The oxidation-reduction potentials of cytochrome o + c4 and cytochrome o purified from Azotobacter vinelandii.

Oxidation-reduction titrations of Azotobacter vinelandii cytochrome o + c4 and cytochrome o were performed with simultaneous potential and absorbance measurements under anaerobic conditions. Cytochrome c4 has a midpoint potential (Em, 7.4) of 260mV and purified cytochrome o has an Em, 7.4 of -18mV. Little change in the midpoint potential of cytochrome o was observed when titrated in the pH range 6.2--9.8.     

Oxygenated cytochrome o (Vitreoscilla) formed by treating oxidized cytochrome with superoxide anion

Oxygenated cytochrome o can be formed experimentally in twoways, i) by reaction of reduced cytochrome o with molecularoxygen, or ii) by reaction of oxidized cytochrome o with superoxideanion generated by the action of the xanthine oxidase system.It is thermodynamically feasible for oxidized cytochrome o plusO₂–, and reduced cytochrome o plus O₂ to appear as intermediatesin reactions i) and ii), respectively. Superoxide dismutase completely inhibits the xanthine oxidase-catalyzedconversion of oxidized cytochrome o into the oxygenated formbut it has relatively little effect on the oxygenated cytochromeo formation in the reaction system consisting of NADH, NADH-cytochromeo reductase, and cytochrome o. Thus, if superoxide anion doesplay a significant role in the latter system it must be efficientlycoupled to react with cytochrome o and inaccessible to superoxidedismutase. Direct electron transfer from the reductase to thecytochrome without the involvement of superoxide anion is analternative mechanism. (Received December 16, 1976; )     

Purification and characterization of cytochrome o from Azotobacter vinelandii

The membrane-bound cytochrome o has been solubilized from the Azotobacter vinelandii electron transport particle and further purified by use of conventional chromatographic procedures. The spectral characteristics as well as the other properties noted for purified cytochrome o are reported herein.     

pH probes respond to redox changes in cytochrome o

N-Phenylnaphthylamine (NPN) has been used previously to probe the fluidity or microviscosity of membrane lipids. We have shown (Sedgwick, E. G., and Bragg, P.D. (1988) FEBS Lett. 229, 127-130) that the fluorescence intensity of this probe abruptly increases upon depletion of the oxygen content of a medium by respiring cytochrome o of Escherichia coli that has been incorporated into soybean phospholipid vesicles. We now show that the pH probes pyranine and quinacrine behave similarly to NPN. The fluorescence change is not due to changes in the pH gradient across the membrane or to a change in the distribution of probe between the vesicles and the external medium. It is insensitive to uncouplers. The fluorescence change with pyranine and quinacrine occurs also with soluble cytochrome o in the absence of added phospholipid. The NPN response requires added phospholipid. Alteration of the redox state of cytochrome o with cyanide suggests that these probes respond to a change in the redox state of the cytochrome, either by alterations in binding of the probe to the cytochrome or by a change in the environment of the probe bound to the cytochrome. This behavior should be considered when pyranine or quinacrine are used to measure changes in the internal pH of membrane vesicles containing redox proteins.     

Biochemical and biophysical properties of cytochrome o of Azotobacter vinelandii

Cytochrome o, solubilized from the membrane of Azotobacter vinelandii, has been purified to homogeneity as judged by ultracentrifugation and polyacrylamide gel electrophoresis. The detergent-containing cytochrome o is composed of one polypeptide chain with a molecular weight of 28 000-29 000, associated by polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate. The enzyme exists as a dimer by gel filtration analysis. The amino analysis which reveals the majority of residues are of hydrophobic nature. The cytochrome o oxidase contains protoheme as its prosthetic group and about 20-40% of phospholipids. The phospholipids are identified as phosphatidylethanolamine and phosphatidylglycerol by radioautographic analysis using 2-dimensional thin-layer chromatography. No copper or nonheme iron can be detected in the purified oxidase preparation by atomic absorption and chemical analyses. Oxidation-reduction titration shows this membrane-bound cytochrome o to be a low-potential component, and Em was determined to be -18 mV in the purified form and -30 mV in the membrane-bound form. Both forms bind CO with a reduced absorption peak at 559 and 557-558 nm in the native and solubilized forms, respectively. A high-spin (g = 6.0) form is assigned to the oxidized cytochrome o by electron paramagnetic resonance analysis, and KCN abolishes this high-spin signal. CO titration of purified cytochrome o in the anaerobic conditions shows the enzyme binds one CO per four protohemes and a dissociation constant is estimated to be 3.2 microM for CO. Cyanide reacts with purified cytochrome o in both oxidized and CO-bound forms, identified by specific spectral compounds absorbed at the Soret region. Cytochrome c, often co-purified with cytochrome c from the membrane, cannot serve as a reductant for cytochrome o in vitro, due to the apparent potential difference of about 300 mV. Upon separation, both cytochrome o and cytochrome c4 show a great tendency of aggregation. Furthermore, the oxidase activity (measured by tetramethyl-p-phenylenediamine oxidation rate) decreases as the cytochrome c concentration is decreased by ammonium sulfate fractionation. All these suggest the structural and functional complex nature of cytochrome c4 and cytochrome o in the membrane of A. vinelandii.     

Immunological characterization of the cytochrome o terminal oxidase from Escherichia coli

The cytochrome o terminal oxidase from Escherichia coli was immunochemically purified and monospecific antiserum toward cytochrome o was obtained. This antiserum is able to precipitate 100% of the ubiquinol-1 oxidase activity in Triton X-100 extracts of membranes from an E. coli strain in which cytochrome o is the only terminal oxidase. Cytochrome o was analyzed and quantitated using crossed immunoelectrophoresis, rocket immunoelectrophoresis, and sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Analysis by sodium dodecyl sulfate-polyacrylamide gel electrophoresis shows that cytochrome o is composed of four subunits of approximate equimolar stoichiometry with molecular weights of 51,000, 28,500, 18,000, and 12,700. The low temperature (77 K) reduced - oxidized spectrum of the immunoprecipitate shows two peaks at 555 and 562 nm, indicating b-type cytochromes. With the anti-cytochrome o and antiserum toward the cytochrome d terminal oxidase complex which was previously obtained, it is possible to immunochemically assay for all the cytochromes in the cytoplasmic membrane of aerobically grown E. coli. Preliminary results indicate that the biosynthesis of cytochrome o is repressed when cytochrome d is induced by lowering the dissolved oxygen concentration during cell growth.     

Purified lac permease and cytochrome o oxidase are functional as monomers

Purified lac permease, the 46.5-kDa product of the lac Y gene that catalyzes lactose/H+ symport, or purified cytochrome o, a terminal oxidase of the Escherichia coli respiratory chain composed of four subunits with a composite molecular mass of 140 kDa, was reconstituted into proteoliposomes individually or in combination. The preparations were then examined by freeze-fracture electron microscopy employing conventional platinum/carbon replicas or by means of a new technique using thin tantalum replicas. In nonenergized proteoliposomes, both proteins appear to reconstitute as monomers based on (i) the variation of intramembrane particle density with protein concentration; (ii) the ratio of particles corresponding to each protein in proteoliposomes reconstituted with a known ratio of permease to oxidase; and (iii) the dimensions of the particles observed in tantalum replicas. The intramembrane particle diameters in tantalum replicas are about 20-25% smaller than those observed in conventional platinum/carbon replicas, indicating that the dimensions of the particles revealed with tantalum more accurately reflect the sizes of lac permease and cytochrome o. The diameters and heights of the permease and cytochrome o in tantalum replicas are 5.1 nm X 2.8 nm and 7.4 nm X 4.2 nm, respectively. Furthermore, a higher percentage of lac permease molecules exhibits a notch or cleft in tantalum replicas relative to platinum/carbon replicas. Importantly, the initial rate of lactose/H+ symport in proteoliposomes varies linearly with the ratio of lac permease to phospholipid, and no change is observed in either the size or distribution of lac permease molecules when the proteoliposomes are energized. The results taken as a whole provide a strong indication that both lac permease and cytochrome o reconstitute into proteoliposomes as monomers, that the permease does not dimerize in the presence of the H+ electrochemical gradient, and that both molecules are completely functional as monomers.     

Metabolic role of flavoprotein in terminal oxidation by cytochrome o from Vitreoscilla

NADH-cytochrome o reductase is associated with purified preparationsof cytochrome o, and these preparations can be separated into"reductase-enriched" and "reductase-poor" fractions by columnchromatography. Direct evidence for the presence of flavin inthese preparations was obtained from fluorescence spectra, andthe intensity of the fluorescence maxima was greater in reductase-enrichedpreparations of cytochrome o than in reductase-poor ones. Exogenouslyadded flavin stimulated the rate of NADH oxidation by molecularoxygen that is catalyzed by preparations of cytochrome o, morestimulation being observed with "reductase-poor" than with "reductase-enriched"preparations. Since reduction of cytochrome o in an aerobicsolution was also stimulated by added flavin, the primary effectof the latter is on the NADH-cytochrome o reductase side ofthe cytochrome. Possible explanations for the observed stimulationof the reduction of cytochrome o in aerobic solutions in thepresence of exogenous flavin are 1) reconstitution of flavin-deficientreductase, 2) flavin acting as a mobile electron carrier betweenthe reductase and the cytochrome, 3) cytochrome o being reducedby superoxide anion generated as an intermediate in the reactionof reduced flavin with oxygen. More direct evidence for theparticipation of a flavor-protein in the reduction of cytochromeo was the observed photochemical reduction of cytochrome o inan anaerobic CO atmosphere without added flavin using EDTA asan electron donor. (Received July 18, 1977; )     

Spectral evidence for cytochrome o in the brown membrane of Halobacterium halobium

The brown membrane domain of Halobacterium halobium is a developmental precursor of the purple membrane and it contains a b-type cytochrome in addition to bacterio-opsin. In this report we provide spectroscopic evidence that the majority of the cytochrome content is a halobacterial cytochrome o. This cytochrome has absorption spectral properties in the oxidized, reduced, and CO liganded states which are characteristic of cytochrome o. The CD spectra show a complex bilobed pattern in the Soret spectral region which reflects the similarity of the heme environment to those of other b-type cytochromes involved in electron transport. We have also demonstrated a positive cooperativity of CO binding which, combined with CD spectral results, suggests two interacting heme moieties per cytochrome o. Size exclusion HPLC of solubilized brown membrane preparations shows a heme b containing protein with an Mr 42,000-46,000. The cytochrome may be easily separated from bacterio-opsin using a hydroxyapatite elution method on Triton X-100 solubilized preparations. The relative ease with which brown membrane disks may be oriented in high optical quality films should make the brown membrane a valuable model system for future spectroscopic investigations of cytochrome o.     

Relationships between membrane-bound cytochrome o from Vitreoscilla and that of Escherichia coli

The cytochrome o terminal oxidases from the bacteria Vitreoscilla and Escherichia coli are structurally and functionally related. They have similar optical spectra, both exhibit ubiquinol-1 oxidase activity and are inhibited similarly. Both enzymes contain four subunits by SDS-polyacrylamide gel electrophoresis analysis and contain protoheme IX and Cu2+ prosthetic groups. Antibodies raised against the oxidase purified from E. coli crossreact with the Vitreoscilla oxidase.     

Potentiometric and spectroscopic properties of the cytochrome o complex of Escherichia coli

Cytochrome o purified from cell membranes of Escherichia coli shows two potentiometrically distinct species with midpoint oxidation-reduction potentials of +265 +/- 5 and +140 +/- 15 mV. The component with the higher potential reacted with carbon monoxide and so likely is the oxygen-reacting heme of the cytochrome o complex. It appears to be responsible for the absorption maximum at 564 nm in reduced minus oxidized difference spectra measured at 77 K. The midpoint potential of the other component was sensitive to oxidation by ferricyanide. This latter component had an absorption maximum at about 554 nm. The inhibitor 2-heptyl-4-hydroxyquinoline N-oxide inhibited reoxidation of reduced cytochrome o by oxygen and modified the spectroscopic behaviour of the 564 nm component. The ratio of the heights of the maxima in the alpha-band region of the absorption spectrum differed in cytochrome o prepared from cloned material from that found in cytochrome o from noncloned sources, in spite of the similar polypeptide compositions of the two preparations.     

Recent studies of the cytochrome o terminal oxidase complex of Escherichia coli

The cytochrome o complex is the predominant terminal oxidase in the aerobic respiratory chain of Escherichia coli when the bacteria are grown under conditions of high aeration. The oxidase is a ubiquinol oxidase and reduces molecular oxygen to water. Electron transport through the enzyme is coupled to the generation of a protonmotive force. The purified cytochrome o complex contains four or five subunits, two protoheme IX (heme b) prosthetic groups, plus at least one Cu. The subunits are all encoded by the cyo operon. Sequence comparisons show that the cytochrome o complex is closely related to the aa3-type cytochrome c oxidase family. Gene fusions have been used to define the topology of each of the gene products. Subunits I, II, III and IV are proposed to have 15, 2, 5 and 3 transmembrane spans, respectively. The fifth gene product (cyoE) encodes a protein with 7 membrane spanning segments, and this may also be a subunit of this enzyme. Fourier transform infrared spectroscopy has been used to monitor CO bound in the active site where oxygen is reduced. These data provide definitive proof that the cytochrome o complex has a heme-copper binuclear center, similar to that present in the aa3-type cytochrome c oxidases. Site-directed mutagenesis is being utilized to define which amino acids are ligands to the heme iron and copper prosthetic groups.     

Expression of cytochrome o in hydrogen uptake constitutive mutants of Rhizobium japonicum.

Mutant strains of Rhizobium japonicum constitutive for H2 uptake activity (Hupc) contained significantly more membrane-bound b-type cytochrome than did the wild type when grown heterotrophically. The Hupc strains contained approximately three times more dithionite- and NADH-reducible CO-reactive b-type cytochrome than did the wild type; the absorption features of the CO spectra were characteristic of cytochrome o. This component, designated cytochrome b', was not reduced by NADH in the presence of cyanide. Cytochrome o from the wild type (SR) and cytochrome b' from mutants SR476 and SR481 bound to CO with similar dissociation constants of 5.4, 7.4, and 5.6 microM, respectively. NADH-dependent reduction of cytochrome b' from SR476 and SR481 and the cytochrome o from SR followed pseudo-first-order kinetics with similar rate constants. Based on these spectral, ligand-binding, and kinetic measurements, it was concluded that cytochrome b' expressed by the Hupc mutants is equivalent to cytochrome o found in the wild type. H2, NADH, and succinate each reduced the same amount of total b-type cytochrome in membranes from SR481, and the rate of H2-dependent cytochrome o reduction was significantly less than with succinate or NADH as the reductants. It was concluded that neither cytochrome o nor any b-type cytochrome expressed by the Hupc mutants was unique to the H2 oxidation system. At low O2 concentrations, the inhibition of H2 and NADH oxidase activities by CO closely paralleled the binding of CO to cytochrome o rather than cytochromes a3 or c'. This suggested that NADH and H2 oxidation involved primarily cytochrome o as the terminal oxidase at low O2 tensions.     

Subunit a of cytochrome o oxidase requires both YidC and SecYEG for membrane insertion

The Escherichia coli YidC protein belongs to the Oxa1 family of membrane proteins that facilitate the insertion of membrane proteins. Depletion of YidC in E. coli leads to a specific defect in the functional assembly of major energy transducing complexes such as the F1F0 ATPase and cytochrome bo3 oxidase. Here we report on the in vitro reconstitution of the membrane insertion of the CyoA subunit of cytochrome bo3 oxidase. Efficient insertion of in vitro synthesized pre-CyoA into proteoliposomes requires YidC, SecYEG, and SecA and occurs independently of the proton motive force. These data demonstrate that pre-CyoA is a substrate of a novel pathway that involves both SecYEG and YidC.