The protein formula of beef heart cytochrome c 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.

Probing the high-affinity site of beef heart cytochrome c oxidase by cross-linking

A covalent complex between cytochrome c oxidase and Saccharomyces cerevisiae iso-1-cytochrome c (called caa3) has been prepared at low ionic strength. Subunit III Cys-115 of beef heart cytochrome c oxidase cross-links by disulphide bond formation to thionitrobenzoate-modified yeast cytochrome c, a derivative shown to bind into the high-affinity site for substrate [Fuller, Darley-Usmar and Capaldi (1981) Biochemistry 20, 7046-7053]. Stopped-flow experiments show that (1) covalently bound yeast cytochrome c cannot donate electrons to cytochrome oxidase, whereas oxidation of exogenously added cytochrome c and electron transfer to cytochrome a are only slightly affected; (2) the steady-state reduction levels of cytochrome c and cytochrome a in the covalent complex caa3 are higher than those found in the native aa3 enzyme. However, (3) K(m) and Vmax values obtained from the non-linear Eadie-Hofstee plots are very similar in both caa3 and aa3. The results imply that cytochrome c bound to the high-affinity site is not in a configuration optimal for electron transfer.

Structure and function of a molecular machine: cytochrome c oxidase

Cytochrome c is responsible for over 90% of the dioxygen consumption in the living cell and contributes to the build-up of a proton electrochemical gradient derived by the vectorial transfer of electrons between cytochrome c and molecular oxygen. The metal ions found in cytochrome oxidases play a crucial role in these processes and have been extensively studied. In this review we present and discuss some of the relevant spectroscopic and kinetic properties of the prosthetic groups of cytochrome c oxidase.

Cytochrome c oxidase: structural studies by electron microscopy of two-dimensional crystals

Cytochrome c oxidase is a complex integral membrane protein consisting of 13 different polypeptide chains and four metal centers having a total molecular weight of approximately 200,000 daltons. It can be isolated in two 2-dimensional crystalline forms differing in aggregation state of the enzyme. One crystal form consists of cytochrome oxidase dimers (approximately 400,000 daltons) embedded unidirectionally in the lipid bilayer of a collapsed vesicle while the other form consists of crystalline sheets of cytochrome oxidase monomers. Both crystal forms have been studied by electron microscopy during the past two decades, and this paper summarizes the results of early structural studies as well as more recent results applying techniques of cryoelectron microscopy and digital image processing. The structure of frozen-hydrated cytochrome oxidase dimers at 20 A resolution is discussed as well as the packing of monomers within dimers and the site of cytochrome c binding.

Stoichiometry and redox behaviour of metals in cytochrome-c oxidase

The early observation of extra copper in preparations of cytochrome-c oxidase has recently lead to a renewed interest in its stoichiometry and possible redox function. In various, pure preparations (heme A contents close to the theoretical value of 9.79 nmol/mg protein for the 13-subunit bovine enzyme) protein-related metal stoichiometries of 3 Cu, 2 Fe, 1 Zn, 1 Mg/monomer with M(r) 204266 were determined. Despite the presence of five potential redox metal ions, reductive and reoxidative titrations indicate the presence of only four one-electron-accepting/donating species in the ligand-free enzyme. Participation of two copper ions in a binuclear copper site acting as one-electron acceptor may explain both the observed copper stoichiometry and the redox behaviour. The homology of the C-terminal sequence of subunit II with one of the copper-binding sites in nitrous-oxide reductases provides possible ligands for complexing two copper ions in a binuclear center.

Subunits VIIa,b,c of human cytochrome c oxidase. Identification of both 'heart-type' and 'liver-type' isoforms of subunit VIIa in human heart

The N-terminal amino acid sequences and the electrophoretic mobilities of the subunits VIIa, VIIb and VIIc of cytochrome c oxidase purified from human heart were investigated and compared with those from human skeletal muscle and from bovine heart. In purified human heart cytochrome c oxidase, both so-called 'heart-type' and 'liver-type' isoforms of subunit VIIa were found. The first 30 residues of the N-terminal amino acid sequences of these 'heart-type' and 'liver-type' subunits VIIa showed nine differences. The two isoforms of subunit VIIa in human heart were present in almost equal amounts, in contrast to the situation in skeletal muscle, where the 'heart-type' subunit VIIa was predominant. Therefore, our results imply that in human heart a cytochrome c oxidase isoform pattern is present that differs from that found in skeletal muscle. Subunits VIIb and VIIc purified from human heart oxidase proved to be very similar to their bovine heart counterparts. Our direct demonstration of the presence of subunit VIIb, the sequence of which has only recently been identified in the bovine heart enzyme, suggests that human cytochrome c oxidase also contains 13 subunits. We found no evidence for the presence of different isoforms of subunit VIIc in cytochrome c oxidase from human heart and skeletal muscle. We observed clear differences in the electrophoretic mobility of the subunits VIIa,b,c between bovine and human cytochrome c oxidase. On Tricine/glycerol/SDS/polyacrylamide gels the 'heart-type' and 'liver-type' subunits VIIa present in human heart cytochrome c oxidase migrated with almost the same electrophoretic mobility. Subunit VIIb migrated only slightly faster than subunit VIIa, whereas VIIc proved to have the highest electrophoretic mobility on Tricine/SDS/glycerol/polyacrylamide gels. Our findings may have implications for the elucidation of certain tissue-specific cytochrome c oxidase deficiencies in man.

Intrinsic tryptophan phosphorescence as a marker of conformation and oxygen diffusion in purified cytochrome oxidase

Cytochrome oxidase exhibits phosphorescence from tryptophan in aqueous solution in the absence of oxygen. The lifetime for the resting reduced enzyme suspended in Tween-20 is around 30 ms at pH 8. The lifetime is longest between pH 7 and 8 and decreases with lowering of pH. Oxygen quenches the phosphorescence with a Stern-Volmer quenching constant of approximately 5 x 10(7) M-1.s-1 at 5 degrees C whereas cytochrome c has no effect. We interpret these results to indicate that room temperature tryptophan phosphorescence arises from tryptophan(s) in structured region(s) remote from the hemes and that the protein does not impose a significant barrier for the diffusion of oxygen.

Cytochrome c oxidase in Paracoccus denitrificans. Protein, chemical, structural, and evolutionary aspects

Preparations and protein chemical characterizations performed with cytochrome c oxidase (E.C. 1.9.3.1) from the purple bacterium Paracoccus denitrificans are reviewed. The simplest catalytically competent complex of the enzyme consists of two subunits of 62012 and 2799 Da. The theoretical heme a/protein ratio of the purified enzyme is 22.0 nmol/mg. The amino acid sequences of both proteins are compared with examples of subunits I and II of mitochondrial terminal oxidases from the main kingdoms of eukaryotes. The significance of the emerging conserved features such as membrane penetration patterns, invariant residues, stoichiometry, and sites of prosthetic groups are discussed. The Paracoccus enzyme represents the only prokaryotic oxidase detailed so far, which is directly related to the mitochondrial oxidases by common ancestry in the growing O2 atmosphere.

Subunit analysis of bovine cytochrome c oxidase by reverse phase high performance liquid chromatography

Bovine cytochrome c oxidase subunits were separated by reverse phase high performance liquid chromatography using a C4 column eluted with water and an acetonitrile gradient, both containing 0.1% trifluoroacetic acid. Subunits I and III precipitated in this solvent and could not be analyzed; the remaining eleven subunits were dissociated, denatured, soluble and could be resolved by elution from the column. The protein subunit eluting in each chromatographic peak was identified by a combination of polyacrylamide gel electrophoresis in sodium dodecyl sulfate, NH2-terminal amino acid sequencing, and amino acid analysis. Each subunit produced a single elution peak with the exception of subunit VIc (nomenclature of Kadenbach et al., 1983, Anal. Biochem. 129, 517-521), which eluted from the column as two well-resolved peaks. Sequence analysis showed that the two subunit VIc elution peaks resulted from partial chemical blockage of the alpha-amino serine residue of subunit VIc. The C4 reverse phase HPLC was used to document specific subunit removal from bovine cytochrome c oxidase either by tryptic digestion or by dodecyl maltoside extraction. The described HPLC method for separating cytochrome c oxidase subunits should be applicable for the analysis of other multisubunit proteins, especially other multisubunit membrane protein complexes.

Cytochrome c oxidases: polypeptide composition, role of subunits, and location of active metal centers

The general structure of the enzyme, its polypeptide composition, and a proposal for a rational nomenclature are discussed. The mitochondrially coded and bacterial cytochrome c oxidase subunits have been analyzed with more attention focused on elucidating the number of metals present in the enzyme and the ligands available for their coordination. The picture of a 2 Cu/2 Fe enzyme has been compared with that of a 3 Cu/2 Fe enzyme and a new model is proposed for the location of the metal centers in the enzyme.

Purification of cytochrome-c oxidase retaining its pulsed form

A new purification procedure for cytochrome-c oxidase from bovine heart mitochondria is described. The enzyme was purified by selective solubilization in Triton X-100 and subsequent hydroxyapatite and gel chromatography. The preparation was highly pure and active. The subunit composition and steady-state kinetics were found to be the same as those reported for other preparations. In contrast to most of the previously published protocols the method presented here resulted in a preparation which had a rapid intramolecular electron transfer from cytochrome a to cytochrome a3, i.e. it was found to have retained its pulsed state. This correlated with monoexponential cyanide-binding kinetics. The formation of resting kinetics and biphasic cyanide-binding kinetics was shown to be induced by a short incubation at pH 5.0.

Kinetic parameters of cytochrome c oxidase in rat skeletal muscle: effect of endurance training

The kinetic properties of cytochrome c oxidase (EC 1.9.3.1) in skeletal muscle tissue of sedentary and endurance-trained rats were studied. The initial velocity of the cytochrome c oxidase reaction was determined polarographically over a large range of cytochrome c concentrations and the maximal velocity (Vmax) and the Michaelis constant (Km) were calculated. The catalytic activity of cytochrome c oxidase in isolated mitochondria was also investigated. The training programme consisted of treadmill running for 2 h a day, 6 days a week, at a speed of 30 m min-1 and 30 degrees elevation, for 4 weeks. Vmax of cytochrome oxidase with respect to cytochrome c increased significantly from 254 to 310 mumol O2 min-1 g-1 protein in response to training (P less than 0.001), whereas Km remained unchanged (18.9 and 18.7 microM). The turnover number (TN) increased from 11.1 S-1 in sedentary rats to 16.6 S-1 in trained rats (P less than 0.001). The results suggest a qualitative change in the enzyme molecule in addition to a true Vmax change of cytochrome c oxidase in response to endurance training.

Characterization and application of a thermostable primary transport system: cytochrome-C oxidase from Bacillus stearothermophilus

Cytochrome-c oxidase from Bacillus stearothermophilus has been purified to homogeneity by detergent extraction followed by DEAE-cellulose, hydroxyapatite- and gel-filtration chromatography. The enzyme is a typical cytochrome-aa3-type oxidase which binds carbon monoxide and is sensitive to classical oxidase inhibitors like cyanide and azide. The purified enzyme is composed of three different subunits (57, 37 and 22 kDa). The subunit with intermediate molecular mass contains a covalently attached heme-c moiety. The enzyme appeared to be extremely thermostable (inactivation temperature = 81 degrees C). Highest turnover rates of the reconstituted enzyme were obtained with Saccharomyces cerevisiae cytochrome c or reduced forms of non-physiological electron donors like N,N,N',N'-tetramethyl-p-phenylenediamine and phenazine methosulphate. The reconstituted enzyme can generate a proton-motive force consisting of a high membrane potential and trans-membrane pH gradient. The high electro-motive force of the enzyme (delta p = -180 to -200 mV) indicates that this enzyme functions as a high-capacity electrogenic proton pump. Liposomes containing the purified thermostable and thermoactive cytochrome-c oxidase were fused with membranes from the fermentative bacterium Clostridium acetobutylicum. In the hybrid system a high proton-motive force can be generated upon oxidation of reduced N,N,N',N'-tetramethyl-p-phenylenediamine by the incorporated oxidase which subsequently can be used to drive secondary transport of amino acids. This demonstrates the applicability of the cytochrome-c oxidase to study solute transport in membranes of fermentative bacteria.

Determination of molecular mass of the aroid alternative oxidase by radiation-inactivation analysis

The functional molecular mass of the cyanide-resistant salicylhydroxamate-sensitive duroquinol oxidase activity from Sympocarpus foetidus (skunk cabbage) and Sauromatum guttatum spadix mitochondria was determined by radiation-inactivation analysis. The functional molecular mass for the oxidase activity was found to be 26,700 Da for skunk cabbage and 29,700 Da for Sauromatum guttatum mitochondria frozen at -70 degrees C. Irradiation of dried mitochondrial samples resulted in a larger target size of 38,000 Da, and in some cases, a stimulation of activity at low dose of radiation. The functional molecular mass of cytochrome c oxidase activity from skunk-cabbage and bovine heart mitochondria was also investigated. Dried and frozen mitochondrial samples from both species yielded similar target sizes, in the range 70,900-73,400 Da. Purified bovine heart cytochrome c oxidase was also irradiated, and yielded a functional molecular mass of 66,400 Da. The target size of cytochrome c oxidase agrees with literature values insofar as the target size is considerably smaller than the molecular mass of the entire complex.

Crystalline cytochrome c oxidase of bovine heart mitochondrial membrane: composition and x-ray diffraction studies

The integral mitochondrial membrane protein cytochrome c oxidase (ferrocytochrome-c:oxygen oxidoreductase, EC 1.9.3.1) was crystallized from solutions of the protein from bovine heart isolated as described earlier [Yoshikawa, S. & Caughey, W. S. (1982) J. Biol. Chem. 257, 412-420]. Crystallinity was demonstrated by x-ray diffraction. Microcrystals (tetragonal prisms, 0.02 mm in the largest dimension) were obtained in high yield with retention of activity and contained Fe, Cu, Zn, and Mg in approximate atom ratios of 1.0:1.25:0.5:0.5, respectively. Analysis of the amino acid residues and the tightly bound detergent support an apparent molecular mass of about 200 kDa, of which 150 kDa is protein (1316 +/- 66 amino acids) and 50 kDa is detergent (Brij 35). Seven major polypeptides are evident by sodium dodecyl sulfate/polyacrylamide gel electrophoresis. Adjustments in buffer concentration and other conditions have yielded much larger green crystals, hexagonal bipyramids; a crystal 0.3 X 0.5 X 0.7 mm gave x-ray diffractions as high as 8-A resolution and a space group of P6(2) or P6(4), and cell dimensions of a = b = 174.5 A, c = 282.2 A, alpha = beta = 90 degrees, and gamma = 120 degrees were obtained. A reasonable value of 3.1 A3/Da for Vm, the average space per dalton of protein in the crystal, was obtained for the asymmetric unit, which contains four irons and is a dimer of two minimal catalytic units. Cylindrical dimers (80 X 100 A) estimated from two-dimensional electron diffraction studies [Fuller, S. D., Capaldi, R. A. & Henderson, R. (1979) J. Mol. Biol. 134, 305-327] pack well in the crystal lattice with the symmetry of the space group of the crystal. The crystallization procedure developed is useful in purification of the enzyme and shows promise for the production of crystals of sufficiently high order to gain improved structural information from x-ray diffraction.

Electrophoretically monodisperse cytochrome c oxidases

A discontinuous gradient polyacrylamide gel electrophoresis under nondenaturing conditions has been used to demonstrate monodispersity of procaryotic and eucaryotic cytochrome c oxidase preparations. Alkaline treated bovine enzyme which contains nine subunits as analysed by subsequent discontinuous SDS-polyacrylamide gel electrophoresis is a monodisperse dimer in 0.1% Triton X-100 and a monomer in 0.1% dodecyl maltoside. The Mr-values corrected for bound detergent are 286,000 in Triton X-100 and 152,000 in dodecyl maltoside respectively. The two-subunit bacterial cytochrome c oxidase of Paracoccus denitrificans is proved to be a monomer with a corrected Mr of 76,000 in both nonionic detergents Triton X-100 and dodecyl maltoside.

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.

Cytochrome c oxidase is a three-copper, two-heme-A protein

Metal contents of preparations of procaryotic (Paracoccus denitrificans) and eucaryotic (beef heart) cytochrome c oxidases have been determined by inductively coupled plasma atomic emission spectroscopy and shown to be stoichiometrically related to the protein contents. The results show that oxidases which possess subunits I and II have three copper atoms besides hemes a and a3 (Paracoccus denitrificans, Cu: 2.97 +/- 0.08 and Fe: 2.09 +/- 0.10; bovine heart, Cu: 2.83 +/- 0.07 and Fe: 1.94 +/- 0.12). Together with data reported for the c1 aa3 oxidase from Thermus thermophilus, the following conclusions can be drawn. Subunit I binds two copper atoms and both hemes a and a3 and thus is the universal terminal oxidase of this spectral type. Subunit II binds one copper and functions as an electron conductor. The mitochondrial respiratory complex IV binds, in addition to three copper and two hemes a, stoichiometric amounts of magnesium and zinc (bovine heart Mg: 0.98 +/- 0.05 and Zn: 1.01 +/- 0.04).

The transmembrane helices of beef heart cytochrome oxidase

The locations of the transmembrane helices in the 12 subunits of beef heart cytochrome oxidase were predicted with a modified form of the von Heijne-Blomberg hydrophobicity scale. Based on ∼20 residues per transmembrane helix, about 480 of the estimated 660 helical residues (36.8% of 1,793 total residues) are expected to be in transmembrane helices that have their axes tilted by a small angle α from the normal to the plane of the membrane. This angle is calculated to be ∼30°, based on the observed overall tilt angle θ of 39° obtained from circular dichroism (CD) measurements on multilamellar films, or about 25°, based on the observed tilt angle θ of 36° obtained from the infrared linear dichroism of films. For 21 residues per transmembrane helix, the calculated values of α become 32° and 28°, respectively, depending upon the value of θ used. Thus, a transmembrane helical tilt angle of ∼30° accounts for the predicted transmembrane stretches in cytochrome oxidase if 20-21 residues are sufficient to span the membrane. Additional helical residues in the lipid head region may deviate by a larger angle from the normal to the plane of the membrane in cytochrome oxidase.

Fluorescein mercuric acetate specifically displaces zinc from cytochrome oxidase

Treatment of beef heart cytochrome oxidase with fluorescein mercuric acetate (FMA) was found to specifically displace zinc from the enzyme and inhibit the steady-state activity in a parallel fashion. The native cytochrome oxidase preparation contained 2.3 Cu: 2.0 Fe: 1.1 Zn: 0.9 Mg. Addition of 2 equivalents of FMA inhibited the activity by 50% and displaced 60% of the zinc from the enzyme, but did not affect the copper, iron or magnesium content. The pre-steady-state reduction of cytochrome oxidase by ferrocytochrome c was not affected by the FMA treatment, in contrast to the inhibition of steady state activity. These results suggest a possible structural or functional role for zinc in cytochrome oxidase.

Sequence homologies and structural similarities between the polypeptides of yeast and beef heart cytochrome c oxidase

The homologous polypeptides in yeast and beef heart cytochrome c oxidase have been identified by sequence comparisons and structural similarities. The properties of individual polypeptides have been used to specify which components are extrinsic, and which intrinsic and bilayer spanning, in the cytochrome c oxidase complex.

Triton X-100 induced dissociation of beef heart cytochrome c oxidase into monomers

Purified beef heart cytochrome c oxidase, when solubilized with at least 5 mg of Triton X-100/mg of protein, was found to be a monodisperse complex containing 180 molecules of bound Triton X-100 with a protein molecular weight of 200 000, a Stokes radius of 66-72 A, and an s(0)20,w = 8.70 S. These values were determined by measurement of the protein molecular weight by sedimentation equilibrium in the presence of D2O, evaluation of the sedimentation coefficient, S(0)20,w, by sedimentation velocity with correction for its dependence upon the concentration of protein and detergent, and measurement of the effective radius by calibrated Sephacryl S-300 gel chromatography. The monomeric complex was judged to be homogeneous and monodisperse since the effective mass of the complex was independent of the protein concentration throughout the sedimentation equilibrium cell and a single protein schlieren peak was observed during sedimentation velocity. These results are interpreted in terms of a fully active monomeric complex that exhibits typical biphasic cytochrome c kinetics and contains 2 heme a groups and stoichiometric amounts of the 12 subunits normally associated with cytochrome c oxidase. With lower concentrations of Triton X-100, cytochrome c oxidase dimers and higher aggregates can be present together with the monomeric complex. Monomers and dimers can be separated by sedimentation velocity but cannot be separated by Sephacryl S-300 gel filtration, probably because the size of the Triton X-100 solubilized dimer is not more than 20% larger than the Triton X-100 solubilized monomer.(ABSTRACT TRUNCATED AT 250 WORDS)

Transient kinetics of subunit-III-depleted cytochrome c oxidase

Cytochrome c oxidase from ox heart was depleted of subunit III and its transient kinetic properties studied by stopped-flow and flash photolysis. It was found that the overall mechanism of electron transfer is very similar for subunit-III-depleted and native oxidase, although significant differences in some kinetic parameters have been detected. These include the second-order rate constant for cytochrome c oxidation and the rate-limiting step of the overall process. Moreover, at low cytochrome c/oxidase ratios (where the number of reducing equivalents is insufficient), the rate of reoxidation of cytochrome a was found to be very slow, even in air, and in fact for the subunit-III-depleted enzyme is even slower than for the native oxidase. The stability of reduced cytochrome a excludes the likelihood that removal of subunit III leads to a new O2-binding site, and the result may be relevant to the lowered vectorial H+/e- stoichiometry. The subunit-III-depleted oxidase can be pulsed under appropriate conditions and its combination with CO is unchanged, as shown by kinetic experiments and difference spectroscopy.

Studies on cytochrome-c oxidase, XIII. Amino-acid sequence of the small membrane polypeptide VIIIc from bovine heart respiratory complex IV

The isolation and complete amino-acid sequence analysis of the cytoplasmically synthesized polypeptide VIIIc from bovine heart cytochrome-c oxidase is described. The protein is a stoichiometric constituent of the mitochondrial respiratory complex IV. Its primary structure is deduced from N-terminal sequencing and peptides obtained by enzymatic cleavage with Staphylococcus aureus proteinase and chemical cleavage with cyanogen bromide. The small protein consists of 56 amino acids summing up to a total Mr of 6243. From position 34 to 51 the chain contains a hydrophobic sequence of 18 residues. This probably membrane-spanning segment also contains the 2 cysteine residues of the chain. The function of this subunit in the respiratory complex IV is still unknown.