Characterization of a cb-type cytochrome c oxidase from Helicobacter pylori

Identification of Na+-Pumping Cytochrome Oxidase in the Membranes of Extremely Alkaliphilic Thioalkalivibrio Bacteria

For the first time, the functioning of the oxygen reductase Na+-pump (Na+-pumping cytochrome c oxidase of the cbb₃-type) was demonstrated by examining the respiratory chain of the extremely alkaliphilic bacterium Thioalkalivibrio versutus [Muntyan, M. S., et al. (2015) Cytochrome cbb₃ of Thioalkalivibrio is a Na+-pumping cytochrome oxidase, Proc. Natl. Acad. Sci. USA, 112, 7695-7700], a product of the ccoNOQP operon. In this study, we detected and identified this enzyme using rabbit polyclonal antibody against the predicted C-terminal amino acid sequence of its catalytic subunit. We found that this cbb₃-type oxidase is synthesized in bacterial cells, where it is located in the membranes. The 48-kDa oxidase subunit (CcoN) is catalytic, while subunits CcoO and CcoP with molecular masses of 29 and 34 kDa, respectively, are cytochromes c. The theoretical pI values of the CcoN, CcoO, and CcoP subunits were determined. It was shown that parts of the CcoO and CcoP subunits exposed to the aqueous phase on the cytoplasmic membrane P-side are enriched with negatively charged amino acid residues, in contrast to the parts of the integral subunit CcoN adjacent to the aqueous phase. Thus, the Na+-pumping cytochrome c oxidase of T. versutus, both in function and in structure, demonstrates adaptation to extremely alkaline conditions.

Molecular Hydrogen Metabolism: a Widespread Trait of Pathogenic Bacteria and Protists

Pathogenic microorganisms use various mechanisms to conserve energy in host tissues and environmental reservoirs. One widespread but often overlooked means of energy conservation is through the consumption or production of molecular hydrogen (H2). Here, we comprehensively review the distribution, biochemistry, and physiology of H2 metabolism in pathogens. Over 200 pathogens and pathobionts carry genes for hydrogenases, the enzymes responsible for H2 oxidation and/or production. Furthermore, at least 46 of these species have been experimentally shown to consume or produce H2 Several major human pathogens use the large amounts of H2 produced by colonic microbiota as an energy source for aerobic or anaerobic respiration. This process has been shown to be critical for growth and virulence of the gastrointestinal bacteria Salmonella enterica serovar Typhimurium, Campylobacter jejuni, Campylobacter concisus, and Helicobacter pylori (including carcinogenic strains). H2 oxidation is generally a facultative trait controlled by central regulators in response to energy and oxidant availability. Other bacterial and protist pathogens produce H2 as a diffusible end product of fermentation processes. These include facultative anaerobes such as Escherichia coli, S Typhimurium, and Giardia intestinalis, which persist by fermentation when limited for respiratory electron acceptors, as well as obligate anaerobes, such as Clostridium perfringens, Clostridioides difficile, and Trichomonas vaginalis, that produce large amounts of H2 during growth. Overall, there is a rich literature on hydrogenases in growth, survival, and virulence in some pathogens. However, we lack a detailed understanding of H2 metabolism in most pathogens, especially obligately anaerobic bacteria, as well as a holistic understanding of gastrointestinal H2 transactions overall. Based on these findings, we also evaluate H2 metabolism as a possible target for drug development or other therapies.

Regulation of Respiratory Pathways in Campylobacterota: A Review

The Campylobacterota, previously known as Epsilonproteobacteria, are a large group of Gram-negative mainly, spiral-shaped motile bacteria. Some members like the Sulfurospirillum spp. are free-living, while others such as Helicobacter spp. can only persist in strict association with a host organism as commensal or as pathogen. Species of this phylum colonize diverse habitats ranging from deep-sea thermal vents to the human stomach wall. Despite their divergent environments, they share common energy conservation mechanisms. The Campylobacterota have a large and remarkable repertoire of electron transport chain enzymes, given their small genomes. Although members of recognized families of transcriptional regulators are found in these genomes, sofar no orthologs known to be important for energy or redox metabolism such as ArcA, FNR or NarP are encoded in the genomes of the Campylobacterota. In this review, we discuss the strategies that members of Campylobacterota utilize to conserve energy and the corresponding regulatory mechanisms that regulate the branched electron transport chains in these bacteria.

Flavonoid Glycosides of Polygonum capitatum Protect against Inflammation Associated with Helicobacter pylori Infection

The antibacterial and anti-inflammatory activities, and protective effects of extracts (flavonoid glycosides) of Polygonum capitatum were investigated to detect the evidence for the utilization of the herb in the clinical therapy of gastritis caused by H. pylori. A mouse gastritis model was established using H. pylori. According to treating methods, model mice were random assigned into a model group (MG group), a triple antibiotics group (TG group, clarithromycin, omeprazole and amoxicillin), low/middle/high concentrations of flavonoid glycosides groups (LF, MF and HF groups) and low/middle/high concentrations of flavonoid glycosides and amoxicillin groups (LFA, MFA and HFA groups). A group with pathogen-free mice was regarded as a control group (CG group). The eradicate rates of H. pylori were 100%, 93%, 89% in TG, MFA and HF groups. The serum levels of IFN-gamma and gastrin were higher in a MG group than those from all other groups (P < 0.05). The serum levels of IFN-gamma and gastrin were reduced significantly in LF, MF and HF groups (P < 0.05) while little changes were observed in LFA, MFA and HFA groups. In contrast, the serum levels of IL-4 were lower and higher in MG and CG groups compared with other groups (P<0.05). The serum levels of IL-4 were increased significantly in LF, MF and HF groups (P < 0.05) while little changes were found in LFA, MFA and HFA groups. According to pathological scores, flavonoid glycosides therapy showed better protection for gastric injuries than the combination of flavonoid glycoside and amoxicillin (P < 0.05). The results suggested that flavonoid glycoside has repairing functions for gastric injuries. The results suggest that the plant can treat gastritis and protect against gastric injuries. The flavonoid glycosides from Polygonum capitatum should be developed as a potential drug for the therapy of gastritis caused by H. pylori.

Biochemical and biophysical characterization of the two isoforms of cbb3-type cytochrome c oxidase from Pseudomonas stutzeri

The cbb3-type cytochrome c oxidases (cbb3-CcOs) are members of the heme-copper oxidase superfamily that couple the reduction of oxygen to translocation of protons across the membrane. The cbb3-CcOs are present only in bacteria and play a primary role in microaerobic respiration, being essential for nitrogen-fixing endosymbionts and for some human pathogens. As frequently observed in Pseudomonads, Pseudomonas stutzeri contains two independent ccoNO(Q)P operons encoding the two cbb3 isoforms, Cbb3-1 and Cbb3-2. While the crystal structure of Cbb3-1 from P. stutzeri was determined recently and cbb3-CcOs from other organisms were characterized functionally, less emphasis has been placed on the isoform-specific differences between the cbb3-CcOs. In this work, both isoforms were homologously expressed in P. stutzeri strains from which the genomic version of the respective operon was deleted. We purified both cbb3 isoforms separately by affinity chromatography and increased the yield of Cbb3-2 to a similar level as Cbb3-1 by replacing its native promoter. Mass spectrometry, UV-visible (UV-Vis) spectroscopy, differential scanning calorimetry, as well as oxygen reductase and catalase activity measurements were employed to characterize both cbb3 isoforms. Differences were found concerning the thermal stability and the presence of subunit CcoQ. However, no significant differences between the two isoforms were observed otherwise. Interestingly, a surprisingly high turnover of at least 2,000 electrons s(-1) and a high Michaelis-Menten constant (Km ~ 3.6 mM) using ascorbate-N,N,N',N'-tetramethyl-p-phenylenediamine dihydrochloride (TMPD) as the electron donor were characteristic for both P. stutzeri cbb3-CcOs. Our work provides the basis for further mutagenesis studies of each of the two cbb3 isoforms specifically.

Structural and functional characterization of HP0377, a thioredoxin-fold protein from Helicobacter pylori

Maturation of cytochrome c is carried out in the bacterial periplasm, where specialized thiol-disulfide oxidoreductases provide the correct reduction of oxidized apocytochrome c before covalent haem attachment. HP0377 from Helicobacter pylori is a thioredoxin-fold protein that has been implicated as a component of system II for cytochrome c assembly and shows limited sequence similarity to Escherichia coli DsbC, a disulfide-bond isomerase. To better understand the role of HP0377, its crystal structures have been determined in both reduced and partially oxidized states, which are highly similar to each other. Sedimentation-equilibrium experiments indicate that HP0377 is monomeric in solution. HP0377 adopts a thioredoxin fold but shows distinctive variations as in other thioredoxin-like bacterial periplasmic proteins. The active site of HP0377 closely resembles that of E. coli DsbC. A reductase assay suggests that HP0377 may play a role as a reductase in the biogenesis of holocytochrome c553 (HP1227). Binding experiments indicate that it can form a covalent complex with HP0518, a putative L,D-transpeptidase with a catalytic cysteine residue, via a disulfide bond. Furthermore, physicochemical properties of HP0377 and its R86A variant have been determined. These results suggest that HP0377 may perform multiple functions as a reductase in H. pylori.

Mechanistic stoichiometry of proton translocation by cytochrome cbb3

Cytochrome cbb(3) belongs to the superfamily of respiratory heme-copper oxidases that couple the reduction of molecular oxygen to proton translocation across the bacterial or mitochondrial membrane. The cbb(3)-type enzymes are found only in bacteria, and are both structurally and functionally the most distant from their mitochondrial counterparts. The mechanistic H(+)/e(-) stoichiometry of proton translocation in these cbb(3)-type cytochrome c oxidases has remained controversial. A stoichiometric efficiency of only one-half that of the mitochondrial aa(3)-type enzyme was recently proposed to be related to adaptation of the organism to microaerobic environments. Here, proton translocation by the Rhodobacter sphaeroides enzyme was studied using purified cytochrome cbb(3) reconstituted into liposomes. An H(+)/e(-) stoichiometry of proton translocation close to unity was observed using the oxygen pulse method, but solely in conditions in which the vast majority of the enzyme was fully reduced in the anaerobic state before the O(2) pulse. These data were compared with results using whole cells or spheroplasts, and the discrepancies in the literature data were discussed. Our results suggest that a proton-pumping efficiency of 1 H(+)/e(-) may be achieved using the single-proton uptake pathway identified in the structure of cytochrome cbb(3). The mechanism of proton pumping thus differs from that of the aa(3)-type oxidases of mitochondria and bacteria.

Novel transporter required for biogenesis of cbb3-type cytochrome c oxidase in Rhodobacter capsulatus

The acquisition, delivery, and incorporation of metals into their respective metalloproteins are important cellular processes. These processes are tightly controlled in order to prevent exposure of cells to free-metal concentrations that could yield oxidative damage. Copper (Cu) is one such metal that is required as a cofactor in a variety of proteins. However, when present in excessive amounts, Cu is toxic due to its oxidative capability. Cytochrome c oxidases (Coxs) are among the metalloproteins whose assembly and activity require the presence of Cu in their catalytic subunits. In this study, we focused on the acquisition of Cu for incorporation into the heme-Cu binuclear center of the cbb(3)-type Cox (cbb(3)-Cox) in the facultative phototroph Rhodobacter capsulatus. Genetic screens identified a cbb(3)-Cox defective mutant that requires Cu(2+) supplementation to produce an active cbb(3)-Cox. Complementation of this mutant using wild-type genomic libraries unveiled a novel gene (ccoA) required for cbb(3)-Cox biogenesis. In the absence of CcoA, the cellular Cu content decreases and cbb(3)-Cox assembly and activity become defective. CcoA shows homology to major facilitator superfamily (MFS)-type transporter proteins. Members of this family are known to transport small solutes or drugs, but so far, no MFS protein has been implicated in cbb(3)-Cox biogenesis. These findings provide novel insights into the maturation and assembly of membrane-integral metalloproteins and on a hitherto-unknown function(s) of MFS-type transporters in bacterial Cu acquisition.

IMPORTANCE

Biogenesis of energy-transducing membrane-integral enzymes, like the heme copper-containing cytochrome c oxidases, and the acquisition of transition metals, like copper, as their catalytic cofactors are vital processes for all cells. These widespread and well-controlled processes are poorly understood in all organisms, including bacteria. Defects in these processes lead to severe mitochondrial diseases in humans and poor crop yields in plants. In this study, using the facultative phototroph Rhodobacter capsulatus as a model organism, we report on the discovery of a novel major facilitator superfamily (MFS)-type transporter (CcoA) that affects cellular copper content and cbb(3)-type cytochrome c oxidase production in bacteria.

Dynamic water networks in cytochrome cbb3 oxidase

Heme-copper oxidases (HCOs) are terminal electron acceptors in aerobic respiration. They catalyze the reduction of molecular oxygen to water with concurrent pumping of protons across the mitochondrial and bacterial membranes. Protons required for oxygen reduction chemistry and pumping are transferred through proton uptake channels. Recently, the crystal structure of the first C-type member of the HCO superfamily was resolved [Buschmann et al. Science 329 (2010) 327-330], but crystallographic water molecules could not be identified. Here we have used molecular dynamics (MD) simulations, continuum electrostatic approaches, and quantum chemical cluster calculations to identify proton transfer pathways in cytochrome cbb(3). In MD simulations we observe formation of stable water chains that connect the highly conserved Glu323 residue on the proximal side of heme b(3) both with the N- and the P-sides of the membrane. We propose that such pathways could be utilized for redox-coupled proton pumping in the C-type oxidases. Electrostatics and quantum chemical calculations suggest an increased proton affinity of Glu323 upon reduction of high-spin heme b(3). Protonation of Glu323 provides a mechanism to tune the redox potential of heme b(3) with possible implications for proton pumping.

The diheme cytochrome c(4) from Vibrio cholerae is a natural electron donor to the respiratory cbb(3) oxygen reductase

The respiratory chain of Vibrio cholerae contains three bd-type quinol oxygen reductases as well as one cbb(3) oxygen reductase. The cbb(3) oxygen reductase has been previously isolated and characterized; however, the natural mobile electron donor(s) that shuttles electrons between the bc(1) complex and the cbb(3) oxygen reductase is not known. The most likely candidates are the diheme cytochrome c(4) and monoheme cytochrome c(5), which have been previously shown to be present in the periplasm of aerobically grown cultures of V. cholerae. Both cytochromes c(4) and c(5) from V. cholerae have been cloned and expressed heterologously in Escherichia coli. It is shown that reduced cytochrome c(4) is a substrate for the purified cbb(3) oxygen reductase and can support steady state oxygen reductase activity of at least 300 e(-1)/s. In contrast, reduced cytochrome c(5) is not a good substrate for the cbb(3) oxygen reductase. Surprisingly, the dependence of the oxygen reductase activity on the concentration of cytochrome c(4) does not exhibit saturation. Global spectroscopic analysis of the time course of the oxidation of cytochrome c(4) indicates that the apparent lack of saturation is due to the strong dependence of K(M) and V(max) on the concentration of oxidized cytochrome c(4). Whether this is an artifact of the in vitro assay or has physiological significance remains unknown. Cyclic voltammetry was used to determine that the midpoint potentials of the two hemes in cytochrome c(4) are 240 and 340 mV (vs standard hydrogen electrode), similar to the electrochemical properties of other c(4)-type cytochromes. Genomic analysis shows a strong correlation between the presence of a c(4)-type cytochrome and a cbb(3) oxygen reductase within the beta- and gamma-proteobacterial clades, suggesting that cytochrome c(4) is the likely natural electron donor to the cbb(3) oxygen reductases within these organisms. These would include the beta-proteobacteria Neisseria meningitidis and Neisseria gonnorhoeae, in which the cbb(3) oxygen reductases are the only terminal oxidases in their respiratory chains, and the gamma-proteobacterium Pseudomonas stutzeri.

Purification of Helicobacter pylori NCTC 11637 cytochrome bc1 and respiration with D-proline as a substrate

Helicobacter pylori is a microaerophilic bacterium associated with gastric inflammation and peptic ulcers. Knowledge of how pathogenic organisms produce energy is important from a therapeutic point of view. We found d-amino acid dehydrogenase-mediated electron transport from d-proline or d-alanine to oxygen via the respiratory chain in H. pylori. Coupling of the electron transport to ATP synthesis was confirmed by using uncoupler reagents. We reconstituted the electron transport chain to demonstrate the electron flow from the d-amino acids to oxygen using the recombinant cytochrome bc(1) complex, cytochrome c-553, and the terminal oxidase cytochrome cbb(3) complex. Upon addition of the recombinant d-amino acid dehydrogenase and d-proline or d-alanine to the reconstituted electron transport system, reduction of cytochrome cbb(3) and oxygen consumption was revealed spectrophotometrically and polarographically, respectively. Among the constituents of H. pylori's electron transport chain, only the cytochrome bc(1) complex had been remained unpurified. Therefore, we cloned and sequenced the H. pylori NCTC 11637 cytochrome bc(1) gene clusters encoding Rieske Fe-S protein, cytochrome b, and cytochrome c(1), with calculated molecular masses of 18 kDa, 47 kDa, and 32 kDa, respectively, and purified the recombinant monomeric protein complex with a molecular mass of 110 kDa by gel filtration. The absorption spectrum of the recombinant cytochrome bc(1) complex showed an alpha peak at 561 nm with a shoulder at 552 nm.

Sequence analysis of the cbb3 oxidases and an atomic model for the Rhodobacter sphaeroides enzyme

The cbb3-type oxidases are members of the heme-copper oxidase superfamily, distant by sequence comparisons, but sharing common functional characteristics. To understand the minimal common properties of the superfamily, and to learn about cbb3-type oxidases specifically, we have analyzed a wide set of heme-copper oxidase sequences and built a homology model of the catalytic subunit of the cbb3 oxidase from Rhodobacter sphaeroides. We conclude that with regard to the active site surroundings, the cbb3 oxidases greatly resemble the structurally known oxidases, while major differences are found in three segments: the additional N-terminal stretch of ca. 60 amino acids, the segment following helix 3 to the end of helix 5, and the C-terminus from helix 11 onward. The conserved core contains the active site tyrosine and also an analogue of the K-channel of proton transfer, but centered on a well-conserved histidine in the lower part of helix 7. Modeling the variant parts of the enzyme suggests that two periplasmic loops (between helices 3 and 4 and between helices 11 and 12) could interact with each other as a part of the active site structure and might have an important role in proton pumping. An analogue of the D-channel is not found, but an alternative channel might form around helix 9. A preliminary packing model of the trimeric enzyme is also presented.

Campylobacter jejuni gene expression in the chick cecum: evidence for adaptation to a low-oxygen environment

Transcriptional profiling of Campylobacter jejuni during colonization of the chick cecum identified 59 genes that were differentially expressed in vivo compared with the genes in vitro. The data suggest that C. jejuni regulates electron transport and central metabolic pathways to alter its physiological state during establishment in the chick cecum.

The bacterial cytochrome cbb3 oxidases

Cytochrome cbb(3) oxidases are found almost exclusively in Proteobacteria, and represent a distinctive class of proton-pumping respiratory heme-copper oxidases (HCO) that lack many of the key structural features that contribute to the reaction cycle of the intensely studied mitochondrial cytochrome c oxidase (CcO). Expression of cytochrome cbb(3) oxidase allows human pathogens to colonise anoxic tissues and agronomically important diazotrophs to sustain N(2) fixation. We review recent progress in the biochemical characterisation of these distinctive oxidases that lays the foundation for understanding the basis of their proposed high affinity for oxygen, an apparent degeneracy in their electron input pathways and whether or not they acquired the ability to pump protons independently of other HCOs.

Identification by RNA profiling and mutational analysis of the novel copper resistance determinants CrdA (HP1326), CrdB (HP1327), and CzcB (HP1328) in Helicobacter pylori

Mechanisms involved in maintaining cytoplasmic metal ion homeostasis play a central role in the adaptation of Helicobacter pylori to the changing gastric environment. An investigation of the global regulatory responses to copper ions by using RNA profiling with a threshold factor of 4.0 revealed that copper induces transcription of 19 H. pylori genes and that only the ferritin gene pfr is repressed. The 57-fold copper induction identified the HP1326 gene encoding an H. pylori-specific protein as a candidate for a novel copper resistance determinant. The HP1326 gene is expressed as a monocistronic unit, and two small HP1326 mRNAs are copper induced. The HP1326 protein is secreted and is required for copper resistance maintained by cytoplasmic copper homeostasis, as H. pylori HP1326 mutants were copper sensitive and displayed increased copper induction of HP1326 transcription as well as elevated copper repression of ferritin synthesis. The clear copper-sensitive phenotype displayed by H. pylori HP1327 and HP1328 mutants provides strong evidence that the HP1326 protein, together with the signal peptide site of the H. pylori-specific protein HP1327, whose gene is located downstream from that encoding HP1326, and the CzcB and CzcA metal efflux system component homologs HP1328 and HP1329, constitutes a novel type of copper efflux pump, as discussed below. The HP1329 gene could not be inactivated, but the 14-fold transcriptional copper induction determined by RNA profiling points towards a function of the encoded CzcA homolog in copper resistance. In summary, results from RNA profiling identified the novel H. pylori-specific copper resistance determinants CrdA (HP1326) and CrdB (HP1327), which are required for adaptation to copper-rich environmental conditions.

The cytochrome cbb3 from Pseudomonas stutzeri displays nitric oxide reductase activity

The cytochrome cbb3 is an isoenzyme in the family of cytochrome c oxidases. This protein purified from Pseudomonas stutzeri displays a cyanide-sensitive nitric oxide reductase activity (Vmax=100+/-9 mol NO x mol cbb3(-1) x min(-1) and Km=12+/-2.5 microm), which is lost upon denaturation. This enzyme is only partially reduced by ascorbate, and readily re-oxidized by NO under anaerobic conditions at a rate consistent with the turnover number for NO consumption. As shown by transient spectroscopy experiments and singular value decomposition (SVD) analysis, these results suggest that the cbb3-type cytochromes, sharing structural features with bacterial nitric oxide reductases, are the enzymes retaining the highest NO reductase activity within the heme-copper oxidase superfamily.

Properties of the detergent solubilised cytochrome c oxidase (cytochrome cbb(3)) purified from Pseudomonas stutzeri

Cytochrome cbb(3) is a cytochrome c-oxidising isoenzyme that belongs to the superfamily of respiratory haem/copper oxidases. We have developed a purification method yielding large amounts of pure cbb(3) complex from the soil bacterium Pseudomonas stutzeri. This cytochrome cbb(3) complex consists of three subunits (ccoNOP) in a 1:1:1 stoichiometry and contains two b-type and three c-type haems. The protein complex behaves as a monomer with an overall molecular weight of 114.0+/-8.9 kDa and a s(0)(20,w) value of 8.9+/-0.3 S as determined by analytical ultracentrifugation. Crystals diffracting to 5.0 A resolution have been grown by the vapour diffusion sitting drop method to an average size of 0.1 x 0.1 x 0.3 mm. This is the first crystallisation report of a (cbb(3))-type oxidase.

The symbiotically essential cbb(3)-type oxidase of Bradyrhizobium japonicum is a proton pump

Purified cbb(3)-type oxidase of Bradyrhizobium japonicum was reconstituted into phospholipid vesicles. Tight vesicles were obtained as shown by the disturbance of deltapH with CCCP and the membrane potential with valinomycin, which led to a six-fold increase in cytochrome c oxidase activity. The vesicles were thus suitable for proton translocation experiments. In the presence of valinomycin, a pulse with reduced cytochrome c caused an acidification with a subsequent alkalinization, whereas the same pulse caused only an alkalinization in the presence of valinomycin plus CCCP. We conclude that the cbb(3)-type oxidase of B. japonicum is a proton pump.

Identification of cytochrome c oxidase in the alkaliphilic, obligately chemolithoautotrophic, sulfur-oxidizing bacterium 'Thioalcalomicrobium aerophilum' strain AL 3

Cytochrome c oxidase from the novel alkaliphilic autotrophic sulfur bacterium 'Thioalcalomicrobium aerophilum' strain AL 3 was isolated and purified 87-fold. Spectroscopic analysis revealed the presence of both c- and b-type hemes as well as copper in a ratio of 3:2:1. The purified enzyme consists of three subunits with apparent molecular masses of 41, 34 and 32 kDa. The two small subunits contain covalently bound heme c. With TMPD as a substrate the pH optimum was determined to be pH 8.0. In the presence of monovalent cations the specific activity of the purified oxidase increased significantly. The enzyme was not able to oxidize external cytochrome c, but accepted electron from its native electron donor. The latter was separated from the other membrane cytochromes during anion-exchange chromatography and was identified as a high potential cytochrome c(551). Overall the data indicate that the cytochrome c oxidase from this alkaliphilic autotrophic bacterium belongs to the heme-copper oxidase superfamily; regarding its subunit composition and content of prosthetic groups, the enzyme is similar in many aspects to the cbb(3)-type cytochrome c oxidases described for several neutrophilic bacteria, including anaerobic phototrophic and aerobic sulfur-oxidizing bacteria.