The number of subunits in bovine cytochrome c oxidase

Evolutionary aspects of cytochrome c oxidase

The presence of additional subunits in cytochrome oxidase distinguish the multicellular eukaryotic enzyme from that of a simple unicellular bacterial enzyme. The number of these additional subunits increases with increasing evolutionary stage of the organism. Subunits I-III of the eukaryotic enzyme are related to the three bacterial subunits, and they are encoded on mitochondrial DNA. The additional subunits are nuclear encoded. Experimental evidences are presented here to indicate that the lower enzymatic activity of the mammalian enzyme is due to the presence of nuclear-coded subunits. Dissociation of some of the nuclear-coded subunits (e.g. VIa) by laurylmaltoside and anions increased the activity of the rat liver enzyme to a value similar to that of the bacterial enzyme. Further, it is shown that the intraliposomal nucleotides influence the kinetics of ferrocytochrome c oxidation by the reconstituted enzyme from bovine heart but not from P. denitrificans. The regulatory function attributed to the nuclear-coded subunits of mammalian cytochrome c oxidase is also demonstrated by the tissue-specific response of the reconstituted enzyme from bovine heart but not from bovine liver to intraliposomal ADP. These enzymes from bovine heart and liver differ in the amino acid sequences of subunits VIa, VIIa, and VIII. The results presented here are taken to indicate a regulation of cytochrome c oxidase activity by nuclear-coded subunits which act like receptors for allosteric effectors and influence the catalytic activity of the core enzyme via conformational changes.

Separation, stability and kinetics of monomeric and dimeric bovine heart cytochrome c oxidase

The stability of monomeric and dimeric bovine heart cytochrome c oxidase in laurylmaltoside-containing buffers of high ionic strength allowed separation of the two forms by gel-filtration high-performance liquid chromatography (HPLC). A solution of the dimeric oxidase could be diluted without monomerisation. Both monomeric and dimeric cytochrome c oxidase showed biphasic steady-state kinetics when assayed spectrophotometrically at low ionic strength. Thus, the biphasic kinetics did not result from negative cooperativity between the two adjacent cytochrome c binding sites of the monomers constituting the dimeric oxidase. On polyacrylamide gels in the presence of sodium dodecyl sulphate (SDS) a fraction of subunit III of the dimeric enzyme migrated as a dimer, a phenomenon not seen with the monomeric enzyme. This might suggest that in the dimeric oxidase subunit III lies on the contact surface between the protomers. If so, the presumably hydrophobic interaction between the two subunits III resisted dissociation by SDS to some extent. Addition of sufficient ascorbate and cytochrome c to the monomeric oxidase to allow a few turnovers induced slow dimerisation (on a time-scale of hours). This probably indicates that one of the transient forms arising upon reoxidation of the reduced enzyme is more easily converted to the dimeric state than the resting enzyme. Gel-filtration HPLC proved to be a useful step in small-scale purification of cytochrome c oxidase. In the presence of laurylmaltoside the monomeric oxidase eluted after the usual trace contaminants, the dimeric Complex III and the much larger Complex I. The procedure is fast and non-denaturing, although limited by the capacity of available columns.

Identification of different quaternary structures of beef heart cytochrome-c oxidase by two-dimensional polyacrylamide gel electrophoresis

A two-dimensional gel electrophoresis is described to identify different quaternary structures of the heart cytochrome-c oxidase. Bovine enzyme was purified and separated by discontinuous gradient polyacrylamide gel electrophoresis under nondenaturing conditions in the 1st dimension into several discrete complexes and thereupon shown to be heterodisperse in Triton X-100 and dodecyl maltoside. A discontinuous SDS-polyacrylamide gel electrophoresis in the 2nd dimension was used to determine the subunit composition of the isolated complexes. One of these represents the intact enzyme with 12 different polypeptides while the others have an incomplete subunit composition.

Studies on the role of the oligomeric state and subunit III of cytochrome oxidase in proton translocation

Anion-exchange fast protein liquid chromatography in the presence of lauryldimethylamine N-oxide (LDAO) was introduced to separate cytochrome oxidase into different complexes that either did or did not contain subunit III. Both kinds of enzyme complex exhibited H+ translocation after reconstitution into phospholipid vesicles, but with a significantly (approx. 50-60%) reduced H+/e- ratio as compared with unchromatographed enzyme. The anion-exchange FPLC fractions of the enzyme (with or without subunit III) sedimented more slowly than the control enzyme upon sucrose gradient centrifugation in the presence of cholate and a high potassium phosphate concentration. When the control enzyme was subjected to the sucrose gradient centrifugation in the presence of LDAO or Triton X-100, instead of cholate, one band containing all subunits was observed, which sedimented slowly like the FPLC fractions. Transfer of this band to cholate medium, and reapplication on the sucrose gradient (with cholate), yielded both a slow- and a fast-migrating band after centrifugation. Enzyme complexes that sedimented slowly or rapidly in the sucrose gradients revealed longer and shorter elution times, respectively, in gel filtration FPLC. This suggests that these complexes corresponds to monomers and dimers of cytochrome oxidase. Solubilization of proteoliposomes and subsequent sucrose gradient centrifugation in cholate yielded one fast-migrating band for the untreated enzyme, but both a fast- and a slow-migrating band for the anion-exchange FPLC-treated enzyme, which was exclusively slow-migrating before reconstitution into liposomes. It is suggested that dimerisation of monomeric cytochrome oxidase may be favoured when the enzyme encounters a membranous milieu, and that the dimeric structure might be necessary for proton translocation.

Labeling of the electron entry site of cytochrome c oxidase using 51Cr2+

51Cr2+ has been used as a probe to locate the electron entry site of bovine cytochrome c oxidase. The results of static titrations, column chromatography, and low pH LDS polyacrylamide gradient gel electrophoresis are reported. Of the protein subunits of cytochrome c oxidase, only subunit II is specifically labeled during electron transfer from Cr2+ to the electron accepting site. We therefore conclude that this site is located in subunit II. Our results provide experimental evidence to corroborate the view that this subunit is associated with redox centers of the enzyme, an hypothesis based on indirect evidence provided by the amino acid sequences and analogy with the bacterial enzyme.

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.

Properties and reconstitution of a cytochrome oxidase deficient in subunit III

Three different preparations of beef heart cytochrome oxidase (EC 1.9.3.1) were reconstituted into the membranes of artificial liposomes, and the electrical charge/electron ratios were determined for charge translocation coupled to enzymic activity. Our previously characterised subunit-III-deficient preparation, which apparently lacks H+ translocation capacity [Saraste et al. (1981) Eur. J. Biochem. 115, 261-268] has a decreased charge/electron ratio (0.9-1.0) as determined from the uptake of potassium in the presence of valinomycin, in contrast to the intact reconstituted cytochrome oxidase (1.9-2.0). A third preparation that was depleted of three minor polypeptides by trypsin treatment (these polypeptides are also removed together with subunit III using the present method), but which retains subunit III, had a K+/e- ratio of 1.5 but also a relatively low respiratory control index. The pH-dependence of the Em of cytochrome a determined in the presence of cyanide is abolished in the subunit-III-deficient enzyme. Electron transfer activities are nearly identical for the original and subunit-III-depleted enzymes at an infinite concentration of cytochrome c in a polarographic assay with supplemented phospholipids. The optical spectral properties are very similar for both preparations, but with a small shift to the blue of the alpha-peak in the modified enzyme. These results support the hypothesis that the removal of subunit III abolishes the H+-translocating function of cytochrome oxidase. This occurs by an intrinsic decoupling of H+ transport from electron transfer, and yields a preparation with only half-maximal efficiency of energy conservation.

Structural and functional properties of cytochrome c oxidase from Bacillus subtilis W23

The terminal component of the electron transport chain, cytochrome c oxidase (ferrocytochrome c: oxygen oxidoreductase) was purified from Bacillus subtilis W23. The enzyme was solubilized with alkyglucosides and purified to homogeneity by cytochrome c affinity chromatography. The enzyme showed absorption maxima at 414 nm and 598 nm in the oxidized form and at 443 nm and 601 nm in the reduced form. Upon reaction with carbon monoxide of the reduced purified enzyme the absorption maxima shifted to 431 nm and 598 nm. Sodium dodecylsulfate polyacrylamide gel electrophoresis indicated that the purified enzyme is composed out of three subunits with apparent molecular weights of 57 000, 37 000 and 21 000. This is the first report on a bacterial aa3-type oxidase containing three subunits. The functional properties of the enzyme are comparable with those of the other bacterial cytochrome c oxidases. The reaction catalyzed by this oxidase was strongly inhibited by cyanide, azide and monovalent salts. Furthermore a strong dependence of cytochrome c oxidase activity on negatively charged phospholipids was observed. Crossed immunoelectrophoresis experiments strongly indicated a transmembranal localization of cytochrome c oxidase.

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.

Quaternary structure of bovine cytochrome oxidase

A hydrodynamically homogeneous preparation of bovine mitochondrial cytochrome c oxidase can be obtained by anion-exchange chromatography of alkaline-treated enzyme, followed by a gel permeation chromatography step, which further removes some (aggregated) apoprotein. The molecular weight, Mr, of the monodisperse enzyme in Triton X-100 was found to be 210000. This complex is composed of six different polypeptides, with Mr summing up to about 110000 in toto, in a relative one-to-one stoichiometry. Two sets of these subunits constitute the 210000-Mr enzyme complex. In contrast to our earlier report [Saraste, Penttilä, Coggins, and Wikström, FEBS Lett. 114 (1980) 35-38] the 210000-Mr enzyme contains four (and not two) haems A, and therefore represents the dimer of cytochrome aa3. One of the proposed seven subunits, number III, is lacking in this enzyme preparation.

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.

Interactions of Ca2+ and H+ with heme A in cytochrome oxidase

Ca2+ ions shift the absorption spectrum of reduced cytochrome a in mitochondria by acting from the outside of the membrane. In isolated cytochrome oxidase the shift may be induced by either Ca2+ or H+, the apparent pK varying between 6.20 and 5.75 depending on the state of cytochrome a3. Studies of the Soret band show that Ca2+ also shifts the spectrum of ferrocytochrome a3 in isolated oxidase in contrast to the situation in mitochondria or isolated oxidase reconstituted into liposomes. Model studies with reduced bis-imidazole heme A reveals an analogous spectral shift induced by Ca2+. Esterification of the propionate carboxyls of heme A abolishes the spectral shift, suggesting that it is due to interaction of Ca2+ with these groups. When taken together with the data with intact mitochondria, this suggests that the propionate side chains of cytochrome a are accessible to Ca2+ and H+ from the outside of the mitochondrial membrane. In the soluble enzyme both hemes a and a3 are accessible. Thus heme a may be located near the outside of the inner membrane whereas heme a3 experiences a different environment in which no Ca2+ shift occurs.

The mechanism of proton translocation driven by the respiratory nitrate reductase complex of Escherichia coli

Low concentrations (1-50mum) of ubiquinol(1) were rapidly oxidized by spheroplasts of Escherichia coli derepressed for synthesis of nitrate reductase using either nitrate or oxygen as electron acceptor. Oxidation of ubiquinol(1) drove an outward translocation of protons with a corrected -->H(+)/2e(-) stoichiometry [Scholes & Mitchell (1970) J. Bioenerg.1, 309-323] of 1.49 when nitrate was the acceptor and 2.28 when oxygen was the acceptor. Proton translocation driven by the oxidation of added ubiquinol(1) was also observed in spheroplasts from a double quinone-deficient mutant strain AN384 (ubiA(-)menA(-)), whereas a haem-deficient mutant, strain A1004a, did not oxidize ubiquinol(1). Proton translocation was not observed if either the protonophore carbonyl cyanide m-chlorophenylhydrazone or the respiratory inhibitor 2-n-heptyl-4-hydroxyquinoline N-oxide was present. When spheroplasts oxidized Diquat radical (DQ(+)) to the oxidized species (DQ(++)) with nitrate as acceptor, nitrate was reduced to nitrite according to the reaction: [Formula: see text] and nitrite was further reduced in the reaction: [Formula: see text] Nitrite reductase activity (2) was inhibited by CO, leaving nitrate reductase activity (1) unaffected. Benzyl Viologen radical (BV(+)) is able to cross the cytoplasmic membrane and is oxidized directly by nitrate reductase to the divalent cation, BV(++). In the presence of CO, this reaction consumes two protons: [Formula: see text] The consumption of these protons could not be detected by a pH electrode in the extra-cellular bulk phase of a suspension of spheroplasts unless the cytoplasmic membrane was made permeable to protons by the addition of nigericin or tetrachlorosalicylanilide. It is concluded that the protons of eqn. (3) are consumed at the cytoplasmic aspect of the cytoplasmic membrane. Diquat radical, reduced N-methylphenazonium methosulphate and its sulphonated analogue N-methylphenazonium-3-sulphonate (PMSH) and ubiquinol(1) are all oxidized by nitrate reductase via a haem-dependent, endogenous quinone-independent, 2-n-heptyl-4-hydroxyquinoline N-oxide-sensitive pathway. Approximate-->H(+)/2e(-) stoichiometries were zero with Diquat radical, an electron donor, 1.0 with reduced N-methylphenazonium methosulphate or its sulphonated analogue, both hydride donors, and 2.0 with ubiquinol(1) (QH(2)), a hydrogen donor. It is concluded that the protons appearing in the medium are derived from the reductant and the observed-->H(+)/2e(-) stoichiometries are accounted for by the following reactions occurring at the periplasmic aspect of the cytoplasmic membrane.: [Formula: see text]

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.