Identification of subunits of bovine heart cytochrome oxidase

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.

Polypeptide components of bovine heart cytochrome c oxidase. Cross-identifications in two high-resolution polyacrylamide-gel electrophoresis systems

Two-dimensional polyacrylamide-gel electrophoresis has been used to correlate polypeptide components of bovine heart cytochrome c oxidase that are resolved by two high resolution systems. The systems utilise chloral hydrate (2,2,2-trichloroethane-1,1-diol), which resolves fifteen components, and sodium dodecyl sulphate and urea, which resolves thirteen components. Seven components have been isolated and identified from their amino acid compositions in terms of polypeptides for which the amino acid sequence is known. Full resolution of all components present in this enzyme cannot be accomplished using any single-dimension system currently available.

Interactions in cytochrome oxidase: functions and structure

Mitochondrial cytochrome c oxidase is an exceedingly complex multistructural and multifunctional membranous enzyme. In this review, we will provide an overview of the many interactions of cytochrome oxidase, stressing developments not covered by the excellent monograph of Wikström, Krab, and Saraste (1981), and continuing into early 1983. First we describe its functions (both in the nominal sense, as a transporter of electrons between cytochrome c and oxygen, and in its role in energy transduction). Then we describe its structure, emphasizing the protein (its structure as a whole, the number and stoichiometry of its subunits, their biosynthetic origin, and their interactions with each other, with other components of the enzyme complex, and with the membrane as a whole). Finally, we present a model in which the protein conformation serves as the focus for the dynamic interaction of its two major functions.

Resolution of bovine heart cytochrome c oxidase into smaller complexes by controlled subunit denaturation

Bovine heart cytochrome c oxidase has been partially denaturated under mild conditions with 0.1-0.25% lithium dodecyl sulfate and 0.05% Triton X-100. From its reactivity towards CO and CN-, an unmasking of the heme a was inferred in this enzyme. The catalytic activity was lost during the denaturation and small spectral differences became visible. Spectra and ligand binding properties of the denatured enzyme were reversed by dilution in 2% Triton X-100. This suggests that during the denaturation procedure the hemes were not displaced from their original sites. By gel filtration of the partially denatured enzyme the following complexes of subunits were obtained: I-III, I-II-III, II-IV-V-VI-VII and IV-V-VI-VII. The first three complexes retained almost all the heme, and their spectral characteristics were very similar to those of the partially denatured cytochrome c oxidase. The data, in combination with the information that subunit III does not contain heme [Saraste et al. (1980) FEBS Lett. 114, 35-38], suggest that the hemes are attached to subunit I and II. After denaturation of cytochrome c oxidase under more drastic conditions some of the heme was also found to be associated with the smaller subunits, but its spectral characteristics were radically altered, becoming almost identical to those of free heme.

Amino acid sequence of bovine heart cytochrome oxidase subunit IV (Albany)

The preliminary data on the amino acid sequence of subunit IV from bovine heart cytochrome oxidase (Albany) is presented. The subunit consists of 97 amino acids linked together in a single polypeptide chain. The sequence was established by the isolation, purification and sequencing of some of the tryptic, chymotryptic and thermolytic and Staphylococcus aureus protease peptides. This subunit is present in all cytochrome oxidase preparations. It corresponds to polypeptide VIa in cytochrome oxidase (Aachen) and subunit a in cytochrome oxidase (Eugene).

Biochemical characterization of the OXI mutants of the yeast Saccharomyces cerevisiae

OXI mutants in Saccharomyces cerevisiae lack a functional cytochrome c oxidase. Wild type and OXI mutants were grown in the presence of radioactive delta-amino[14C]levulinic acid, a precursor of porphyrin and heme, and [3H]mevalonic acid, a precursor of the alkyl side-chain of heme a. SDS polyacrylamide gel electrophoresis of the delipidated mitochondria showed that delta-amino[14C]levulinic acid was distributed into three bands migrating in the regions of Mr 28 000, 13 500, and 10 000, while [3H]mevalonic acid was found in a single band with apparent Mr of 10 000. The immunoprecipitates obtained by incubating the solubilized mitochondria of any OXI mutant with antibodies against cytochrome c oxidase, showed, after delipidation, a high specific radioactivity due to delta-amino[14C]levulinic acid and [3H]mevalonic acid. This suggested that a prophyrin a was present in all these OXI mutants. HCl fractionation confirmed the presence of porphyrin a in the apooxidase of these mutants. Atomic absorption spectra of the immunoprecipitate of cytochrome c oxidase showed that copper was not detectable in the mutant OXI IIIa which lacked subunit 1, but was present in the mutant OXI IIIb, which exhibited a minor alteration in the electrophoretic mobility of subunit 1. In OXI I and II mutants there was a 50% reduction in the amount of copper in the immunoprecipitated cytochrome c oxidase. These observations may be interpretable as follows: (1) alterations in polypeptide biosynthesis due to the OXI mutations lead to an improper configuration of cytochrome c oxidase, so that ferrochelatase cannot transfer iron into porphyrin a; (2) subunit I is the binding site for copper, but the mutations in subunits II and III alter the binding site of one of the two copper atoms in subunit I.

The reactivity of thiol groups in bovine heart cytochrome c oxidase towards 5,5'-dithiobis(2-nitrobenzoic acid)

Bovine heart cytochrome c oxidase consists of 12 stoicheiometric polypeptide chains of at least 11 different types. The enzyme contains 14--16 cysteine residues; the distribution of nearly all cysteine residues over the subunits has been established. In native cytochrome c oxidase two thiol groups reacted rapidly and stoicheiometrically with 5,5'-dithiobis(2-nitrobenzoic acid) (DTNB). These thiol groups are located in subunits I and III, respectively. This implies that subunit I is not fully buried in the hydrophobic core of the enzyme. After dissociation of the enzyme by sodium dodecyl sulphate more thiol groups became available to DTNB, in addition to those in subunits I and III, at least one in subunit II, two in fraction V/VI and one to two in the smallest subunit fraction. It is shown that separation of the subunits of cytochrome c oxidase by gel permeation chromatography in the presence of sodium dodecyl sulphate depends on the pH of the elution medium. The elution volume of subunits I, III and VII is dependent on pH, that of the others independent.

Subunit stoichiometry of cytochrome c oxidase of bovine heart

Cytochrome c oxidase from bovine heart was dissociated into its protein subunits by sodium dodecylsulphate, the subunits were separated on a preparative scale by sodium dodecylsulphate gel permeation chromatography. The subunits elute upon gel chromatography in order of decreasing apparent relative molecular mass (I, 40000; II, 26000; III, 21000; IV, 17000; V and VI, 12000; VII, 10000 and VIII, 6000). The very hydrophobic subunits I and III tend to form small aggregates both in the presence and absence of sodium dodecylsulphate. The molar ratio of the subunits was determined by two methods: firstly by quantitative amino acid analysis of each subunit peak, and secondly from the absorbance of each subunit at 280 nm caused by tryptophan and tyrosine. We conclude that the subunits I to VI are present in 1 : 1 ratio; our fraction VII contains two stoichiometric polypeptides which may or may not be identical. Fraction VIII contains enough protein for four stoichiometric chains which may belong to three different types. The 12 stoichiometric chains add up to a Mr of about 170000 if the sizes of I and III are 40000 and 21000, respectively. After correction for the presence of aggregates, subunits I and III appear to be present in more than 1 : 1 stoichiometric amounts with respect to other subunits, which probably means that subunits I and III are considerably larger than hitherto assumed. This is in line with recently published mtDNA sequence work [Anderson, S. et al. (1981) Nature, 290, 457--465].

Additional components of bovine heart cytochrome c oxidase demonstrated by high-resolution polyacrylamide-gel electrophoresis in the presence of chloral hydrate

We have shown that aq. 100% (w/v) chloral hydrate (2,2,2-trichloroethane-1,1-diol) dissociates bovine heart cytochrome c oxidase. We have developed new procedures of polyacrylamide-gel electrophoresis in the presence of chloral hydrate that permit variation in the pH of the separation, and, by using these procedures, we have observed 15 components in preparations of the enzyme. This number contrasts with the eight bands that were seen on electrophoresis in the presence of SDS (sodium dodecyl sulphate) and urea. We have isolated material from these eight bands and have characterized each by electrophoresis in the presence of chloral hydrate. Twelve of the fifteen components that were seen by electrophoresis in chloral hydrate were identified as constituents of the eight bands seen by electrophoresis in the presence of SDS and urea. Two-dimensional electrophoretic separations confirmed these identifications ans showed that the other three components which were resolved as discrete bands by electrophoresis in the presence of chloral hydrate appeared to be diffusely present in the electrophoretic separations performed in the presence of SDS and urea, which suggested anomalous behaviour in that detergent. Trypsin treatment of cytochrome c oxidase caused total loss, as observed by electrophoretic separations in the presence of chloral hydrate, of a number of components. The trypsin-sensitive components included all of those that behaved anomalously in the presence of SDS and urea. Chloral hydrate is a potent non-ionic dissociating agent for cytochrome c oxidase and its use in polyacrylamide-gel electrophoresis, with variation in the pH of the gel, permits charge-dependent separations that should have general application in the analysis of membrane proteins.

Immunological and chemical characterization of rat liver cytochrome c oxidase

Rat liver cytochrome c oxidase (ferrocytochrome c: oxygen oxidoreductase; EC 1.9.3.1) was separated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis into 12 different polypeptide chains. Specific antisera against the holoenzyme and against purified subunits IV and VIII were used to characterize the enzyme complex. The antiserum against subunit IV precipitates from sodium dodecyl sulfate-dissociated mitochondria only subunit IV and from Triton X-100-dissolved mitochondria all 12 polypeptide chains, indicating their integral location within the enzyme complex. Different antisera against the holoenzyme only precipitate subunits IV, V and VIb from sodium dodecyl sulfate-dissociated mitochondria, suggesting the location of these subunits on the surface layer of the complex. Subunit VIII is thought to be located within the complex, since a specific antiserum does not precipitate the complex. The amino acid composition of all 12 protein subunits is different, thus excluding their origin from proteolytic degradation. The proteolytic degradation of subunit IV into IV during isolation of the enzyme was corroborated by the very similar amino acid composition of both proteins.

Studies on the resolution of cytochrome oxidase

Cytochrome oxidase has been resolved in acetic acid and high salt/detergent media. In 0.5% acetic acid, the smaller subunits of the enzyme are selectively extracted with retention of an insoluble protein fraction containing subunits I-IV, VII. This fraction retains all the heme and copper of the original enzyme in a spectrally unaltered state, and possesses enzymic activity comparable to the unresolved enzyme. The further removal of subunit IV from this fraction results in migration of heme and copper and modification of their spectral characteristics. Resolution of the enzyme in a high salt/detergent medium extracts smaller subunits (V-VII) together with subunit IV and some heme and copper. The heme associated with this enzymically active extract has spectral characteristics that are partially suggestive of heme a3. It is suggested that the fraction of subunits I-IV,VII, resolved in dilute acetic acid, may represent the limit of resolution of the cytochrome oxidase complex that remains actively and spectrally indistinguishable from the original enzyme.

Synthesis of cytochrome oxidase in isolated rat hepatocytes

1. The synthesis of cytochrome oxidase was studied in isolated rat hepatocytes labeled in vitro. Labeled whole cells, isolated mitochondria, microsomes and the post microsomal supernatant were treated with antisera to rat liver holo-cytochrome oxidase, and the subunits were adsorbed onto Sepharose-protein A. 2. Seven peptides, corresponding to subunits of rat liver cytochrome oxidase, were immunoabsorbed from mitochondria isolated from cells labeled in the absence of inhibitors. Two peptides, corresponding to subunits I (45 500 daltons) and II (26 000 daltons), were labeled in mitochondria isolated from cycloheximide-treated cells. Labeling of these peptides was inhibited by chloramphenicol. Peptides I and II correspond to the two most heavily labeled mitochondrial translation products found in submitochondrial particles. Possible explanations for the lack of labeling of a third mitochondrially translated subunit are discussed. Labeling of the five smallest peptides was inhibited by cyclohexamide but not by chloramphenicol. 3. Peptide I appears in the holoenzyme later than the other six peptides after a pulse-chase. It is not labeled in the immunoabsorbed cytochrome oxidase after a 30 min pulse with [35S]-methionine, but appears after a 3 h chase with unlabeled methionine. Labeling of the other subunits showed no further increase after the chase.

Cytochrome a-type terminal oxidase derived from Thiobacillus novellus. Molecular and enzymatic properties

Cytochrome a-type terminal oxidase was purified from Thiobacillus novellus to an electrophoretically homogeneous state and some of its properties were studied. The enzyme shows absorption peaks at 428 and 602 nm in the oxidized form, and at 442 and 602 nm in the reduced form. The CO compound of the reduced enzyme shows peaks at 431 and 599 nm. The enzyme has 1 mol of haem a and 1 g-atom of copper per 55600 g and is composed of two kinds of subunit, of 32000 and 23000 daltons, respectively. The enzyme reacts rapidly with tuna, bonito and yeast cytochromes c as well as with T. novellus cytochrome c, while it reacts slowly with horse and cow cytochromes c. The reduction product of oxygen catalysed by the enzyme is water.

The subunit composition of mammalian cytochrome c oxidase

Cytochrome c oxidase from rat liver mitochondria was separated into 12 different protein subunits by application of a highly resolving sodium dodecylsulfate/gel electrophoretic system of different compositions. The 12 protein subunits are shown to represent integral components of mammalian type cytochrome c oxidase for the following reasons. 1. All 12 subunits copurify through various purification procedures. 2. The subunit composition of the isolated enzyme is identical to that of the immunoprecipitated one. 3. All 12 subunits are present in the complex at one to one stoichiometric amounts. 4. A similar composition of 12 subunits was also found for cytochrome c oxidase from rat kidney, pig heart, rabbit liver and stone-marten liver. The difference between our results and all other published data on the subunit composition of mammalian-type cytochrome c oxidase, based on gel electrophoretic analysis, is due to the insufficient resolving power of previously used gel systems and the very similar molecular weight of subunits VIa, b, c, and VIIa, b, c.

Catalytic activity and arrangement of subunit polypeptides in rat liver cytochrome c oxidase as studied by proteolysis

Cytochrome c oxidase from rat liver was incubated with various proteinases of different specificities and the enzymic activity was measured after various incubation times. A loss of catalytic activity was found after digestion with proteinase K, aminopeptidase M and a mitochondrial proteinase from rat liver. In each case the decrease in enzymic activity was compared with the changes in intensities of the polypeptide pattern obtained after sodium dodecyl sulfate polyacrylamide gel electrophoresis. The susceptibilities of the subunit polypeptides of the soluble cytochrome c oxidase to proteinases were very different. Whereas subunit I was most susceptible, subunits V--VII were rather resistant to degradation. From the relative inaccessibility of subunits V--VII to proteinases it is likely that these polypeptides are buried in the interior of the enzyme complex.

The isolation of bovine-heart cytochrome c oxidase subunits. Dependence on phospholipid and cholate-content

The polypeptide chains of bovine-heart cytochrome c oxidase were preparatively isolated by a simple large-scale procedure based on gel permeation chromatography in the presence of sodium dodecyl sulphate. The resolution of the subunits as a function of the cholate and phospholipid content of the preparation was investigated. Cholate, and to a lesser extent, phospholipids interfere with the separation of the subunits; however, they do not prevent dissociation of the enzyme by SDS. Bovine-heart cytochrome c oxidase consists of six major subunits (estimated molecular weights in thousands: 40, 25, 20, 14, 12 and 10). In addition, the enzyme preparation contains at least five minor constituents, present in less than stoichiometric amounts. The first two of the three large subunits, all of which are hydrophobic, have amino-terminal N-formylmethionine. Subunit III, however, has a free methionine N-terminus.

Properties of protease-treated cytochrome c oxidase from beef heart

About 45% of the protein can be removed from oxidized cytochrome c oxidase by treatment with proteolytic enzymes under a variety of conditions, leading to an increased heme to protein ratio. The principal spectroscopic parameters of cytochrome c oxidase are retained in the protease-treated enzyme. Of the overall catalytic activity 20% remained after digestion; the electron-transfer reactions were impaired but the affinity for cytochrome c appeared unchanged. Proteolysis resulted in removal of the hydrophobic subunit III and most of the smaller hydrophilic subunits, leaving a core, which basically consists of the two largest subunits I and II. The subunits I and/or II carry the prosthetic groups of the enzyme and at least one of the cytochrome c binding sites. The smaller subunits, however, are essential for optimal electron transfer and possibly have other functions as well.

Analysis of electrophoretic behavior of cytochrome c oxidase subunits. Retardation coefficient versus molecular weight in dodecyl sulfate urea gels

1. Standard proteins were examined by electrophoresis in highly cross-linked polyacrylamide gels containing sodium dodecyl sulfate and urea. Their behavior was analyzed at a single gel concentration (by molecular weight vs. relative mobility) and at several gel concentrations (molecular weight vs. retardation coefficient). The validity of the latter method of analysis was established for this gel system. 2. Cytochrome c oxidase was subjected to analysis by this method. Compared to standards, subunits I and III showed increased free electrophoretic mobility, while that of subunit V was slightly decreased. The molecular weight values derived were: I, 44 600; II, 22 700; III, 23 500; IV, 16 900; V, 9400; VI, 7600; VII, 4300. The standard errors were all less than +/- 7%. 3. Isolated V and VI were analyzed by two dimensional dodecyl sulfate electrophoresis, in which the second dimension also contained urea. In contrast to their behavior when the holo-enzyme was examined, these isolated subunits V and VI no longer exchange migrating positions relative to each other during the two dimensional analysis. The molecular weight values of the isolated subunits agree with those of the holo-enzyme.

Electron nuclear double resonance of cytochrome oxidase: nitrogen and proton ENDOR from the 'copper' EPR signal

Proton ENDOR resonances have been found from at least two different protons with fairly large and isotropic couplings of about 12 and 19 MHz. It is possible that such protons are attached to carbons that are one bond removed from the point of ligation to copper. A number of weakly coupled protons with anisotropic couplings have also been seen. None of the protons, either weakly or strongly coupled, appears to exchange with 2H2O. We have obtained nitrogen ENDOR from at least one nitrogen with a hyperfine coupling large enough for the nitrogen to be a ligand of copper. We have not yet demonstrated experimentally ENDOR characteristic of the copper nucleus itself.

Immunoprecipitation of a cytoplasmic precursor of rat-liver cytochrome oxidase

A simple method for the isolation of rat liver cells is described. The cells are shown, by an isotope dilution method, to maintain a constant rate of protein synthesis for 8 h of incubation. Antibodies to purified rat liver cytochrome oxidase were raised in rabbits and used to investigate the labeling of cytochrome oxidase in isolated rat liver cells and in vivo. The data demonstrate the occurrence of a precursor of the subunits of cytochrome oxidase that are synthesized in the cytoplasm. 1. Dodecylsulfate gel electrophoresis of the immunoprecipitates from isolated rat liver cells that had been labeled with [35S]methionine for 1 h showed a single radioactive peak with a molecular weight of 50000. 2. Judged by the effects of cycloheximide and chloramphenicol the labeled protein is synthesized on cytoplasmic ribosomes. 3. After labeling for 1 h in vivo with [3H]leucine the labeled protein appears to be exclusively associated with the hepatic microsomal fraction. 4. Ouchterlony double-diffusion analysis demonstrated immunological relationship between the precipitates from microsomes and cytochrome oxidase. In addition to the precipitates derived from mitochondria and microsomes immunoprecipitates were also obtained from the cytosol in comparable amounts; these again were immunologically related. The occurrence of large amounts of precursor(s) (or degradation products) of cytochrome oxidase in rat liver fractions is interpreted in terms of a regulatory pool for amino acid homeostasis in the organism.

Subunit structure of the reconstitutively active cytochrome b-c1 complex. Determination of amino acids and molar distribution of subunit fractions from gel electrophoresis

A quantitative method has been developed to analyze the amino acid composition of protein subunits directly from the Coomassie Blue-stained band of polyacrylamide gel columns after electrophoresis. It is an improved method originally reported by Houston (Houston, L. L. (1971) Anal. Biochem. 44, 81--88). The results obtained can be thus used for the calculation of the molar ratios of subunit components of protein. The manipulation of the method and computation of the results are illustrated by a very complicated lipoprotein complex. The subunit molar ratios of the reconstitutively active cytochrome b-c1 complex were determined to be 2, 2, 2, 3, 2, 2, and 5 among the seven bands of the corresponding molecular weights of 53 000, 50 000, 37 000, 30 000, 28 000, 17 000, and 15 000, from gel electrophoretic columns. The amino acid composition of each subunit fraction determined directly from hydrolysis of gel was comparable with that obtained by actual isolation of each subunit.