Current issues in the chemistry of cytochromec oxidase

Some contemporary issues relevant to the chemistry of mammalian cytochromec oxidase are discussed. These include the optical properties of heme A and the spectroscopic consequences of the differences in side-chain substitution compared to heme B; a common fallacy concerning the electrostatic exchange interaction between cytochromea ₃ and Cu_B; the question of the number and location of the copper components of the enzyme; and the mode of binding of ligands such as cyanide and azide.     

Cytochromec oxidase ofEuglena gracilis: Purification,characterization, and identification of mitochondrially synthesized subunits

Cytochromec oxidase was purified from mitochondria ofEuglena gracilis and separated into 15 different polypeptide subunits by polyacrylamide gel electrophoresis. All 15 subunits copurify through various purification procedures, and the subunit composition of the isolated enzyme is identical to that of the immunoprecipitated one. Therefore, the 15 protein subunits represent integral components of theEuglena oxidase. In anin vitro protein-synthesizing system using isolated mitochondria, polypeptides 1–3 were radioactive labeled in the presence of [³⁵S]methionine. This further identifies these polypeptides with the three largest subunits of cytochromec oxidse encoded by mitochondrial DNA in other eukaryotic organisms. By subtraction, the other 12 subunits can be assigned to nuclear genes. The isolatedEuglena oxidase was highly active withEuglena cytochromec ₅₅₈ and has monophasic kinetics. Using horse cytochromec ₅₅₀ as a substrate, activity of the isolated oxidase was rather low. These findings correlate with the oxidase activity of mitochondrial membranes. Again, reactivity was low with cytochromec ₅₅₀ and 35-fold higher with theEuglena cytochromec ₅₅₈. The data show that the cytochromec oxidase of the protistEuglena is different from other eukaryotic cytochromec oxidases in number and size of subunits, and also with regard to kinetic properties and substrate specificity.Abbreviations kDa kilodalton - PAGE polyacrylamide gel electrophoresis - SDS sodium dodecyl sulfate - TN turnover number     

Functional binding of cardiolipin to cytochromec oxidase

Bovine cytochromec oxidase usually contains 3–4 mol of tightly bound cardiolipin per cytochromeaa ₃ complex. At least two of these cardiolipins are required for full electron transport activity. Without the tightly bound cardiolipin, cytochromec oxidase has only 40–50% of its original activity when assayed in detergents that support activity, e.g., dodecyl maltoside. By measuring the restoration of electron transport activity, functional binding constants for cardiolipin and a number of cardiolipin analogues have been evaluated (K _d,app=1 µM for cardiolipin). These binding constants agree reasonably well with direct measurement of the binding using [¹⁴C]-acetyl-cardiolipin (K _d <0.1 µM) when the enzyme is solubilized with Triton X-100. These data are discussed in relationship to the wealth of data that is known about the association of cardiolipin with cytochromec oxidase and the other mitochrondrial electron transport complexes and transporters.     

Effect of trypsin on the kinetic properties of reconstituted beef heart cytochromec oxidase

Isolated beef heart cytochromec oxidase was reconstituted in liposomes by the cholate dialysis method with 85% of the binding site for cytochromec oriented to the outside. Trypsin cleaved specifically subunit VIa and half of subunit IV from the reconstituted enzyme. The kinetic properties of the reconstituted enzyme were changed by trypsin treatment if measured by the spectrophotometric assay but not by the polarographic assay. It is concluded that subunit VIa and/or subunit IV participate in the electron transport activity of cytochromec oxidase.     

Generation of reducing power in chemosynthesis. VI. Energy-linked reactions in the chemoautotroph,Thiobacillus neapolitanus

Electron donors such as thiosulfate, sulfite, and ascorbate have been shown to enter the respiratory chain ofT. neapolitanus at the level of cytochromec. The enzymatic oxidation of these substrates catalyzed by the cytochrome oxidase (E. C. 1.9.3.1.) ofT. neapolitanus cell-free extracts was coupled to the generation of energy which could be utilized to drive the reverse electron flow from cytochromec to pyridine nucleotides.The reduction of endogenous or added flavin by thiosulfate or ascorbate has been shown to be ATP-dependent; likewise the reduction of cytochromeb by these electron donors also required energy. The rate of ATP-driven reversal of electron transfer from cytochromec to the pyridine nucleotides was much faster compared with the rate of electron reversal catalyzed by the substrate-linked generated energy. The pathway of energy-linked reversal of electron transfer from cytochrome c to pyridine nucleotides involved cytochromeb and flavoproteins.NADH oxidation byT. neapolitanus cell-free extracts is mediated by the flavoprotein and cytochrome systems and this process also appears to be coupled with energy generation. The NADH oxidase (NADH₂: cytochromec oxidoreductase) was partially inhibited by amytal or rotenone, antimycin A or HOQNO, and was relatively insensitive to cyanide or azide.This investigation was supported in part by a National Science Foundation Grant No. GB 6649 and in part by the Department of Interior, Office of Water Resources Research No. A-016-KY.     

The reactions of the oxidase and reductases ofParacoccus denitrificans with cytochromesc

Electron transport in theParacoccus denitrificans respiratory chain system is considerably more rapid when it includes the membrane-bound cytochromec ₅₅₂ than with either solubleParacoccus c ₅₅₀ or bovine cytochromec; a pool function for cytochromec is not necessary. Low concentrations ofParacoccus or bovine cytochromec stimulate the oxidase activity. This observation could explain the multiphasic Scatchard plots which are obtained. A negatively charged area on the back side ofParacoccus c which is not present in mitochondrialc could be a control mechanism forParacoccus reactions.Paracoccus oxidase and reductase reactions with bovinec show the same properties as mammalian systems; and this is true ofParacoccus oxidase reactions with its own soluble cytochromec if added polycation masks the negatively charged area. Evidence for different oxidase and reductase reaction sites on cytochromec include: (1) stimulation of the oxidase but not reductase by a polycation; (2) differences in the inhibition of the oxidase and reductases by monoclonal antibodies toParacoccus cytochromec; and (3) reaction of another bacterial cytochromec withParacoccus reductases but not oxidase. Rapid electron transport occurs in cytochromec-less mutants ofParacoccus, suggesting that the reactions result from collision of diffusing complexes.     

Proteolipids. VI. the dual role of phospholipid in cytochrome oxidase reaction

After being deprived of solubilizing agent, the lipid-free cytochrome oxidase requires Triton X100 and additional phospholipid to obtain maximal activity. High levels of Triton X100 affect the interaction of phospholipid and cytochrome oxidase, thus decreasing the activity. In the terminal segment of the electron transport system, phospholipid serves not only to enhance the interaction between cytochromec and cytochromea, but also to maintain favorable molecular arrangements of reacting groups in both hemoproteins. The relationship between the enzyme activity and phospholipid content as well as the ultrastructure of the enzyme is discussed.Supported under a research grant from the National Institute for Arthritis and Metabolic Diseases AM04663.F. L. Crane is supported by career Grant K6-21, 839 from the National Institute for General Medical Research.     

Phospholipids in the cytochrome oxidase reaction

Phospholipids and Emasol activate cytochrome oxidase by increasing its affinity for its substrate, cytochromec. Cardiolipin was most effective in activating cytochrome oxidase among phospholipids tested. Prior formation of a cytochromec-cytochrome oxidase complex changes the effect of phospholipids. In addition to their structural role in the last segment of the electron transport system, phospholipids can protect the enzyme from heat treatment and mercurial inhibition. They facilitate the interaction between cytochrome oxidase and cytochromec, as well as the cytochromec analogue, protamine.     

Interactions of cytochromec with phospholipid membranes

Summary Cytochromec added during the formation of lecithin-cardiolipin liquid crystals in 0.015m KCl is readily bound. After successive washings with 0.15m KCl, only about 50% of this bound cytochromec is removed. The remaining cytochromec is resistant to further salt extraction, and the amount of this cytochromec that is bound varies with the concentration of added cytochromec to a maximum binding ratio of 170, mole ratio cytochromec to phospholipid. This binding appears to be electrostatic; it is competitively inhibited by increasing the initial molarity of KCl from 0.015 to 0.10m. Binding of cytochromec is insignificant in the absence of cardiolipin, and is affected by varying the pH. Electron microscope studies of osmium tetroxide-stained thin sections show that the liquid crystals consist of vesicles, each of which contains a large number of concentric, alternating light and dense lines. The dense lines have been identified by other workers with the polar head groups of the phospholipids on the surface of a bilayer, and the light area represents the hydrophobic interior. The addition of cytochromec causes an average decrease in the number of lines per vesicle. It increases the center-to-center distance between two neighboring light or dense lines and the width of the dense lines. On the basis of this evidence and electrostatic binding, it is concluded that cytochromec is binding on the polar surfaces of the phospholipid bilayers comprising the liquid crystalline vesicles.     

Regulation of respiration and ATP synthesis in higher organisms: Hypothesis

The present view on the regulation of respiration and ATP synthesis in higher organisms implies only Michaelis-Menten type kinetics and respiratory control as regulatory principles. Recent experimental observations, suggesting further regulatory mechanisms at respiratory chain complexes, are reviewed. A new hypothesis is presented implying regulation of respiration and ATP synthesis in higher organisms mainly via allosteric modification of respiratory chain complexes, in particular of cytochromec oxidase. The allosteric effectors, e.g., metabolites, cofactors, ions, hormones, and the membrane potential are suggested to change the activity and the coupling degree of cytochromec oxidase by binding to specific sites at nuclear coded subunits. Recent results on the structure and activity of cytochromec oxidase, supporting the hypothesis, are reviewed.Dedicated to Professor Dr. Carl Martius on the occasion of his 80th birthday.     

The effect of sulphide on cytochromeaa3 Isosteric and allosteric shifts of the reduced α-peak

1. Sulphide, like cyanide, is a slow-binding inhibitor of cytochromeaa₃ with a high affinity (K_d < 0.1 μM).2. Unlike cyanide binding, the binding of sulphide is apparently independent of the redox state of components of the oxidase other than cytochromea₃and shows no anomalous kinetics during complex formation.3. Sulphide binding to cytochrome a₃³⁺ is accompanied by a blue-shift in the α-peak of the reduced enzyme (a²⁺ a₃³⁺H₂S), similar to but smaller than that induced by azide.4. The reduced sulphide-inhibited system shows a much higher Soret peak at 445 nm than the corresponding cyanide and azide complexes, suggesting that partial electron transfer from sulphide to haem may occur in the complex. No evidence was obtained for the formation of any sulfhaem derivatives of cytochromea₃.5. The influence of energization on the spectrum of mitochondrial cytochrome oxidase, and the effects of calcium on the α-peak of isolated cytochromeaa₃ (Wikstro¨m, M. K. F. (1974) Ann. N. Y. Acad. Sci. 227, 146–158) are distinct from the action of the cytochromea₃ligands.6. A classification of peak shifts in the α-region in terms of isosteric and allosteric ligands is proposed.     

The interaction of rubroskyrin, a modified bis-anthraquinone pigment fromPenicillium islandicum Sopp, with respiratory chain of liver mitochondria

Summary  Rubroskyrin, a modified bisanthraquinone pigment from an yellow rice moldPenicillium islandicum Sopp, was examined for its redox-interaction with the mitochondrial respiratory chain by using rat liver submitochondrial particles (SMP) and was compared with luteoskyrin and rugulosin. Rubroskyrin showed a redox interaction with the NAD-linked respiratory chain of SMP, promoting NADH oxidase in the presence of rotenone, a specific inhibitor to coupling site I of the respiratory chain. Rubroskyrin-mediated NADH oxidase was not inhibited by antimycin A and cyanide, inhibitors to coupling sites II and III, respectively, indicating a generation of an electron transport shunt from a rotenone-insensitive site of NADH dehydrogenase (complex I) to dissolved oxygen. An electrontransport shunt to cytochromec oxidase from complex I was also observed in the experiment with cytochromec and antimycin A. Rubroskyrin did not interact with succinate-linked respiratory chain. Such enzymatic redox response which generates electron transport shunt was not detected for luteoskyrin and rugulosin in the present study.     

Structure of cytochromec oxidase from baker's yeast—a progress report. Preparation of four subunits for amino acid sequence determination and attempts to localize the cytochromec binding site

Summary Cytochromec oxidase from the inner membrane of yeast mitochondria consists of seven nonidentical protein subunits, three being synthesized on mitochondrial ribosomes (molecular weights I: 43 K, II: 34 K, and III: 24 K) and four being made on cytoplasmic ribosomes (molecular weights IV: 14 K, V: 12 K, VI: 12 K, and VII: 4.5 K).In the present study all four cytoplasmically synthesized subunits of the enzyme were isolated on a large scale using ion exchange chromatography and gel filtartion. Their amino acid composition as well as their amino- and carboxy-terminal amino acid residues have been determined. Sequence determinations of sub-units IV and VI are already in an advanced state. The sequence of subunit VI is characterized by a large amino-terminal stretch dominated by charged amino acid residues followed by a cluster of hydrophobic amino acids.The binding site of yeast cytochrome oxidase for cytochromec was studied by chemical crosslinking experiments. The formation of a disulfide bridge between the two proteins was observed by using cytochromec from yeast modified with 5-thionitrobenzoate at the cysteinyl residue in position 107. Alternatively, a disulfide between yeast cytochromec and the oxidase could be formed directly by oxidation with copper phenanthroline. Gel electrophoresis of the crosslinked complexes in sodium dodecyl sulfate revealed a new protein band with an apparent molecular weight of 38 K. This new band appears to be derived from cytochromec and from subunit III of cytochrome oxidase.Recipient of a fellowship from the Swiss National Science Foundation. Present address: Department of Biology, University of California at San Diego, La Jolla, Calif. 92037 (USA).     

Overexpression and Purification of CytochromecOxidase fromRhodobacter sphaeroides

Theaa₃-type cytochromecoxidase ofRhodobacter sphaeroideshas been overexpressed up to seven fold over that in wild-type strains by engineering a multicopy plasmid with all the required oxidase genes and by establishing optimum growth conditions. The two operons containing the three structural genes and two assembly genes for cytochromecoxidase were ligated into a pUC19 vector and reintroduced into several oxidase-deletedR. sphaeroidesstrains. Under conditions of relatively high pH and maximal aeration, high levels of expression were observed. A smaller expression vector, pBBR1MCS, and a fructose promoter (fruP)⁵were found not to enhance cytochromecoxidase expression inR. sphaeroides.An improved cytochromecoxidase purification protocol is reported, which combines histidine elution from a nickel affinity column and anion-exchange chromatography, and results in a higher yield and purity than previously obtained.     

The oxygen tolerance of sulfate-reducing bacteria isolated from North Sea waters

Five strains of sulfate-reducing bacteria, previously isolated from North Sea waters and identified asDesulfovibrio vulgaris, were investigated for their abilities to survive in aerobic natural seawater. Viable organisms of all strains were recoverable after exposure to oxygen for more than 72 h. The level of the protective enzymes superoxide dismutase and catalase detectable in these strains and the low rates at which oxygen was reduced probably account at least in part for their considerable abilities to survive in aerobic environments. The autoxidizable nature of cytochromec ₃ and KCN-inhibitable cytochromec oxidase activity present in these bacteria are postulated to act as possible oxygen-scavenging mechanisms analogous to the activity of NADH oxidase present in certain other strict anaerobes.     

Some properties of cytochromec' and other hemoproteins ofThiocapsa roseopersicina

Cytochromec' ofThiocapsa roseopersicina was partially purified by DEAE and Sephadex chromatography (highest purity index A 275 nm ox./A 396 nm ox.=0.54). It is autoxidizable, thermostable, and is located in the soluble fraction. The reduced cytochromec' reacts with carbon monoxide and has a γ-band at 417 nm and a shoulder at 435 nm. Cyanide (10⁻²−10⁻³ M) does not inhibit the reduction of the cytochromec' by sulfide; only at higher concentrations of cyanide did the shoulder at 435 nm disappear. When the cytochromec' is reduced by dithionite, only one broad α-band at about 550 nm appears next to the γ-band. When is is reduced by sulfide, the absorption spectrum shows an additional β-band at 521 nm. Cytochromec' is not reduced by thiosulfate.     

The metabolism ofStreptomyces griseus

The electron transfer pathway in the respiratory particles ofStreptomyces griseus was studied. Vitamins K₃ and K₅,α- andβ-naphthoquinones, served as the hydrogen acceptors in succinate oxidation, and succinate- and reduced nicotinamide adenine dinucleotide (NADH)-cytochromec reductase activities, but were ineffective for NADH oxidase activity. Vitamin K seemed to mediate the hydrogen from NADH-diaphorase to cytochromec. Chlorpromazine inhibited electron transfer in the respiratory particles. Cyanide completely inhibited the electron transfer system initially, however, oxygen consumption increased gradually with time. AlthoughS. griseus possesses cytochromesa, b, c and pigment 625 (probablyd), the electron transfer chain was complicated. Two terminal oxidase activities (cytochromec oxidase and cytochromec peroxidase activities) were detected in the respiratory particles ofS. griseus. Dedicated to Prof. Shoichiro Usami celebrating his sexagenary birthday.     

STUDIES ON COMPONENTS OF RESPIRATORY SYSTEM OF BEGONIA LEAVES

Investigations were made of the properties of diaphorase, cytochromec reductases, cytochrome c oxidase, and other components ofelectron transfer system in various fractions of leaf homogenateof Begonia semperflorens.

1. All the fractions tested showed the existence of cytochromec oxidase, succinic- and reduced diphosphopyridine nucleotide-cytochromec reductases, and diaphorase. Activities of these enzymes werefound to be associated mainly with the particulate fractions.The particulate fractions showed, in particular, a capacityof reducing oxidized cytochrome c with fumarate, malate, -ketoglutarate,ß-hydroxy-butyrate, and citrate.
2. Optimum pH foroxidation of cytochrome c by the particulatefractions was foundto be 5.5, while that for reduction was7.2.
3. The activityof cytochrome c reductase was partially suppressedby malonate.Partial inhibition of cytochrome c oxidase wascaused by azideand cyanide, the inhibitory effects observedbeing strongerwith particulate fractions than with solublefractions.

(Received August 11, 1962; )     

Isolation and characterization of human heart cytochromec oxidase

Cytochromec oxidase was isolated from human hearts and separated by SDS gel electrophoresis. The identity of polypeptide bands with known subunits was demonstrated by immunoblotting with monospecific antisera to rat liver cytochromec oxidase subunits. The polarographically determined kinetics of cytochromec oxidation were similar to those reported for the bovine heart enzyme.     

The kinetics of electron entry in cytochromec oxidase

Summary The kinetics of electron entry in beef heart cytochromec oxidase have been studied by stopped-flow spectroscopy following chemical modification of the Cu_A site with mercurials. In this derivative Cu_A is no longer reducible by cytochrome c while cytochromea may accept electrons from the latter with rates comparable to the native enzyme. The results indicate that Cu_A is not the exclusive electron entry site in cytochromec oxidase.