect that was diminished by classic inhibitors of mPT, including high Mg(2+), ADP and cyclosporine A. Zn(2+) also depolarized mitochondria, but only at relatively high concentrations. Furthermore Zn(2+)-induced depolarization was slower, partial and sometimes reversible, and was not affected by inhibitors of mPT. We also compared the effects of Ca(2+) and Zn(2+) in a calcein-retention assay. Consistent with the well-documented ability of Ca(2+) to induce mPT, we found that it caused rapid and substantial loss of matrix calcein. In contrast, calcein remained in Zn(2+)-treated mitochondria. Considered together, our results suggest that Ca(2+) and Zn(2+) depolarize mitochondria by considerably different mechanisms, that opening of the mPTP is not a direct consequence of Zn(2+)-induced depolarization, and that Zn(2+) is not a particularly potent mitochondrial inhibitor.

Effects of cadmium and zinc ions on purified lamb kidney cortex glucose-6-phosphate dehydrogenase activity

Glucose-6-phosphate dehydrogenase (G-6-PD) is the first enzyme in the pentose phosphate pathway. Cadmium is a toxic heavy metal that inhibits several enzymes. Zinc is an essential metal but overdoses of zinc have toxic effects on enzyme activities. In this study G-6-PD from lamb kidney cortex was competitively inhibited by zinc both with respect to glucose-6-phosphate (G-6-P) and NADP+ with Ki values of 1.066 +/- 0.106 and 0.111 +/- 0.007 mM respectively whereas cadmium was a non-competitive inhibitor with respect to both G-6-P and NADP+ Ki values of 2.028 +/- 0.175 and 2.044 +/- 0.289 mM respectively.

The role of particulate matter-associated zinc in cardiac injury in rats

BACKGROUND

Exposure to particulate matter (PM) has been associated with increased cardiovascular morbidity; however, causative components are unknown. Zinc is a major element detected at high levels in urban air.

OBJECTIVE

We investigated the role of PM-associated zinc in cardiac injury.

METHODS

We repeatedly exposed 12- to 14-week-old male Wistar Kyoto rats intratracheally (1x/week for 8 or 16 weeks) to a) saline (control); b) PM having no soluble zinc (Mount St. Helens ash, MSH); or c) whole-combustion PM suspension containing 14.5 microg/mg of water-soluble zinc at high dose (PM-HD) and d ) low dose (PM-LD), e) the aqueous fraction of this suspension (14.5 microg/mg of soluble zinc) (PM-L), or f ) zinc sulfate (rats exposed for 8 weeks received double the concentration of all PM components of rats exposed for 16 weeks).

RESULTS

Pulmonary inflammation was apparent in all exposure groups when compared with saline (8 weeks > 16 weeks). PM with or without zinc, or with zinc alone caused small increases in focal subepicardial inflammation, degeneration, and fibrosis. Lesions were not detected in controls at 8 weeks but were noted at 16 weeks. We analyzed mitochondrial DNA damage using quantitative polymerase chain reaction and found that all groups except MSH caused varying degrees of damage relative to control. Total cardiac aconitase activity was inhibited in rats receiving soluble zinc. Expression array analysis of heart tissue revealed modest changes in mRNA for genes involved in signaling, ion channels function, oxidative stress, mitochondrial fatty acid metabolism, and cell cycle regulation in zinc but not in MSH-exposed rats.

CONCLUSION

These results suggest that water-soluble PM-associated zinc may be one of the causal components involved in PM cardiac effects.

The inhibition kinetics of yeast glutathione reductase by some metal ions

Glutathione reductase (GR, type IV, Baker's yeast, E.C 1.6.4.2) is a flavoprotein that catalyzes the NADPH-dependent reduction of oxidized glutathione (GSSG) to reduced glutathione (GSH). In this study some metal ions have been tested on GR; lithium, manganese, molybdate, aluminium, barium, zinc, calcium, cadmium and nickel. Cadmium, nickel and calcium showed a good to moderate inhibitory effect on yeast GR. GR is inhibited non-competitively by Zn2+ (up to 2 mM) and activated above this concentration. Ca2+ inhibition was non-competitive with respect to GSSG and uncompetitive with respect to NADPH. Nickel inhibition was competitive with respect to GSSG and uncompetitive with respect to NADPH. The inhibition constants for these metals on GR were determined. The chelating agent EDTA recovered 90% of the GR activity inhibited by these metals.

A histidine residue acting as a controlling site for dioxygen reduction and proton pumping by cytochrome c oxidase

Cytochrome c oxidase transfers electrons and protons for dioxygen reduction coupled with proton pumping. These electron and proton transfers are tightly coupled with each other for the effective energy transduction by various unknown mechanisms. Here, we report a coupling mechanism by a histidine (His-503) at the entrance of a proton transfer pathway to the dioxygen reduction site (D-pathway) of bovine heart cytochrome c oxidase. In the reduced state, a water molecule is fixed by hydrogen bonds between His-503 and Asp-91 of the D-pathway and is linked via two water arrays extending to the molecular surface. The microenvironment of Asp-91 appears in the x-ray structure to have a proton affinity as high as that of His-503. Thus, Asp-91 and His-503 cooperatively trap, on the fixed water molecule, the proton that is transferred through the water arrays from the molecular surface. On oxidation, the His-503 imidazole plane rotates by 180 degrees to break the hydrogen bond to the protonated water and releases the proton to Asp-91. On reduction, Asp-91 donates the proton to the dioxygen reduction site through the D-pathway. The proton collection controlled by His-503 was confirmed by partial electron transfer inhibition by binding of Zn2+ and Cd2+ to His-503 in the x-ray structures. The estimated Kd for Zn2+ binding to His-503 in the x-ray structure is consistent with the reported Kd for complete proton-pumping inhibition by Zn2+ [Kannt A, Ostermann T, Muller H, Ruitenberg M (2001) FEBS Lett 503:142-146]. These results suggest that His-503 couples the proton transfer for dioxygen reduction with the proton pumping.

The inhibitory binding site(s) of Zn2+in cytochromecoxidase

EXAFS analysis of Zn binding site(s) in bovine-heart cytochrome c oxidase and characterization of the inhibitory effect of internal zinc on respiratory activity and proton pumping of the liposome reconstituted oxidase are presented. EXAFS identifies tetrahedral coordination site(s) for Zn(2+) with two N-histidine imidazoles, one N-histidine imidazol or N-lysine and one O-COOH (glutamate or aspartate), possibly located at the entry site of the proton conducting D pathway in the oxidase and involved in inhibition of the oxygen reduction catalysis and proton pumping by internally trapped zinc.

The inhibition of mitochondrial complex I (NADH:ubiquinone oxidoreductase) by Zn2+

NADH:ubiquinone oxidoreductase (complex I) from bovine heart mitochondria is a highly complicated, membrane-bound enzyme. It is central to energy transduction, an important source of cellular reactive oxygen species, and its dysfunction is implicated in neurodegenerative and muscular diseases and in aging. Here, we describe the effects of Zn2+ on complex I to define whether complex I may contribute to mediating the pathological effects of zinc in states such as ischemia and to determine how Zn2+ can be used to probe the mechanism of complex I. Zn2+ inhibits complex I more strongly than Mg2+, Ca2+, Ba2+, and Mn2+ to Cu2+ or Cd2+. It does not inhibit NADH oxidation or intramolecular electron transfer, so it probably inhibits either proton transfer to bound quinone or proton translocation. Thus, zinc represents a new class of complex I inhibitor clearly distinct from the many ubiquinone site inhibitors. No evidence for increased superoxide production by zinc-inhibited complex I was detected. Zinc binding to complex I is mechanistically complicated. During catalysis, zinc binds slowly and progressively, but it binds rapidly and tightly to the resting state(s) of the enzyme. Reactivation of the inhibited enzyme upon the addition of EDTA is slow, and inhibition is only partially reversible. The IC50 value for the Zn2+ inhibition of complex I is high (10-50 microm, depending on the enzyme state); therefore, complex I is unlikely to be a major site for zinc inhibition of the electron transport chain. However, the slow response of complex I to a change in Zn2+ concentration may enhance any physiological consequences.

Zinc dynamics in the myocardial redox signaling network

Zinc plays a vital role in various cellular functions. Zinc deprivation is associated with severe disorders related to growth, maturation, and stress responses. In the heart, zinc affects differentiation and regeneration of cardiac muscle, cardiac conductance, acute stress responses, and recovery of heart transplants. Recent discoveries of the molecular players in zinc homeostasis revealed that the amount of intracellular free zinc is tightly controlled on the level of uptake, intracellular sequestration, redistribution, storage, and elimination, consequently creating a narrow window of optimal zinc concentration in the cells. Most of intracellular zinc is bound to numerous structural and regulatory proteins, with metabolically active, labile zinc present in picoto nanomolar concentrations. The central position of zinc in the redox signaling network is built on its unique chemical nature. The redox inert zinc creates a redox active environment when it binds to a sulfur ligand. The reversible oxidation of the sulfur ligand is coupled to the reversible zinc release from the protein, thereby executing the task of so-called protein "redox zinc switch." Clearly, the impairment of zinc homeostasis will have far reaching physiological consequences.

Inhibition of proton pumping by zinc ions during specific reaction steps in cytochrome c oxidase

Cytochrome c oxidase (CytcO) is a redox-driven proton pump in the respiratory chain of mitochondria and many aerobic bacteria. The results from several studies have shown that zinc ions interfere with both the uptake and release of protons, presumably by binding near the orifice of the proton entrance and exit pathways. To elucidate the effect of Zn2+ binding on individual electron and proton-transfer reactions, in this study, we have investigated the reaction of the fully reduced R. sphaeroides CytcO with O2, both with enzyme in detergent solution and reconstituted in phospholipid vesicles, and, with and without, Zn2+. The results show that addition of Zn2+ at concentrations of < or = 250 microM to the outside of the vesicles did not alter the transition rates between intermediates PR (P3)-->F3-->O4. However, proton pumping was impaired specifically during the P3-->F3, but not during the F3-->O4 transition at Zn2+ concentrations of < or = 25 microM. Furthermore, proton pumping during the P3-->F3 transition was typically impaired with the "as isolated" CytcO, which was found to contain Zn2+ ions at microM concentration. As has already been shown, Zn2+ was also found to obstruct proton uptake during the P3-->F3 transition, presumably by binding to a site near the orifice of the D-pathway. In this work we found a KI of approximately 1 microM for this binding site. In conclusion, the results show that Zn2+ ions bind on both sides of CytcO and that binding of Zn2+ at the proton output side selectively impairs proton release during the P3-->F3 transition.