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

Respiratory chain components are required for peptidoglycan recognition protein-induced thiol depletion and killing in Bacillus subtilis and Escherichia coli

Mammalian peptidoglycan recognition proteins (PGRPs or PGLYRPs) kill bacteria through induction of synergistic oxidative, thiol, and metal stress. Tn-seq screening of Bacillus subtilis transposon insertion library revealed that mutants in the shikimate pathway of chorismate synthesis had high survival following PGLYRP4 treatment. Deletion mutants for these genes had decreased amounts of menaquinone (MK), increased resistance to killing, and attenuated depletion of thiols following PGLYRP4 treatment. These effects were reversed by MK or reproduced by inhibiting MK synthesis. Deletion of cytochrome aa₃-600 or NADH dehydrogenase (NDH) genes also increased B. subtilis resistance to PGLYRP4-induced killing and attenuated thiol depletion. PGLYRP4 treatment also inhibited B. subtilis respiration. Similarly in Escherichia coli, deletion of ubiquinone (UQ) synthesis, formate dehydrogenases (FDH), NDH-1, or cytochrome bd-I genes attenuated PGLYRP4-induced thiol depletion. PGLYRP4-induced low level of cytoplasmic membrane depolarization in B. subtilis and E. coli was likely not responsible for thiol depletion. Thus, our results show that the respiratory electron transport chain components, cytochrome aa₃-600, MK, and NDH in B. subtilis, and cytochrome bd-I, UQ, FDH-O, and NDH-1 in E. coli, are required for both PGLYRP4-induced killing and thiol depletion and indicate conservation of the PGLYRP4-induced thiol depletion and killing mechanisms in Gram-positive and Gram-negative bacteria.

Bacillus subtilis EdmS (formerly PgsE) participates in the maintenance of episomes

Extrachromosomal DNA maintenance (EDM) is an important process in molecular breeding and for various applications in the construction of genetically engineered microbes. Here we describe a novel Bacillus subtilis gene involved in EDM function called edmS (formerly pgsE). Functional gene regions were identified using molecular genetics techniques. We found that EdmS is a membrane-associated protein that is crucial for EDM. We also determined that EdmS can change a plasmid vector with an unstable replicon and worse-than-random segregation into one with better-than-random segregation, suggesting that the protein functions in the declustering and/or partitioning of episomes. EdmS has two distinct domains: an N-terminal membrane-anchoring domain and a C-terminal assembly accelerator-like structure, and mutational analysis of edmS revealed that both domains are essential for EDM. Further studies using cells of Bacillus megaterium and itsedmS (formerly capE) gene implied that EdmS has potential as a molecular probe for exploring novel EDM systems.

Isolation of a cytochrome aa(3) gene from Bradyrhizobium japonicum

Bradyhizobium japonicum strain LO501 is a Tn5-induced mutant that does not express the terminal oxidase cytochrome aa(3) (cytochrome-c oxidase, EC 1.9.3.1). Two and one-half kilobase pairs of LO501 genomic DNA that flanks the transposon was isolated and used as a hybridization probe to obtain the wild-type gene from a cosmid library. Two subcloned fragments from two of the isolated cosmids were ligated into broad host range vectors, and restriction maps of these fragments were generated. The resultant plasmids, pCA1 and pBL33, each contained DNA homologous to that mutated in strain LO501. The two plasmids were each introduced into strain LO501 by conjugal transfer, and it was found that pCA1, but not pBL33, complemented the oxidase mutant. The transconjugant strain LO501[pCA1] expressed wild-type levels of cytochrome aa(3), as discerned spectrophotometrically, and had restored N,N,N',N'-tetramethyl-p-phenylenediamine oxidase activity. Furthermore, the frequency of complementation of LO501 cells that received pCA1 by conjugation was 1.0, demonstrating that pCA1 complemented the mutant in trans. The results show that pCA1 contains the entire wild-type gene that was mutated in strain LO501, and this gene is required for cytochrome aa(3) expression.

Methylobacillus flagellatus KT contains a novel cbo-type cytochrome oxidase

The o-type oxidase from the methanol-grown obligate methylotroph Methylobacillus flagellatus KT has been purified to homogeneity. The complex is composed of four subunits (57, 40, 35 and 30 kDa). It contains six haems (4C:1B:1O) and one copper atom per molecule. It is proposed that the haem O-Cu(B) binuclear centre and a low-spin haem B are located in subunit I (57 kDa), two haems C reside in the cytochrome c homodimer (35 kDa), two haems C belong to the dihaem cytochrome c (30 kDa). The presented data provide evidence that cytochrome cbo is a novel representative of the haem-copper oxidase superfamily.

Expression, purification, and characterization of recombinant forms of membrane-bound cytochrome c-550nm from Bacillus subtilis

Bacillus subtilis expresses a cytochrome c-550nm that participates in respiratory electron transfer and is an integral membrane protein. Analysis of the B. subtilis cytochrome c-550nm amino acid sequence predicts a single N-terminal transmembrane helix attached to a water-soluble heme binding domain [C. von Wachenfeldt and L. Hederstedt (1990) J. Biol. Chem. 265, 13939-13948]. We have purified cytochrome c-550nm from wild-type B. subtilis and B. subtilis transformed with the shuttle vector pHP13 containing the gene for B. subtilis cytochrome c-550nm (cccA). In B. subtilis transformed with pHP13/cccA there is better than eightfold more membrane-bound cytochrome c-550nm than in wild-type B. subtilis. The overexpressed cytochrome c-550nm can be purified by chromatography on hydroxylapatite and Q-Sepharose media. A six-histidine tag has been added to the C-terminus of cytochrome c-550nm from B. subtilis as a further aid for purification. This strain produces cytochrome c-550nm to a level fourfold greater than wild type and allows for one-step purification using metal affinity chromatography. UV-Vis spectroscopy detects no change in the heme C spectrum due to the addition of six histidines. Neither form of B. subtilis cytochrome c-550nm is stable in its reduced state in aerated buffer, unless EDTA is added. The two forms, wild-type and his-tagged, of cytochromes c have similar midpoint redox potentials of 195 and 185 mV, respectively, and are equally good substrates for B. subtilis cytochrome c oxidase. We conclude that the addition of the histidine tag eases the purification of cytochrome c-550nm from B. subtilis plasma membranes and that the additional metal binding site does not compromise the stability or functional properties of the protein.

High-yield purification of cytochrome aa3 and cytochrome caa3 oxidases from Bacillus subtilis plasma membranes

When grown in aerated shaking culture, Bacillus subtilis expresses two different haem A-containing terminal oxidases: cytochrome aa3-quinol oxidase and cytochrome caa3 oxidase. This paper describes a high-yield conventional procedure for purifying the two haem A-containing oxidases from the same aerobic culture of Bacillus subtilis. Yields of close to 40% of the total haem A are achieved and about 6 mg of each of the purified oxidases is obtained from 4 litres of liquid culture. Both of the purified enzymes have two subunits, with apparent molecular masses of 71.6 kDa and 34.3 kDa for the cytochrome caa3 oxidase, and 67.6 kDa and 37.2 kDa for aa3-quinol oxidase. These features are in agreement with the sequence data for the corresponding structural genes in the aa3 and caa3 operons of B. subtilis. Some spectral and enzymic features of the two purified oxidases are reported that are consistent with the inclusion of both of these enzymes as members of the cytochrome oxidase superfamily.

The menaquinol oxidase of Bacillus subtilis W23

The quinol oxidase appears to be mainly responsible for the oxidation of the bacterial MKH2 in Bacillus subtilis W23 growing with either glucose or succinate. The activity of the enzyme was maximum with dimethylnaphthoquinol, a water-soluble analogue of the bacterial menaquinol. Menadiol or duroquinol were less actively respired, and naphthoquinol was not oxidized at all. After fourtyfold purification the isolated enzyme contained 5.3 mumol cytochrome aa3 per gram of protein and negligible amounts of cytochrome b and c. The turnover number based on cytochrome aa3 was about 10(3) electrons.s-1 at pH 7 and 37 degrees C. The preparation consisted mainly of a M(r) 57,000 and a M(r) 36,000 polypeptide. The N-terminal amino acid sequence of the latter polypeptide differed from that predicted by the qoxA gene of B. subtilis strain 168 (Santana et al. 1992), in that asp-14 predicted by qoxA was missing in the M(r) 36,000 polypeptide.

Molecular biology of Bacillus subtilis cytochromes

Bacillus subtilis cells must have cytochromes for growth and can synthesize cytochromes of a-, b-, c-, d-, and o-types. After a long lag, our knowledge of the structure, genetics and specific role for these cytochromes is now growing exponentially as the result of recent research. This progress is reviewed here and includes, for example, the discovery of two different cytochrome a systems and genes required for their biogenesis.

Bacillus subtilis cytochrome oxidase mutants: biochemical analysis and genetic evidence for two aa3-type oxidases

The ctaBCDEF genes coding for cytochrome c oxidase were found to reside adjacent to a regulatory gene ctaA at 127 degrees on the Bacillus subtilis chromosome. The structural genes for subunits I and II, ctaD and ctaC, were deleted by gene-replacement using a phleomycin-resistance marker. The mutant was unable to oxidize N,N,N',N'-tetramethyl-p-phenylene-diamine and oxidized cytochrome c at a significantly lower rate. Absorption spectra of the mutant and wild-type membranes confirmed the presence of two haem A-containing enzymes in B. subtilis. Another mutant, with a spontaneous deletion upstream from ctaC, was found to express neither of these enzymes. Radioactive haem-labelling was used to identify subunit II, which contains a haem C, and cytochrome c-550 among the membrane-bound c-type cytochromes of B. subtilis.

Two structurally different cytochromes c from Bacillus azotoformans: on the evolution of gram-positive bacteria

c-552 and split-alpha c-555 cytochromes from Bacillus azotoformans are classified on the basis of partial sequence information. The haem-containing polypeptides are postulated to be structurally equivalent to small IC and ID subclass cytochromes found in purple bacteria.

Bacillus subtilis expresses two kinds of haem-A-containing terminal oxidases

The expression of two different aa3-type cytochrome oxidases is demonstrated in Bacillus subtilis. One of them (denoted caa3-605), was predicted by DNA-sequencing of Bacillus cytochrome oxidase genes, but has not been found previously. It contains covalently bound haem C in subunit II and is very similar to the enzyme previously described in the thermophilic bacterium PS3. The other oxidase (denoted aa3-600) deviates from most known oxidases of aa3 type, and is probably identical with the oxidase described by de Vrij et al. [de Vrij, W., Azzi, A. & Konings, W. N. (1983) Eur. J. Biochem. 131, 97-103]. It shows no immunological cross-reactivity to the PS3 enzyme and differs from this spectroscopically; it contains no CuA and does not oxidise cytochrome c despite of its haem-A chromophores. It catalyses oxidation of quinols, which is proposed to be its physiological function.

The Bacillus subtilis cytochrome-c oxidase. Variations on a conserved protein theme

The structural genes of cytochrome-c oxidase in Bacillus subtilis have been isolated and sequenced. Five genes, ctaB-F, are closely spaced. ctaC, ctaD, ctaE and ctaF are the genes for subunits II, I, III and IVB, respectively, ctaB, which may encode an assembly factor, is separated and upstream from the others. In comparison to its mitochondrial counterparts, subunit I has an extended C-terminus with two additional transmembrane segments, whereas subunit III has lost two such segments from its N-terminus. The C-terminal extension in subunit II is a covalent cytochrome-c domain, previously characterized only in the thermophilic oxidases. Subunit IVB, a small hydrophobic protein, is a novel subunit. These predictions suggest that the B. subtilis cytochrome-c oxidase is structurally more related to the four-subunit Escherichia coli cytochrome-bo complex than, for instance, to the Paracoccus denitrificans enzyme. Cytochrome aa3, which was previously isolated from B. subtilis [de Vrij, W., Azzi, A. & Konings, W. N. (1983) Eur. J. Biochem. 131, 97-103] is not encoded by the ctaC-F genes; thus, there seems to be two different cytochrome-aa3-type oxidases in this Gram-positive bacterium.

Purification and characterization of two-subunit cytochrome aa3 from Bacillus cereus

Cytochrome c-oxidase type aa3 (EC 1.9.3.1) was purified to homogeneity from vegetative Bacillus cereus by ion-exchange and hydroxylapatite chromatography in the presence of Triton X-100. Gel filtration analysis suggested a dimeric structure apparently 172 kDa in size; however, only a monomer of 81 kDa was detected when analysed by non-denaturing gel electrophoresis. Denaturing gel electrophoresis analysis of the protein showed the presence of two subunits (51 and 30 kDa). Atomic absorption and visible spectroscopy showed typical aa3 redox centres with haem a iron and copper in a ratio of 22 nmol and 35 ng-atom per mg protein, respectively. No haem c was found associated with the purified enzyme in the conditions reported here. Oxidase activity was fully reconstituted by phospholipids in the presence of N,N,N',N'-tetramethyl-p-phenylenediamine or reduced yeast cytochrome c (but not horse cytochrome c) as electron donors. This activity was abolished by cyanide and carbon monoxide.

A fourth subunit is present in cytochrome c oxidase from the thermophilic bacterium PS3

A new putative subunit was found in cytochrome c oxidase (aa3-type) of the thermophilic bacterium PS3. The N-terminal amino acid sequence of this approximately 12 kDa protein coincides with the deduced sequence of an open reading frame found downstream from the gene encoding subunit I of the PS3 cytochrome oxidase [(1988) J. Biochem. 103, 606-610]. This small hydrophobic protein, composed of 109 amino acid residues after the initial methionine residue has been processed, shows homology with the subunit IV (cyoD product) of cytochrome bo-type quinol oxidase of Escherichia coli.

Isolation and sequence of ctaA, a gene required for cytochrome aa3 biosynthesis and sporulation in Bacillus subtilis

Cytochrome aa3 is one of two terminal oxidase complexes in the Bacillus subtilis electron transport chain. A novel genetic strategy was devised which permitted the isolation of B. subtilis mutants lacking cytochrome aa3 by selection for streptomycin-resistant clones which failed to oxidize the artificial electron donor N,N,N',N'-tetramethyl-p-phenylenediamine. Two mutations were studied intensively. Spectroscopic examination showed that each mutant lacked cytochrome aa3; they were also asporogenous and unable to grow on lactate as the sole carbon and energy source. These mutations were mapped to a locus designated ctaA, located at 127 degrees between pyrD and metC on the B. subtilis chromosome. Both ctaA mutations were closely linked by transformation to the pycA locus. The ctaA locus and a portion of the pycA locus were cloned from a B. subtilis integration library constructed in Escherichia coli. A recombinant plasmid containing a 4.0-kilobase insert of B. subtilis DNA could transform both ctaA mutants to CtaA+. Gene disruption and complementation experiments with subcloned fragments revealed that the ctaA locus consisted of a single transcriptional unit about 1.35 kilobase pairs in size. The nucleotide sequence of the ctaA transcriptional unit contains a single open reading frame capable of coding for a protein with a predicted molecular weight of 34,065. The predicted protein is extremely hydrophobic, with several probable membrane-spanning domains. No sequence similiarity was found between ctaA and the highly conserved procaryotic and mitochondrial oxidase polypeptides. Cloning and sequence analysis of two ctaA mutations revealed that one allele is a nonsense mutation in the carboxy terminus and the other is a missense mutation in the amino terminus; this indicates that the pleiotropic phenotype conferred by each mutation was caused by loss of CtaA or of its activity. Genetic evidence suggests that the ctaA gene product is required as an accessory protein in the genetic expression, posttranslational biogenesis, or both, of the cytochrome aa3 complex and during an early stage of sporogenesis.

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.

Comparative study of energy-transducing properties of cytoplasmic membranes from mesophilic and thermophilic Bacillus species

The properties of enzymes involved in energy transduction from a mesophilic (Bacillus subtilis) and a thermophilic (B. stearothermophilus) bacterium were compared. Membrane preparations of the two organisms contained dehydrogenases for NADH, succinate, L-alpha-glycerophosphate, and L-lactate. Maximum NADH and cytochrome c oxidation rates were obtained at the respective growth temperatures of the two bacteria. The enzymes involved in the oxidation reactions in membranes of the thermophilic species were more thermostable than those of the mesophilic species. The apparent microviscosities of the two membrane preparations were studied at different temperatures. At the respective optimal growth temperatures, the apparent microviscosities of the membranes of the two organisms were remarkably similar. The transition from the gel to the liquid-crystalline state occurred at different temperatures in the two species. In the two species, the oxidation of physiological (NADH) and nonphysiological (N,N,N',N'-tetramethyl-p-phenylenediamine or phenazine methosulfate) electron donors led to generation of a proton motive force which varied strongly with temperature. At increasing temperatures, the efficiency of energy transduction declined because of increasing H+ permeability. At the growth temperature, the efficiency of energy transduction was lower in B. stearothermophilus than in the mesophilic species. Extremely high respiratory activities enabled B. stearothermophilus to maintain a high proton motive force at elevated temperatures. The pH dependence of proton motive force generation appeared to be similar in the two membrane preparations. The highest proton motive forces were generated at low external pH, mainly because of a high pH gradient. At increasing external pH, the proton motive force declined.

Immunological and spectral characterization of partly purified cytochrome oxidase from the cyanobacterium Synechocystis 6714

Membranes were isolated by French pressure cell extrusion of lysozyme-preincubated cells of the cyanobacterium Synechocystis 6714 after growth in the presence of 0.4 M NaCl for 4 days. These cells showed up to 6-fold respiratory activity (oxygen uptake) when compared to control cells. Separation of plasma and thylakoid membranes revealed that the major part of cytochrome c oxidase was associated with the latter. Immunoblotting of sodium dodecylsulfate polyacrylamide gel electrophorized membranes with antisera raised against subunit I, subunit II, and the holoenzyme of the aa3-type cytochrome oxidase from Paracoccus denitrificans gave specific and complementary cross-reactions at apparent molecular weights of about 25 and 17-18 kDa, respectively. Crude membranes were solubilized also with n-octyl glucoside, and the cytochrome oxidase was separated from the extract by affinity chromatography using immobilized cytochrome c from Saccharomyces cerevisiae. The enzyme was eluted with KCl/octyl glucoside. Dialysed and concentrated enzyme solution, which was free of b- and c-type cytochromes, gave reduced alpha- and gamma-peaks around 603 and 443 nm, respectively. Upon treatment of the sample with carbon monoxide the peaks were found at 593 and 433 nm, respectively. Photodissociation spectra of the CO-complexed enzyme were in full agreement with cytochrome aa3 being a functional cytochrome oxidase in Synechocystis 6714.

Isolation and partial characterization of the cytochrome c oxidase of Micrococcus luteus (lysodeikticus)

The cell membrane of Micrococcus luteus (lysodeikticus) contains a respiratory chain composed of hemes a, b, and c, which contain 171, 457, and 407 pmol/mg protein, respectively. Cytochrome c oxidase, the heme a containing component, has been purified after solubilization in Triton X-100, by gel filtration on Sepharose 4B-CL ammonium sulfate precipitation and ion-exchange and affinity chromatographies on a yeast cytochrome c-Sepharose 4B column. The purified complex, which contains three polypeptides of apparent Mr 47,000, 31,000, and 19,000, has CN-sensitive ferrocytochrome c oxidase activity (Ki = 0.35 microM) and a characteristic absorption spectrum with maxima in the oxidized form at 595 and 426 nm and in the reduced form at 601 and 444 nm. The purified enzyme contains 17.4 nmol/mg protein and its copper content is 23.2 nmol/mg protein. The enzyme was purified about 100-fold with respect to its content in crude membranes. The total heme a yield, also with respect to crude membranes content, was 6.8%.

Kinetic characterization of cytochrome c oxidase from Bacillus subtilis

Bacillus subtilis aa3-type cytochrome c oxidase is capable of oxidizing cytochrome c from different origins. The kinetic properties of the enzyme are influenced by ionic strength. The affinity for Saccharomyces cerevisiae cytochrome c declines with increasing ionic strength whereas the Vmax remains almost constant. An increase of Vmax is observed when the enzyme is incorporated in artificial membranes. Negatively charged phospholipids allow high turnover rates of the aa3-type oxidase. The effect of ionic strength on oxidation of horse heart cytochrome c results in significant changes of both Km and Vmax. These effects can be explained by disturbances of enzyme-substrate interactions and are not related to changes in the aggregation state of the enzyme. The respiration control index of the enzyme reconstituted in artificial membranes appeared to be dependent on phospholipid composition, protein/lipid ratios and also on the external pH. The action of the ionophores nigericin and valinomycin, at various pH values, on the enzyme activity and proton-permeability measurements of the membranes indicate that both components of the proton-motive force, the membrane potential and the pH gradient, can in principle regulate enzyme activity in the reconstituted state.

Spectral and potentiometric analysis of cytochromes from Bacillus subtilis

Bacillus subtilis cytoplasmic membranes contain several cytochromes which are linked to the respiratory chain. At least six different cytochromes have been separated and identified by ammonium sulphate fractionation and ion-exchange chromatography. They include two terminal oxidases with CO-binding properties and cyanide sensitivity. One of these is an aa3-type cytochrome c oxidase which has characteristic absorption maxima in the reduced-oxidized difference spectrum at 601 nm in the alpha-band and at 443 nm in the Soret band regions. In the alpha-band two separate electron transitions with Em = +205 mV and Em = +335 mV can be discriminated by redox potentiometric titration. The other CO-binding cytochrome c oxidase contains two cytochrome b components with alpha-band maxima at 556 nm and 559 nm. Cytochrome b556 can be reduced by ascorbate and has an Em + +215 mV, whereas cytochrome b559 has an Em = +140 mV. Furthermore a complex consisting of a cytochrome b564 (Em = +140 mV) associated with a cytochrome c554 (Em = +250 mV) was found. This cytochrome c554, which can be reduced by ascorbate, appears to have an asymmetrical alpha-peak and stains for heme-catalyzed peroxidase activity on SDS-containing polyacrylamide gels. A protein with a molecular mass of about 30 kDa is responsible for this activity. A cytochrome b559 (Em = +65 mV) appears to be an essential part of succinate dehydrogenase. Finally a cytochrome c550 component with an apparent mid-point potential of Em = +195 mV has been detected.

Homology between bacterial DNA and bovine mitochondrial DNA encoding cytochrome c oxidase subunit III

A segment of mitochondrial DNA encoding the bovine cytochrome c oxidase subunit III gene was isolated and inserted into an Escherichia coli plasmid vector. A 556 base pair fragment of the insert DNA representing about 70% of the 3'-end of the subunit III gene was used to search for homology with bacterial DNA from strains that contain heme aa3-type cytochrome c oxidases. Bacillus subtilis, Thermus thermophilus, and PS3 DNAs all showed strong hybridization to the probe, whereas Paracoccus denitrificans and Rhodopseudomonas sphaeroides DNAs showed only weak hybridization to the probe, even under low stringency conditions.

Measurements of the proton motive force generated by cytochrome c oxidase from Bacillus subtilis in proteoliposomes and membrane vesicles

Cytochrome c oxidase from Bacillus subtilis was reconstituted in liposomes and its energy-transducing properties were studied. The reconstitution procedure used included Ca2+-induced fusion of pre-formed membranes. The orientation of the enzyme in liposomes is influenced by the phospholipid composition of the membrane. Negatively charged phospholipids are essential for high oxidase activity and respiratory control. Analyses of the proteoliposomes by gel filtration, density gradient centrifugation and electron microscopy indicated a heterogeneity of the proteoliposomes with respect to size and respiratory control. Cytochrome c oxidase activity in the proteoliposomes resulted in the generation of a proton motive force, internally negative and alkaline. In the presence of the electron donor, ascorbate/N,N,N',N'-tetramethyl-p-phenylenediamine/cytochrome c or ascorbate/phenazine methosulphate, the reconstituted enzyme generated an electrical potential of 84 mV which was increased by the addition of nigericin to 95 mV and a pH gradient of 32 mV which was increased by the addition of valinomycin to 39 mV. Similar results were obtained with beef-heart cytochrome c oxidase reconstituted in liposomes. The maximal proton motive force which could be generated, assuming no endogenous ion leakage, varied over 110-140 mV. From this the efficiency of energy transduction by cytochrome c oxidase was calculated to be 18-23%, indicating that the oxidase is an efficient proton-motive-force-generating system.

Structure and function of cytochrome-c oxidase

Recent works on the structure and the function of cytochrome-c oxidase are reviewed. The subunit composition of the mitochondrial enzyme depends on the species and is comprised of between 5 and 13 subunits. It is reduced to 1 to 3 subunits in prokaryotes. The complete amino acid composition has been derived from protein sequencing. Gene sequences are partially known in several eukaryote species. Metal centers are only located in subunits I and II. The mitochondrial cytochrome-c oxidase is Y-shaped; the arms of the Y cross the inner membrane, the stalk protrudes into the intermembrane space. The bacterial enzyme has a simpler, elongated shape. A number of data have been accumulated on the subunit topology and on their location within the protein. All available spectrometric techniques have been used to investigate the environment of the metal centers as well as their interactions. From the literature, attention must be paid to what may be considered or not as an active form. The steady improvement of the instrumentation has yielded evidence for different kinds of heterogeneities which could reflect the in vivo situation. The 'pulsed' and 'resting' conformers have been well characterized. The 'oxygenated' form has been identified as a peroxide derivative of the fully oxidized cytochrome-c oxidase. The mammalian enzyme has been isolated in fully active monomeric form which does not preclude the initially suggested dimeric behavior in situ. The role of the lipids is still largely investigated, mainly through reconstitution experiments. Kinetic studies of electron transfer between cytochrome c and cytochrome-c oxidase lead to a single catalytic site model to account for the multiphasic kinetics. Results related to the low temperature investigation of the intermediate steps in the reaction between oxygen and cytochrome-c oxidase received a sound confirmation by the resolution of compound A at room temperature. It is also pointed out that the so-called mixed valence state might not be a transient state in the catalytic reduction of oxygen. The functioning of cytochrome-c oxidase as a proton pump has been supported by a number of experimental results. Subunit III would be involved in this process. The redox link to the proton pump has been suggested to be at the Fea-CuA site. The molecular mechanism responsible for the proton pumping is still unknown.

A comparative survey of several bacterial aa3-type cytochrome c oxidases

The aa3-type cytochrome c oxidases purified from Nitrobacter agilis, Thiobacillus novellus, Nitrosomonas europaea, and Pseudomonas AM 1 were compared. They have haem a and copper atom as the prosthertic groups and show alpha and gamma absorption peaks at around 600 and 440 nm, respectively. Each oxidase molecule is composed of two kinds of subunits. The N. agilis oxidase has 2 moles of haem a and 2 atoms of copper in the minimal structural unit composed of one molecule each of the two kinds of subunits, while the T. novellus enzyme seems to contain one molecule of the haem and one atom of the metal in the unit. The N. europaea oxidase shows very low affinity for carbon monoxide. Each oxidase reacts rapidly with some eukaryotic cytochromes c as well as with its native cytochrome c. The cytochrome c oxidase activity of the N. agilis oxidase is 50% inhibited by 1 microM KCN, while 50% inhibition of the activity requires 100 microM KCN in the case of the N. europaea enzyme.

Kinetics of cytochrome c and TMPD oxidation by cytochrome c oxidase from the thermophilic bacterium, PS3

Cytochrome caa3 (cytochrome oxidase) from the thermophilic bacterium PS3 can exhibit full catalytic activity in the presence of ascorbate and TMPD or other electron donors and in the absence of added soluble c-type cytochromes. It appears to possess only a low-affinity and not a high-affinity site for the soluble cytochromes. Proteoliposomal cytochrome caa3 develops an effective membrane potential in the presence of ascorbate and TMPD or PMS, in the absence of added soluble cytochrome c. Reduction of the a3 centre is blocked in the presence of cyanide. During reductive titrations of the cyanide-inhibited enzyme, electrons initially equilibrate among three centres, the c haem, the a haem and one of the associated Cu atoms. During steady-state turnover, electrons probably enter the complex via the bound c haem; the a haem and perhaps an associated CuA atom are reduced next. It is concluded that, despite its size and hydrophobic association with the aa3 complex, the haem c-containing subunit can behave in an analogous way to that of mammalian cytochrome c, bound at the high-affinity site of the eucaryotic enzyme.

Purification and partial characterization of two cytochrome oxidases (caa3 and o) from the thermophilic bacterium PS3

Two cytochrome oxidases, cytochrome aa3 (EC 1.9.3.1) and cytochrome o, have been purified from the membranes of a thermophilic bacterium, PS3. The enzymes were solubilized with Triton X-100 and purified to apparent homogeneity on anion-exchange columns. The properties of the three-subunit cytochrome oxidase complex caa3 obtained here are compared with the same enzyme isolated by Sone, N. and Yanagita, Y. (1982) (Biochim. Biophys. Acta 682, 216-226). On storage, the purified caa3 enzyme undergoes denaturation; a shoulder at 432 nm seen in (CO-reduced)-minus-reduced difference spectra may be due in part to denaturation products of the enzyme. The purified cytochrome o is more stable. At room temperature, the reduced-minus-oxidized difference spectrum shows absorbance maxima at 427 and 559 nm; at 77 K, its alpha-band is split into 554 and 557 nm components. At room temperature, the CO-reduced-minus-reduced spectrum shows troughs at 430 nm and 560 nm. Dissociating polyacrylamide gel electrophoresis suggests that the purified cytochrome o is composed of one type of subunit with an apparent molecular mass of 47 000-48 000. Metal analysis of the purified enzyme demonstrated the lack of copper. Both oxidases, purified in the presence of Triton X-100, exist in highly polydisperse forms.

Purification and characterization of the cytochrome oxidase from alkalophilic Bacillus firmus RAB

A cytochrome oxidase was purified 52-fold from membranes of alkalophilic Bacillus firmus RAB by extraction with Triton X-100, ion-exchange and hydroxyapatite chromatography, and gel filtration. On denaturing gels, the purified enzyme dissociated into two subunits of 56,000 and 40,000 Mr as well as a cytochrome c with an Mr of approximately 14,000. Heme contents calculated for an enzyme with a molecular weight of 110,000 were found to be 2 mol of heme a and 1 mol of heme c per mol of cytochrome oxidase; approximately 2 mol of copper per mol of purified enzyme was also found. Enzyme activity was observed in assays using reduced yeast or horse heart cytochrome c. Activity of the purified enzyme was optimal at pH 6.0 and in the presence of added lipids. Impure, membrane-associated activity exhibited a broader pH range for optimal activity extending to alkaline values.

Interaction of reduced and oxidized cytochrome c with the mitochondrial cytochrome c oxidase and bc1-complex

Making use of a hetero-bifunctional reagent (succinimidyl 4-(p-male-imidophenyl)butyrate, SMPB), yeast cytochrome c was linked through a thioether bond to the maleimide group whereas the active N-hydroxy-succinimide ester site of the SMPB was used for the reaction with the primary amino groups of Affi-gel 102. The capacity and stability (also to reducing agents) of the column were greatly improved relative to previous systems. This new gel allowed the study of the interactions of cytochrome c oxidase and reductase with reduced and oxidized cytochrome c. For cytochrome c oxidase a significant difference in the interaction with ferri- and ferro-cytochrome c was observed but no such a difference was seen in the case of cytochrome c reductase.