Subfractionation and characterization of soluble c-type cytochromes from Paracoccus denitrificans cultured under various limiting conditions in the chemostat

Soluble c-type cytochromes were partially purified from Paracoccus denitrificans cells grown in succinate- and methanol-limited aerobic, nitrate-limited anaerobic and oxygen-limited chemostat cultures. Five c types could be distinguished with the following apparent molecular masses, absorption maxima and midpoint potentials. (a) 9.2 kDa, 549 nm and +190 mV; (b) 14 kDa, 549 nm and +227 mV; (c) 22 kDa, 552 nm and +190 mV; (d) 30 kDa, 552.7 nm and +160 mV; (e) 45 kDa, a dihaem: 555 nm, +128 mV and 551 nm, -163 mV. The 14-kDa polypeptide was present under all growth conditions examined and most probably is the already well characterized cytochrome c550. In methanol-limited grown cells three additional cytochromes were found, the 9.2-kDa, 22-kDa and 30-kDa ones. Under oxygen-limited conditions the 45-kDa and under anaerobic growth conditions small quantities of the 30-kDa and 45-kDa cytochromes c were present. Based on the apparent molecular masses the 14-kDa, 22-kDa, 30-kDa and 45-kDa cytochromes may also be present in membrane-fractions.     

Isolation of ubiquinol oxidase from Paracoccus denitrificans and resolution into cytochrome bc1 and cytochrome c-aa3 complexes

An enzyme complex with ubiquinol-cytochrome c oxidoreductase, cytochrome c oxidase, and ubiquinol oxidase activities was purified from a detergent extract of the plasma membrane of aerobically grown Paracoccus denitrificans. This ubiquinol oxidase consists of seven polypeptides and contains two b cytochromes, cytochrome c1, cytochrome aa3, and a previously unreported c-type cytochrome. This c-type cytochrome has an apparent Mr of 22,000 and an alpha absorption maximum at 552 nm. Retention of this c cytochrome through purification presumably accounts for the independence of ubiquinol oxidase activity on added cytochrome c. Ubiquinol oxidase can be separated into a 3-subunit bc1 complex, a 3-subunit c-aa3 complex, and a 57-kDa polypeptide. This, together with detection of covalently bound heme and published molecular weights of cytochrome c1 and the subunits of cytochrome c oxidase, allows tentative identification of most of the subunits of ubiquinol oxidase with the prosthetic groups present. Ubiquinol oxidase contains cytochromes corresponding to those of the mitochondrial bc1 complex, cytochrome c oxidase complex, and a bound cytochrome c. Ubiquinol-cytochrome c oxidoreductase activity of the complex is inhibited by inhibitors of the mitochondrial bc1 complex. Thus it seems likely that the pathway of electron transfer through the bc1 complex of ubiquinol oxidase is similar to that through the mitochondrial bc1 complex. The number of polypeptides present is less than half the number in the corresponding mitochondrial complexes. This structural simplicity may make ubiquinol oxidase from P. denitrificans a useful system with which to study the mechanisms of electron transfer and energy transduction in the bc1 and cytochrome c oxidase sections of the respiratory chain.     

Respiratory Development in Saccharomyces cerevisiae Grown at Controlled Oxygen Tension

Saccharomyces cerevisiae was grown in batch culture over a wide range of oxygen concentrations, varying from the anaerobic condition to a maximal dissolved oxygen concentration of 3.5 muM. The development of cells was assayed by measuring amounts of the aerobic cytochromes aa(3), b, c, and c(1), the cellular content of unsaturated fatty acids and ergosterol, and the activity of respiratory enzyme complexes. The half-maximal levels of membrane-bound cytochromes aa(3), b, and c(1), were reached in cells grown in O(2) concentrations around 0.1 muM; this was similar to the oxygen concentration required for half-maximal levels of unsaturated fatty acid and sterol. However, the synthesis of ubiquinone and cytochrome c and the increase in fumarase activity were essentially linear functions of the dissolved oxygen concentration up to 3.5 muM oxygen. The synthesis of the succinate dehydrogenase, succinate cytochrome c reductase, and cytochrome c oxidase complexes showed different responses to changes in O(2) concentration in the growth medium. Cyanide-insensitive respiration and P(450) cytochrome content were maximal at 0.25 muM oxygen and declined in both more anaerobic and aerobic conditions. Cytochrome c peroxidase and catalase activities in cell-free homogenates were high in all but the most strictly anaerobic cells.     

Methanol and ethanol oxidase respiratory chains of the methylotrophic acetic acid bacterium, Acetobacter methanolicus.

Acetobacter methanolicus is a unique acetic acid bacterium which has a methanol oxidase respiratory chain, as seen in methylotrophs, in addition to its ethanol oxidase respiratory chain. In this study, the relationship between methanol and ethanol oxidase respiratory chains was investigated. The organism is able to grow by oxidizing several carbon sources, including methanol, glycerol, and glucose. Cells grown on methanol exhibited a high methanol-oxidizing activity and contained large amounts of methanol dehydrogenase and soluble cytochromes c. Cells grown on glycerol showed higher oxygen uptake rate and dehydrogenase activity with ethanol but little methanol-oxidizing activity. Furthermore, two different terminal oxidases, cytochrome c and ubiquinol oxidases, have been shown to be involved in the respiratory chain; cytochrome c oxidase predominates in cells grown on methanol while ubiquinol oxidase predominates in cells grown on glycerol. Both terminal oxidases could be solubilized from the membranes and separated from each other. The cytochrome c oxidase and the ubiquinol oxidase have been shown to be a cytochrome co and a cytochrome bo, respectively. Methanol-oxidizing activity was diminished by several treatments that disrupt the integrity of the cells. The activity of the intact cells was inhibited with NaCl and/or EDTA, which disturbed the interaction between methanol dehydrogenase and cytochrome c. Ethanol-oxidizing activity in the membranes was inhibited with 2-heptyl-4-hydroxyquinoline N-oxide, which inhibited ubiquinol oxidase but not cytochrome c oxidase. Alcohol dehydrogenase has been purified from the membranes of glycerol-grown cells and shown to reduce ubiquinone-10 as well as a short side-chain homologue in detergent solution.(ABSTRACT TRUNCATED AT 250 WORDS)     

Membrane-bound cytochromes in a sulfate-reducing strict anaerobe Desulfovibrio vulgaris Miyazaki F

Cytoplasmic membranes were isolated from the cells of a sulfate-reducing strict anaerobe Desulfovibrio vulgaris Miyazaki F and membrane-bound cytochromes were characterized. Redox difference spectra at 77 K revealed the presence of cytochromes with the alpha peaks at 552 and 556 nm while CO-binding difference spectra showed the presence of o-type cytochrome(s). Partial purification of the cytochromes demonstrated that the membranes contain cytochromes c550, c551, c556 and possibly d1 besides high molecular mass cytochrome c and cytochrome c3. It turned out that two kinds of novel CO-binding c-type cytochromes are present in the membrane. The membranes and a partially purified fraction showed weak ubiquinol-1 oxidase activity but no cytochrome c oxidase activity. Results suggest that D. vulgaris does not express the heme-copper terminal oxidase under our growth conditions in spite of the presence of the col gene, which is homologous to the gene of subunit I of the aa3-type oxidase.     

Studies on cytochrome c oxidase activity of the cytochrome c1aa3 complex from Thermus thermophilus

Cytochrome oxidase from T. thermophilus is isolated as a noncovalent complex of cytochromes c1 and aa3 in which the four redox components of aa3 appear to be associated with a single approximately 55,000-D subunit while the heme C is associated with a approximately 33,000-D peptide (Yoshida, T., Lorence, R. M., Choc, M. G., Tarr, G. E., Findling, K. L., and Fee, J. A. (1983) J. Biol. Chem. 258, 112-123). We have examined the steady state transfer of electrons from ascorbate to oxygen by cytochrome c1aa3 as mediated by horse heart, Candida krusei, and T. thermophilus (c552) cytochromes c as well as tetramethylphenylenediamine (TMPD). These mediators exhibit simple Michaelis-Menten kinetic behavior yielding Vmax and KM values characteristic of the experimental conditions. Three classes of kinetic behavior were observed and are qualitatively discussed in terms of a reaction scheme. The data show that tetramethylphenyldiamine and cytochromes c react with the enzyme at independent sites; it is suggested that cytochrome c1 may efficiently transfer electrons to cytochrome aa3. When incorporated into phospholipid vesicles, the highly purified cytochrome c1aa3 was found to translocate one proton into the exterior medium for each molecule of cytochrome c552 oxidized. The combined results suggest that this bacterial enzyme functions in a manner generally identical with the more complex eucaryotic enzyme.     

Two types of cytochrome cd1 in the aerobic photosynthetic bacterium, Erythrobacter sp. OCh 114

Components I and II of cytochrome cd1 which had different spectral features were purified from the aerobic photosynthetic bacterium, Erythrobacter sp. strain OCh 114. Component I showed an absorption maxima at 700 and 406 nm in the oxidized form, and at 621, 552.5, 548 and 416 nm in the reduced form. Component II showed an absorption maxima at 635 and 410 nm in the oxidized form and at 628, 552.5, 548 and 417 nm in the reduced form. The relative molecular mass, Mr, of both cytochromes was determined to be 135,000 with two identical subunits. Components I and II showed pI values of 7.6 and 6.8, respectively. The redox potential of hemes ranged from +234 mV to +242 mV, except for the heme d1 of component I (Em7 = +134 mV). Components I and II showed both cytochrome c oxidase and nitrite reductase activities. Cytochrome c oxidase activity was strongly inhibited by a low concentration of nitrite and cyanide. Erythrobacter cytochromes c-551 and c-552 were utilized as electron donors for the cytochrome c oxidase reaction. The high affinity of cytochrome c-552 to component II (Km = 1.27 microM) suggested a physiological significance for this cytochrome. Erythrobacter cytochromes cd1 are unique in their presence in cells grown under aerobic conditions as compared to other bacterial cytochromes cd1 which are formed only under denitrifying conditions.     

Specificity of the interaction between the Paracoccus denitrificans oxidase and its substrate cytochrome c: comparing the mitochondrial to the homologous bacterial cytochrome c(552), and its truncated and site-directed mutants

Under in vitro conditions, bacterial cytochrome c oxidases may accept several nonhomologous c-type electron donors, including the evolutionarily related mitochondrial cytochrome c. Several lines of evidence suggest that in intact membranes the heme aa(3) oxidase from Paracoccus denitrificans receives its electrons from the membrane-bound cytochrome c(552). Both the structures of the oxidase and of a heterologously expressed, soluble fragment of the c(552) have been determined recently, but no direct structural information about a static cocomplex is available. Here, we analyze the kinetic properties of the isolated oxidase with the full-size c(552), with two truncated soluble forms, and with a set of site-specific mutants within the presumed docking site of the cytochrome, all heterologously expressed in Escherichia coli. Our data indicate that all three forms, the wild type and both truncations, are fully competent kinetically and exhibit biphasic kinetic behavior, however, under widely different ionic strength conditions. When mutations in lysine residues clustered around the interaction domain were introduced into the smallest fragment of c(552), both kinetic parameters, K(M) and k(cat), were drastically influenced. On the other hand, when the nonmutated truncated form was used to donate electrons to a set of oxidase mutants with replacements clustered along the docking site on subunit II, we observe distinct differences when comparing the kinetic properties of the widely used horse heart cytochrome c with those of the bacterial c(552). We conclude that the specific docking sites for the two types of cytochromes differ to some extent.     

Cytochrome o (bo) is a proton pump in Paracoccus denitrificans and Escherichia coli

Spheroplasts from aerobically grown wild-type Paracoccus denitrificans cells respire with succinate despite specific inhibition of the cytochrome bc1 complex by myxothiazol. Coupled to this activity, which involves only b-type cytochromes, there is translocation of 1.5-1.9 h+/e- across the cytoplasmic membrane. Similar H+ translocation ratios are observed during oxidation of ubiquinol in spheroplasts from aerobically grown mutants of Paracoccus lacking cytochrome c oxidase, or deficient in cytochrome c, as well as in a strain of E. coli from which cytochrome d was deleted. These observations show that the cytochrome o complex is a proton pump much like cytochrome aa3 to which it is structurally related.     

Comparison of cytochromes from anaerobically and aerobically grown cells of Pseudomonas perfectomarinus.

Pseudomonas perfectomarinus (ATCC 14405) is a facultative anaerobe capable of either oxygen respiration or anaerobic nitrate respiration, i.e., denitrification. A comparative study of the electron transfer components of cells revealed five c-type cytochromes and cytochrome cd in the soluble fraction from anaerobically grown cells and four c-type cytochromes in the soluble fraction from aerobically grown cells. Purification procedures yielded three c-type cytochromes (designated c-551, c-554, and acidic c-type) from both kinds of cells as indicated by similarities in absorption spectra, molecular weight, and electrophoretic mobility. Cytochrome cd, a diheme c-type cytochrome (cytochrome c-552), and a split-alpha c-type cytochrome were recovered only from anaerobically grown cells. A c-type cytochrome with a low ratio of alpha to beta absorption peak heights was uniquely present in the aerobically grown cells. Liquid N2 temperature absorption spectroscopy on the membrane fraction from anaerobically grown cells revealed residual cytochrome cd as well as differences in the relative amounts of c-type and b-type cytochromes in membranes prepared from cells grown under the two different conditions.     

Cytochromes c550, c552, and c1 in the Electron Transport Network of Paracoccus denitrificans: Redundant or Subtly Different in Function?

Paracoccus denitrificans strains with mutations in the genes encoding the cytochrome c(550), c(552), or c(1) and in combinations of these genes were constructed, and their growth characteristics were determined. Each mutant was able to grow heterotrophically with succinate as the carbon and free-energy source, although their specific growth rates and maximum cell numbers fell variably behind those of the wild type. Maximum cell numbers and rates of growth were also reduced when these strains were grown with methylamine as the sole free-energy source, with the triple cytochrome c mutant failing to grow on this substrate. Under anaerobic conditions in the presence of nitrate, none of the mutant strains lacking the cytochrome bc(1) complex reduced nitrite, which is cytotoxic and accumulated in the medium. The cytochrome c(550)-deficient mutant did denitrify provided copper was present. The cytochrome c(552) mutation had no apparent effect on the denitrifying potential of the mutant cells. The studies show that the cytochromes c have multiple tasks in electron transfer. The cytochrome bc(1) complex is the electron acceptor of the Q-pool and of amicyanin. It is also the electron donor to cytochromes c(550) and c(552) and to the cbb(3)-type oxidase. Cytochrome c(552) is an electron acceptor both of the cytochrome bc(1) complex and of amicyanin, as well as a dedicated electron donor to the aa(3)-type oxidase. Cytochrome c(550) can accept electrons from the cytochrome bc(1) complex and from amicyanin, whereas it is also the electron donor to both cytochrome c oxidases and to at least the nitrite reductase during denitrification. Deletion of the c-type cytochromes also affected the concentrations of remaining cytochromes c, suggesting that the organism is plastic in that it adjusts its infrastructure in response to signals derived from changed electron transfer routes.     

A high-affinity cbb3-type cytochrome oxidase terminates the symbiosis-specific respiratory chain of Bradyrhizobium japonicum.

It has been a long-standing hypothesis that the endosymbiotic rhizobia (bacteroids) cope with a concentration of 10 to 20 nM free O2 in legume root nodules by the use of a specialized respiratory electron transport chain terminating with an oxidase that ought to have a high affinity for O2. Previously, we suggested that the microaerobically and anaerobically induced fixNOQP operon of Bradyrhizobium japonicum might code for such a special oxidase. Here we report the biochemical characteristics of this terminal oxidase after a 27-fold enrichment from membranes of anaerobically grown B. japonicum wild-type cells. The purified oxidase has TMPD (N,N,N',N'-tetramethyl-p-phenylenediamine) oxidase activity as well as cytochrome c oxidase activity. N-terminal amino acid sequencing of its major constituent subunits confirmed that presence of the fixN,fixO, and fixP gene products. FixN is a highly hydrophobic, heme B-binding protein. FixO and FixP are membrane-anchored c-type cytochromes (apparent Mrs of 29,000 and 31,000, respectively), as shown by their peroxidase activities in sodium dodecyl sulfate-polyacrylamide gels. All oxidase properties are diagnostic for it to be a member of the cbb3-type subfamily of heme-copper oxidases. The FixP protein was immunologically detectable in membranes isolated from root nodule bacteroids, and 85% of the total cytochrome c oxidase activity in bacteroid membranes was contributed by the cbb3-type oxidase. The Km values for O2 of the purified enzyme and of membranes from different B. japonicum wild-type and mutant strains were determined by a spectrophotometric method with oxygenated soybean leghemoglobin as the sole O2 delivery system. The derived Km value for O2 of the cbb3-type oxidase in membranes was 7 nM, which is six- to eightfold lower than that determined for the aerobic aa3-type cytochrome c oxidase. We conclude that the cbb3-type oxidase supports microaerobic respiration in endosymbiotic bacteroids.     

Molecular aspects of the electron transfer system which participates in the oxidation of ferrous ion by Thiobacillus ferrooxidans

Abstract: The enzymes and redox proteins, which participate in the oxidation of ferrous ion by the acidophilic iron-oxidizing bacterium Thiobacillus ferrooxidans , have been isolated and characterized. They are Fe(II)-cytochrome c oxidoreductase, cytochromes c -552(s), c -552(m) and c -550(m), rusticyanin, and cytochrome c oxidase. On the basis of the interactions of these components, an electron transfer system has been proposed which seems to function in the oxidation of ferrous ion by the bacterium.     

Electron transport reactions in a cytochrome c-deficient mutant of Paracoccus denitrificans

A mutant of Paracoccus denitrificans which is deficient in c-type cytochromes grows aerobically with generation times similar to those obtained with a wild-type strain. The aa3-type oxidase is functional in the mutant as judged by spectrophotometric assays of cytochrome c oxidation using the membrane particles and cytochrome aa3 reduction in whole cells. The cytochrome c oxidase (aa3-type) of the c-less mutant oxidizes soluble cytochrome c at rates equivalent to those obtained with the wild-type. NADH and succinate oxidase activities of the membrane preparations of the mutant and wild-type are also comparable in the absence of detergent treatment. Exogenous soluble cytochrome c can be both reduced by NADH- and succinate-linked systems and oxidized by cytochrome aa3 present in membranes of the mutant strain. Rapid overall electron transport can occur in the c-less mutant, suggesting that reactions result from collision of diffusing complexes.     

Effect of aerobic growth conditions on the soluble cytochrome content of the purple phototrophic bacterium Rhodobacter sphaeroides: induction of cytochrome c554

When grown anaerobically in the light, Rhodobacter sphaeroides contains appreciable quantities of cytochromes c2 and c', but smaller amounts of other soluble cytochromes such as cytochrome c551.5, cytochrome c554, and an oxygen-binding heme protein. When R. sphaeroides is mass cultured aerobically in the dark to stationary phase, the content of cytochrome c2 does not change appreciably, whereas cytochrome c554 is approximately 8-fold more abundant, cytochrome c' is at least 10-fold less abundant, and cytochrome c551.5 is fivefold lower than in the phototrophically grown cells. These observations confirm previous literature reports that in this organism a cytochrome c553 (or c554 in our experience) is more abundant when cells are grown aerobically. Furthermore, the aerobic cytochrome c554 is positively identified with the previously characterized minor cytochrome c554 component of anaerobic photosynthetic cells. Preliminary sequence results show that cytochrome c554 is a member of the cytochrome c' structural family, but differs from normal cytochromes c' in having a methionine sixth ligand to the heme. The levels of electron carrier proteins of low redox potential had previously been reported to be less in aerobic than in photoheterotrophic cells and we have verified that observation for the specific examples of cytochromes c' and c551.5. The oxygen binding heme protein, SHP, is not induced by aerobic growth.     

Isolation of a Rhizobium phaseoli cytochrome mutant with enhanced respiration and symbiotic nitrogen fixation.

Cultured cells of a Rhizobium phaseoli wild-type strain (CE2) possess b-type and c-type cytochromes and two terminal oxidases: cytochromes o and aa3. Cytochrome aa3 was partially expressed when CE2 cells were grown on minimal medium, during symbiosis, and in well-aerated liquid cultures in a complex medium (PY2). Two cytochrome mutants of R. phaseoli were obtained and characterized. A Tn5-mob-induced mutant, CFN4201, expressed diminished amounts of b-type and c-type cytochromes, showed an enhanced expression of cytochrome oxidases, and had reduced levels of N,N,N',N'-tetramethyl-p-phenylenediamine, succinate, and NADH oxidase activities. Nodules formed by this strain had no N2 fixation activity. The other mutant, CFN4205, which was isolated by nitrosoguanidine mutagenesis, had reduced levels of cytochrome o and higher succinate oxidase activity but similar NADH and N,N,N',N'-tetramethyl-p-phenylenediamine oxidase activities when compared with the wild-type strain. Strain CFN4205 expressed a fourfold-higher cytochrome aa3 content when cultured on minimal and complex media and had twofold-higher cytochrome aa3 levels during symbiosis when compared with the wild-type strain. Nodules formed by strain CFN4205 fixed 33% more N2 than did nodules formed by the wild-type strain, as judged by the total nitrogen content found in plants nodulated by these strains. Finally, low-temperature photodissociation spectra of whole cells from strains CE2 and CFN4205 reveal cytochromes o and aa3. Both cytochromes react with O2 at -180 degrees C to give a light-insensitive compound. These experiments identify cytochromes o and aa3 as functional terminal oxidases in R. phaseoli.     

Respiratory electron transport and light-induced energy transduction in membranes from the aerobic photosynthetic bacterium Roseobacter denitrificans

Membrane fragments isolated from the aerobic phototrophic bacterium Roseobacter denitrificans were examined. Ninety-five percent of the total NADH-dependent oxidative activity was inhibited either by antimycin A or myxothiazol, two specific inhibitors of the cytochrome bc1 complex, which indicates that the respiratory electron transport chain is linear. In agreement with this finding, light-induced oxygen uptake, an electron transport activity catalyzed by the "alternative quinol oxidase pathway" in membranes of several facultative phototrophic species, was barely detectable in membranes of Rsb. denitrificans. Redox titrations at 561-575 nm, 552-540 nm, and 602-630 nm indicated the presence of three b-type cytochromes (Em,7 of +244 +/- 8, +24 +/- 3, -163 +/- 11 mV), four c-type cytochromes (Em,7 of +280 +/- 10, +210 +/- 5, +125 +/- 8, and 20 +/- 3 mV) and two a-type cytochromes (Em,7 of +335 +/- 15, +218 +/- 18 mV). The latter two a-type hemes were shown to be involved in cytochrome c oxidase activity, which was inhibited by both cyanide (I50 = 2 microM) and azide (I50 = 1 mM), while a soluble cytochrome c (c551, Em,7 = +217 +/- 2 mV) was shown to be the physiological electron carrier connecting the bc1 complex to the cytochrome c oxidase. A comparison of the ATP synthesis generated by continuous light in membranes of Rsb. denitrificans and Rhodobacter capsulatus showed that in both bacterial species photophosphorylation requires a membrane redox poise at the equilibrium (Eh > or = +80 < or = +140 mV), close to the oxidation-reduction potential of the ubiquinone pool. These data, taken together, suggest that, although the photosynthetic apparatus of Rsb. denitrificans is functionally similar to that of typical anoxygenic phototrophs, e.g. Rba. capsulatus, the in vivo requirement of a suitable redox state at the ubiquinone pool level restricts the growth capacity of Rsb. denitrificans to oxic conditions.     

Analysis of the respiratory chain in Ethanologenic Zymomonas mobilis with a cyanide-resistant bd-type ubiquinol oxidase as the only terminal oxidase and its possible physiological roles

The respiratory chain of the ethanologenic bacterium Zymomonas mobilis was investigated, in which the pyruvate-to-ethanol pathway has been demonstrated to be mainly responsible for NADH oxidation and the tricarboxylic acid cycle is incomplete. Membranes from cells cultivated under aerobic or anaerobic growth conditions showed dehydrogenase and oxidase activities for NADH, D-lactate and D-glucose and ubiquinol oxidase activity. Intriguingly, the NADH oxidase activity level of membrane fractions from cells grown aerobically was found to be higher than that of membrane fractions from Escherichia coli or Pseudomonas putida grown aerobically, indicating a crucial role of the respiratory chain in NADH oxidation in the organism. Cyanide-resistant terminal oxidase activity was observed and appeared to be due to a bd-type ubiquinol oxidase as the only terminal oxidase encoded by the entire genome. The terminal oxidase with a relatively strong ubiquinol oxidase activity exhibited remarkably weak signals of cytochrome d. Considering these findings and the presence of a type-II NADH dehydrogenase but not a type-I, a simple respiratory chain that generates less energymay have evolved in Z. mobilis.     

Properties of a cytochrome c-enriched light particulate fraction isolated from the photosynthetic bacterium Rhodopseudomonas spheroides.

Differential centrifugation of suspensions of French-press-disrupted Rhodopseudomonas spheroides yielded a light particulate fraction that was different in many properties from the bulk membrane fraction. It was enriched in cytochrome c and had a low cytochrome b content. When prepared from photosynthetically grown cells this fraction had a very low specific bacteriochlorophyll content. The cytochrome c of the light particles differed in absorption maxima at 77K from cytochrome c2 attached to membranes; there was pronounced splitting of the alpha-band, as is found in cytochrome c2 free in solution. Potentiometric titration at A552--A540 showed the presence of two components that fitted an n = 1 titration; one component had a midpoint redox potential of +345mV, like cytochrome c2 in solution, and the second had E0' at pH 7.0 of +110 mV, and they were present in a ratio of approx. 2:3. Difference spectroscopy at 77K showed that the spectra of the two components were very similar. More of a CO-binding component was present in particles from photosynthetically grown cells. Light membranes purified by centrifugation on gradients of 5--60% (w/w) sucrose retained the two c cytochromes; they contained no detectable succinate-cytochrome c reductase or bacteriochlorophyll and very little ubiquinone, but they contained NADH-cytochrome c reductase and some phosphate. Electrophoresis on sodium dodecyl sulphate/polyacrylamide gels showed that the light membranes of aerobically and photosynthetically grown cells were very similar and differed greatly from other membrane fractions of R. spheroides.     

The mixed valence state of the oxidase binuclear centre: how Thermus thermophilus cytochrome ba3 differs from classical aa3 in the aerobic steady state and when inhibited by cyanide

In the aerobic steady state of the classical eukaryotic cytochrome c oxidase, three aa(3) redox metal centres (cytochrome a, CuA and CuB) are partially reduced while the fourth, cytochrome a(3), remains almost fully oxidized. Turnover depends primarily upon the rate of cytochrome a(3) reduction. When prokaryotic cytochrome c-552 oxidase (ba(3)) of Thermus thermophilus turns over, three different metal centres (cytochromes b, a(3) and CuA) share the steady state electrons; it is the fourth, CuB, that apparently remains almost fully oxidized until anaerobiosis. Cytochrome a(3) stays partially reduced during turnover and a possible P/F state may also be populated. Cyanide traps the aerobic ba(3) CuB centre in the a(3)(2+)CNCuB(2+) state; the corresponding eukaryotic cyanide trapped state is a(3)(3+)CNCuB(+). Both states become the fully reduced a(3)(2+)CNCuB(+) upon anaerobiosis. The different reactivities of the aa(3) and ba(3) binuclear centres may be correlated with the very different proximal histidine N-Fe distances in the two enzymes (3.3 A for ba(3) compared to 1.9 A for aa(3)) which may in turn relate to the functioning of thermophilic Thermus cytochrome ba(3) in vivo at a very elevated temperature. But the differences may also just exemplify how evolution can find surprisingly different solutions to the common problem of electron transfer to oxygen. Some of these alternatives were potentially enshrined in a model of the oxidase reaction already adopted by Gerry Babcock in the early 1990s.