A structural-dynamical characterization of human Cox17

Human Cox17 is a key mitochondrial copper chaperone responsible for supplying copper ions, through the assistance of Sco1, Sco2, and Cox11, to cytochrome c oxidase, the terminal enzyme of the mitochondrial energy transducing respiratory chain. A structural and dynamical characterization of human Cox17 in its various functional metallated and redox states is presented here. The NMR solution structure of the partially oxidized Cox17 (Cox17(2S-S)) consists of a coiled coil-helix-coiled coil-helix domain stabilized by two disulfide bonds involving Cys(25)-Cys(54) and Cys(35)-Cys(44), preceded by a flexible and completely unstructured N-terminal tail. In human Cu(I)Cox17(2S-S) the copper(I) ion is coordinated by the sulfurs of Cys(22) and Cys(23), and this is the first example of a Cys-Cys binding motif in copper proteins. Copper(I) binding as well as the formation of a third disulfide involving Cys(22) and Cys(23) cause structural and dynamical changes only restricted to the metal-binding region. Redox properties of the disulfides of human Cox17, here investigated, strongly support the current hypothesis that the unstructured fully reduced Cox17 protein is present in the cytoplasm and enters the intermembrane space (IMS) where is then oxidized by Mia40 to Cox17(2S-S), thus becoming partially structured and trapped into the IMS. Cox17(2S-S) is the functional species in the IMS, it can bind only one copper(I) ion and is then ready to enter the pathway of copper delivery to cytochrome c oxidase. The copper(I) form of Cox17(2S-S) has features specific for copper chaperones.     

Mia40 Combines Thiol Oxidase and Disulfide Isomerase Activity to Efficiently Catalyze Oxidative Folding in Mitochondria

Mia40 (a mitochondrial import and assembly protein) catalyzes disulfide bond formation in proteins in the mitochondrial intermembrane space. By using Cox17 (a mitochondrial copper-binding protein) as a natural substrate, we discovered that, in the presence of Mia40, the formation of native disulfides is strongly favored. The catalytic mechanism of Mia40 involves a functional interplay between the chaperone site and the catalytic disulfide. Mia40 forms a specific native disulfide in Cox17 much more rapidly than other disulfides, in particular, non-native ones, which originates from the recently described high affinity for hydrophobic regions near target cysteines and the long lifetime of the mixed disulfide. In addition to its thiol oxidase function, Mia40 is active also as a disulfide reductase and isomerase. We found that species with inadvertently formed incorrect disulfides are rebound by Mia40 and reshuffled, revealing a proofreading mechanism that is steered by the conformational folding of the substrate protein.     

Functional role of two interhelical disulfide bonds in human Cox17 protein from a structural perspective

Human Cox17 is the mitochondrial copper chaperone responsible for supplying copper ions, through the assistance of Sco1, Sco2, and Cox11, to cytochrome c oxidase, the terminal enzyme of the mitochondrial energy-transducing respiratory chain. It consists of a coiled coil-helix-coiled coil-helix domain stabilized by two disulfide bonds and binds one copper(I) ion through a Cys-Cys motif. Here, the structures and the backbone mobilities of two Cox17 mutated forms with only one interhelical disulfide bond have been analyzed. It appears that the inner disulfide bond (formed by Cys-36 and Cys-45) stabilizes interhelical hydrophobic interactions, providing a structure with essentially the same structural dynamic properties of the mature Cox17 state. On the contrary, the external disulfide bond (formed by Cys-26 and Cys-55) generates a conformationally flexible α-helical protein, indicating that it is not able to stabilize interhelical packing contacts, but is important for structurally organizing the copper-binding site region.     

Cox17 is functional when tethered to the mitochondrial inner membrane

Cox17 is an essential protein in the assembly of cytochrome c oxidase within the mitochondrion. Cox17 is implicated in providing copper ions for formation of CuA and CuB sites in the oxidase complex. To address whether Cox17 is functional in shuttling copper ions to the mitochondrion, Cox17 was tethered to the mitochondrial inner membrane by a fusion to the transmembrane domain of the inner membrane protein, Sco2. The copper-binding domain of Sco2 that projects into the inter-mitochondrial membrane space was replaced with Cox17. The Sco2/Cox17 fusion protein containing the mitochondrial import sequence and transmembrane segment of Sco2 is exclusively localized within the mitochondrion. The Sco2/Cox17 protein restores respiratory growth and normal cytochrome oxidase activity in cox17Delta cells. These studies suggest that the function of Cox17 is confined to the mitochondrial intermembrane space. Domain mapping of yeast Cox17 reveals that the carboxyl-terminal segment of the protein has a function within the intermembrane space that is independent of copper ion binding. The essential C-terminal function of Cox17 maps to a candidate amphipathic helix that is important for mitochondrial uptake and retention of the Cox17 protein. This motif can be spatially separated from the N-terminal copper-binding functional motif. Possible roles of the C-terminal motif are discussed.     

Yeast cox17 solution structure and Copper(I) binding

Cox17 is a 69-residue cysteine-rich, copper-binding protein that has been implicated in the delivery of copper to the Cu(A) and Cu(B) centers of cytochrome c oxidase via the copper-binding proteins Sco1 and Cox11, respectively. According to isothermal titration calorimetry experiments, fully reduced Cox17 binds one Cu(I) ion with a K(a) of (6.15 +/- 5.83) x 10(6) M(-1). The solution structures of both apo and Cu(I)-loaded Cox17 reveal two alpha helices preceded by an extensive, unstructured N-terminal region. This region is reminiscent of intrinsically unfolded proteins. The two structures are very similar overall with residues in the copper-binding region becoming more ordered in Cu(I)-loaded Cox17. Based on the NMR data, the Cu(I) ion has been modeled as two-coordinate with ligation by conserved residues Cys(23) and Cys(26). This site is similar to those observed for the Atx1 family of copper chaperones and is consistent with reported mutagenesis studies. A number of conserved, positively charged residues may interact with complementary surfaces on Sco1 and Cox11, facilitating docking and copper transfer. Taken together, these data suggest that Cox17 is not only well suited to a copper chaperone function but is specifically designed to interact with two different target proteins.     

Yeast Cox11, a protein essential for cytochrome c oxidase assembly,is a Cu(I)-binding protein

Cox11 is a protein essential for respiratory growth and has been implicated in the assembly of the Cu(B) site of cytochrome c oxidase. In the present study, we demonstrate that Cox11 is a copper-binding protein. The soluble C-terminal domain of Cox11 forms a dimer that coordinates one Cu(I) per monomer via three thiolate ligands. The two Cu(I) ions in the dimer exist in a binuclear cluster and appear to be ligated by three conserved Cys residues. Mutation of any of these Cys residues reduces Cu(I) binding and confers respiratory incompetence. Cytochrome c oxidase activity is reduced in these mutants. Thus, the residues important for Cu(I) binding correlate with in vivo function, suggesting that Cu(I) binding is important in Cox11 function.     

The P174L mutation in human Sco1 severely compromises Cox17-dependent metallation but does not impair copper binding

Sco1 is a metallochaperone that is required for copper delivery to the Cu(A) site in the CoxII subunit of cytochrome c oxidase. The only known missense mutation in human Sco1, a P174L substitution in the copper-binding domain, is associated with a fatal neonatal hepatopathy; however, the molecular basis for dysfunction of the protein is unknown. Immortalized fibroblasts from a SCO1 patient show a severe deficiency in cytochrome c oxidase activity that was partially rescued by overexpression of P174L Sco1. The mutant protein retained the ability to bind Cu(I) and Cu(II) normally when expressed in bacteria, but Cox17-mediated copper transfer was severely compromised both in vitro and in a yeast cytoplasmic assay. The corresponding P153L substitution in yeast Sco1 was impaired in suppressing the phenotype of cells harboring the weakly functional C57Y allele of Cox17; however, it was functional in sco1delta yeast when the wild-type COX17 gene was present. Pulse-chase labeling of mitochondrial translation products in SCO1 patient fibroblasts showed no change in the rate of CoxII translation, but there was a specific and rapid turnover of CoxII protein in the chase. These data indicate that the P174L mutation attenuates a transient interaction with Cox17 that is necessary for copper transfer. They further suggest that defective Cox17-mediated copper metallation of Sco1, as well as the subsequent failure of Cu(A) site maturation, is the basis for the inefficient assembly of the cytochrome c oxidase complex in SCO1 patients.     

Isolation and characterization of Cox17p from porcine heart by determining its survival-promoting activity in NIH3T3 cells

We have found that the gel filtration fraction of porcine heart extract clearly promoted the survival of NIH3T3 fibroblast cells in the serum-free medium condition. A structural analysis showed that the active fraction contained a novel peptide, porcine Cox17p (p-Cox17p), which was recently reported by Chen et al. as dopuin (Z. W. Chen et al., Eur. J. Biochem. 249 (1997) 518-522). Porcine Cox17p/dopuin possesses high sequence homology to the product of human COX17 gene (h-Cox17p). Although Cox17p has been implied to be involved in copper recruitment to mitochondria and in the functional assembly of cytochrome oxidase in yeast, its role in mammalian cells is unknown. In this study, we chemically synthesized p-Cox17p to investigate its biological effects. Refolding experiments of synthesized linear p-Cox17p revealed the existence of mostly one pattern of three intrachain disulfide bridges similar to that of native p-Cox17p, because the main oxidized p-Cox17p was completely co-eluted with the natural product. The addition of heavy metal ions such as copper, zinc and cadmium significantly inhibited the formation of the oxidized form, suggesting that reduced p-Cox17p may interact directly with these metal ions. The reduced and oxidized forms of p-Cox17p were also confirmed to promote the survival of NIH3T3 cells in serum-free medium as observed with the natural product, indicating that Cox17p may be a bioactive peptide.     

Oxidative modifications of cardiac mitochondria and inhibition of cytochrome c oxidase activity by 4-hydroxynonenal

4-hydroxynonenal (HNE) is a highly toxic product of lipid peroxidation (LPO). Its role in the inhibition of cytochrome c oxidase activity and oxidative modifications of mitochondrial lipids and proteins were investigated. The exposure of mitochondria isolated from rat heart to HNE resulted in a time- and concentration-dependent inhibition of cytochrome c oxidase activity with an IC50 value of 8.3 +/- 1.0 microM. Immunoprecipitation-Western blot analysis showed the formation of HNE adducts with cytochrome c oxidase subunit I. The loss of cytochrome c oxidase activity was also accompanied by reduced thiol group content and increased HNE-lysine fluorescence. Furthermore, there was a marked increase in conjugated diene formation indicating LPO induction by HNE. Fluorescence measurements revealed the formation of bityrosines and increased surface hydrophobicity of HNE-treated mitochondrial membranes. Superoxide dismutase + catalase and the HO* radical scavenger mannitol partially prevented inhibition of cytochrome c oxidase activity and formation of bityrosines. These findings suggest that HNE induces formation of reactive oxygen species and its damaging effect on mitochondria involves both formation of HNE-protein adducts and oxidation of membrane lipids and proteins by free radicals.     

Mutational analysis of the Saccharomyces cerevisiae cytochrome c oxidase assembly protein Cox11p

Cox11p is an integral protein of the inner mitochondrial membrane that is essential for cytochrome c oxidase assembly. The bulk of the protein is located in the intermembrane space and displays high levels of evolutionary conservation. We have analyzed a collection of site-directed and random cox11 mutants in an effort to further define essential portions of the molecule. Of the alleles studied, more than half had no apparent effect on Cox11p function. Among the respiration deficiency-encoding alleles, we identified three distinct phenotypes, which included a set of mutants with a misassembled or partially assembled cytochrome oxidase, as indicated by a blue-shifted cytochrome aa(3) peak. In addition to the shifted spectral signal, these mutants also display a specific reduction in the levels of subunit 1 (Cox1p). Two of these mutations are likely to occlude a surface pocket behind the copper-binding domain in Cox11p, based on analogy with the Sinorhizobium meliloti Cox11 solution structure, thereby suggesting that this pocket is crucial for Cox11p function. Sequential deletions of the matrix portion of Cox11p suggest that this domain is not functional beyond the residues involved in mitochondrial targeting and membrane insertion. In addition, our studies indicate that Deltacox11, like Deltasco1, displays a specific hypersensitivity to hydrogen peroxide. Our studies provide the first evidence at the level of the cytochrome oxidase holoenzyme that Cox1p is the in vivo target for Cox11p and suggest that Cox11p may also have a role in the response to hydrogen peroxide exposure.     

Folding studies of Cox17 reveal an important interplay of cysteine oxidation and copper binding

Cox17 is a key mitochondrial copper chaperone involved in the assembly of cytochrome c oxidase (COX). The NMR solution structure of the oxidized apoCox17 isoform consists of a coiled-coil conformation stabilized by two disulfide bonds involving Cys(26)/Cys(57) and Cys(36)/Cys(47). This appears to be a conserved tertiary fold of a class of proteins, localized within the mitochondrial intermembrane space, that contain a twin Cys-x(9)-Cys sequence motif. An isomerization of one disulfide bond from Cys(26)/Cys(57) to Cys(24)/Cys(57) is required prior to Cu(I) binding to form the Cu(1)Cox17 complex. Upon further oxidation of the apo-protein, a form with three disulfide bonds is obtained. The reduction of all disulfide bonds provides a molten globule form that can convert to an additional conformer capable of binding up to four Cu(I) ions in a polycopper cluster. This form of the protein is oligomeric. These properties are framed within a complete model of mitochondrial import and COX assembly.     

Protein disulfide-isomerase is a substrate for thioredoxin reductase and has thioredoxin-like activity

We have demonstrated that calf liver protein disulfide-isomerase (Mr 57,000) is a substrate for calf thymus thioredoxin reductase and catalyzes NADPH-dependent insulin disulfide reduction. This reaction can be used as a simple assay for protein disulfide-isomerase during purification in place of the classical method of reactivation of incorrectly oxidized ribonuclease A. Protein disulfide-isomerase contains two redox-active disulfides/molecule which were reduced by NADPH and calf thioredoxin reductase (Km approximately 35 microM). The isomerase was a poor substrate for NADPH and Escherichia coli thioredoxin reductase, but the addition of E. coli thioredoxin resulted in rapid reduction of two disulfides/molecule. Tryptophan fluorescence spectra were shown to monitor the redox state of protein disulfide-isomerase. Fluorescence measurements demonstrated that thioredoxin--(SH)2 reduced the disulfides of the isomerase and allowed the kinetics of the reaction to be followed; the reaction was also catalyzed by calf thioredoxin reductase. Equilibrium measurements showed that the apparent redox potential of the active site disulfide/dithiols of the thioredoxin domains of protein disulfide-isomerase was about 30 mV higher than the disulfide/dithiol of E. coli thioredoxin. Consistent with this, experiments using dithiothreitol or NADPH and thioredoxin reductase-dependent reduction and precipitation of insulin demonstrated differences between protein disulfide-isomerase and thioredoxin, thioredoxin being a better disulfide reductase but less efficient isomerase. Protein disulfide-isomerase is thus a high molecular weight member of the thioredoxin system, able to interact with both mammalian NADPH-thioredoxin reductase and reduced thioredoxin. This may be important for nascent protein disulfide formation and other thiol-dependent redox reactions in cells.     

Catalytic activity of cytochrome oxidase and cytochrome c in apolar solvents containing phospholipids and low amounts of water

Cytochrome c and cytochrome oxidase, in bovine heart submitochondrial particles and in their purified forms, were transferred to a ternary system that contained phospholipids (10 mg/ml toluene), the apolar solvent toluene, and water at concentrations of 13-15 microliters (high water) and 3 microliters (low water) per milliliter of toluene. When the enzymes were transferred back to an all water system, they exhibited full catalytic capacity. In the low water ternary system, cytochrome c could be reduced by ascorbate introduced via inverted micelles. Also in this system, cytochrome oxidase was reduced by ascorbate and cytochrome c but its oxidation was highly impaired. Data on the kinetics of reduction by ascorbate of cytochrome c and cytochrome oxidase under these conditions are presented. Cytochrome oxidase reduced in the organic solvent by ascorbate failed to form a complex with CO, but formed a complex with cyanide introduced via inverted micelles. The oxidized and the ascorbate-reduced cytochrome oxidase-cyanide complex exhibited a trough at 415 nm and a peak at 433 nm. The extent and rate of formation of the cyanide complex were higher with the reduced form of cytochrome oxidase. To achieve protein-protein interactions (cytochrome c-cytochrome oxidase) in the ternary system, it was necessary to extract the two proteins together. There was no functional interaction when they were extracted separately and mixed. In the high water ternary system reduced cytochrome oxidase was not detected, and it oxidized ascorbate at a higher rate than in the low water system; however, this rate was several orders of magnitude lower than in aqueous media.     

Solution structure of Cox11, a novel type of beta-immunoglobulin-like fold involved in CuB site formation of cytochrome c oxidase

Cytochrome c oxidase assembly process involves many accessory proteins including Cox11, which is a copper-binding protein required for Cu incorporation into the Cu(B) site of cytochrome c oxidase. In a genome wide search, a number of Cox11 homologs are found in all of the eukaryotes with complete genomes and in several Gram-negative bacteria. All of them possess a highly homologous soluble domain and contain an N-terminal fragment that anchors the protein to the membrane. An anchor-free construct of 164 amino acids was obtained from Sinorhizobium meliloti, and the first structure of this class of proteins is reported here. The apoform has an immunoglobulin-like fold with a novel type of beta-strand organization. The copper binding motif composed of two highly conserved cysteines is located on one side of the beta-barrel structure. The apoprotein is monomeric in the presence of dithiothreitol, whereas it dimerizes in the absence of the reductant. When copper(I) binds, NMR and extended x-ray absorption fine structure (EXAFS) data indicate a dimeric protein state with two thiolates bridging two copper(I) ions. The present results advance the knowledge on the poorly understood molecular aspects of cytochrome c oxidase assembly.     

Kinetic studies on the reaction between cytochrome c oxidase and ferrocytochrome c.

In stopped-flow experiments in which oxidized cytochrome c oxidase was mixed with ferrocytochrome c in the presence of a range of oxygen concentrations and in the absence and presence of cyanide, a fast phase, reflecting a rapid approach to an equilibrium, was observed. Within this phase, one or two molecules of ferrocytochrome were oxidized per haem group of cytochrome a, depending on the concentration of ferrocytochrome c used. The reasons for this are discussed in terms of a mechanism in which all electrons enter through cytochrome a, which, in turn, is in rapid equilibrium with a second site, identified with 'visible' copper (830 nm-absorbing) Cud (Beinert et al., 1971). The value of the bimolecular rate constant for the reaction between cytochromes c2+ and a3+ was between 10(6) and 10(7) M(-1)-S(-1); some variability from preparation to preparation was observed. At high ferrocytochrome c concentrations, the initial reaction of cytochrome c2+ with cytochrome a3+ could be isolated from the reaction involving the 'visible' copper and the stoicheiometry was found to approach one molecule of cytochrome c2+ oxidized for each molecule of cytochrome a3+ reduced. At low ferrocytochrome c concentrations, however, both sites (i.e. cytochrome a and Cud) were reduced simultaneously and the stoicheiometry of the initial reaction was closer to two molecules of cytochrome c2+ oxidized per molecule of cytochrome a reduced. The bleaching of the 830 nm band lagged behind or was simultaneous with the formation of the 605 nm band and does not depend on the cytochrome c concentration, whereas the extinction at the steady-state does. The time-course of the return of the 830 nm-absorbing species is much faster than the bleaching of the 605 nm-absorbing component, and parallels that of the turnover phase of cytochrome c2+ oxidation. Additions of cyanide to the oxidase preparations had no effect on the observed stoicheiometry or kinetics of the reduction of cytochrome a and 'visible' copper, but inhibited electron transfer to the other two sites, cytochrome a3 and the undetectable copper, Cuu.     

Regulation of apoptosis by the redox state of cytochrome c

Cytochrome c, released from mitochondria into the cytosol, triggers formation of the apoptosome resulting in activation of caspases. This paper reviews the evidence for and against the redox state of cytochrome c regulating apoptosis, and possible mechanisms of this. Three research groups have found that the oxidized form of cytochrome c (Fe(3+)) can induce caspase activation via the apoptosome, while the reduced form (Fe(2+)) cannot. It is unclear whether this is due to the oxidized and reduced forms of cytochrome c having: (i) different affinities for Apaf-1, (ii) different abilities to activate Apaf-1 once bound, or (iii) different affinities for other components of the cell. Experiments replacing the Fe of cytochrome c with redox-inactive metals indicate that cytochrome c does not have to change redox states to activate caspases. In healthy cells, cytosolic cytochrome c is rapidly reduced by various enzymes and/or reductants, which may function to block apoptosis. However, in apoptotic cells, cytosolic cytochrome c is rapidly oxidized by mitochondrial cytochrome oxidase, to which it has access due to permeabilization of the outer membrane. Regulation of the redox state of cytochrome c potentially enables regulation of the intrinsic pathway of apoptosis at a relatively late stage.     

The disulfide relay system of mitochondria is connected to the respiratory chain

All proteins of the intermembrane space of mitochondria are encoded by nuclear genes and synthesized in the cytosol. Many of these proteins lack presequences but are imported into mitochondria in an oxidation-driven process that relies on the activity of Mia40 and Erv1. Both factors form a disulfide relay system in which Mia40 functions as a receptor that transiently interacts with incoming polypeptides via disulfide bonds. Erv1 is a sulfhydryl oxidase that oxidizes and activates Mia40, but it has remained unclear how Erv1 itself is oxidized. Here, we show that Erv1 passes its electrons on to molecular oxygen via interaction with cytochrome c and cytochrome c oxidase. This connection to the respiratory chain increases the efficient oxidation of the relay system in mitochondria and prevents the formation of toxic hydrogen peroxide. Thus, analogous to the system in the bacterial periplasm, the disulfide relay in the intermembrane space is connected to the electron transport chain of the inner membrane.     

Oxidation-reduction and activation properties of chloroplast fructose 1,6-bisphosphatase with mutated regulatory site.

The concentration of Mg(2+) required for optimal activity of chloroplast fructose 1,6-bisphosphatase (FBPase) decreases when a disulfide, located on a flexible loop containing three conserved cysteines, is reduced by the ferredoxin/thioredoxin system. Mutation of either one of two regulatory cysteines in this loop (Cys155 and Cys174 in spinach FBPase) produces an enzyme with a S(0.5) for Mg(2+) (0.6 mM) identical to that observed for the reduced WT enzyme and significantly lower than the S(0.5) of 12.2 mM of oxidized WT enzyme. E(m) for the regulatory disulfide in WT spinach FBPase is -305 mV at pH 7.0, with an E(m) vs pH dependence of -59 mV/pH unit, from pH 5.5 to 8.5. Aerobic storage of the C174S mutant produces a nonphysiological Cys155/Cys179 disulfide, rendering the enzyme partially dependent on activation by thioredoxin. Circular dichroism spectra and thiol titrations provide supporting evidence for the formation of nonphysiological disulfide bonds. Mutation of Cys179, the third conserved cysteine, produces FBPase that behaves very much like WT enzyme but which is more rapidly activated by thioredoxin f, perhaps because the E(m) of the regulatory disulfide in the mutant has been increased to -290 mV (isopotential with thioredoxin f). Structural changes in the regulatory loop lower S(0.5) for Mg(2+) to 3.2 mM for the oxidized C179S mutant. These results indicate that opening the regulatory disulfide bridge, either through reduction or mutation, produces structural changes that greatly decrease S(0.5) for Mg(2+) and that only two of the conserved cysteines play a physiological role in regulation of FBPase.     

The effect of mitochondrial energization on cytochrome c oxidase kinetics as measured at low temperatures. I. The reaction with carbon monoxide.

The kinetics of CO binding by the cytochrome c oxidase of pigeon heart mitochondria were studied as a function of membrane energization at temperatures from 180 to 280 degrees K in an ethylene glycol/water medium. Samples energized by ATP showed acceleration of CO binding compared to those untreated or uncoupled by carbonylcyanide p-trifluoromethyoxyphenylhydrazone but only at relatively low temperatures and high CO concentrations. Experiments using samples in a "mixed valency" (partially oxidized) state showed that the acceleration of ligand binding is not due to the formation of a partially oxidized state via reverse electron transport. It is concluded that in the deenergized state one CO molecule can be closely associated with the cytochrome a3 heme site without actually being bound to the heme iron; in the energized state, two or more ligand molecules can occupy this intermediate position. The change in the apparent ligand capacity of a region near the heme iron in response to energization is evidence for an interaction between cytochrome oxidase and the ATPase system. Furthermore, these results suggest a control mechanism for O2 binding.