The carboxy-terminal insert in the Q-loop is needed for functionality of Escherichia coli cytochrome bd-I

Cytochrome bd, a component of the prokaryotic respiratory chain, is important under physiological stress and during pathogenicity. Electrons from quinol substrates are passed on via heme groups in the CydA subunit and used to reduce molecular oxygen. Close to the quinol binding site, CydA displays a periplasmic hydrophilic loop called Q-loop that is essential for quinol oxidation. In the carboxy-terminal part of this loop, CydA from Escherichia coli and other proteobacteria harbors an insert of ~60 residues with unknown function. In the current work, we demonstrate that growth of the multiple-deletion strain E. coli MB43∆cydA (∆cydA∆cydB∆appB∆cyoB∆nuoB) can be enhanced by transformation with E. coli cytochrome bd-I and we utilize this system for assessment of Q-loop mutants. Deletion of the cytochrome bd-I Q-loop insert abolished MB43∆cydA growth recovery. Swapping the cytochrome bd-I Q-loop for the Q-loop from Geobacillus thermodenitrificans or Mycobacterium tuberculosis CydA, which lack the insert, did not enhance the growth of MB43∆cydA, whereas swapping for the Q-loop from E. coli cytochrome bd-II recovered growth. Alanine scanning experiments identified the cytochrome bd-I Q-loop insert regions Ile³¹⁸-Met³²², Gln³³⁸-Asp³⁴², Tyr³⁵³-Leu³⁵⁷, and Thr³⁶⁸-Ile³⁷² as important for enzyme functionality. Those mutants that completely failed to recover growth of MB43∆cydA also lacked oxygen consumption activity and heme absorption peaks. Moreover, we were not able to isolate cytochrome bd-I from these inactive mutants. The results indicate that the cytochrome bd Q-loop exhibits low plasticity and that the Q-loop insert in E. coli is needed for complete, stable, assembly of cytochrome bd-I.

Enhanced Immunoaffinity Purification of Human Neutrophil Flavocytochrome B for Structure Determination by Electron Microscopy

Determination of the structure of human neutrophil (PMN) flavocytochrome b (Cytb) is a necessary step for the understanding of the structure-function essentials of NADPH oxidase activity. This understanding is crucial for structure-driven therapeutic approaches addressing control of inflammation and infection. Our work on purification and sample preparation of Cytb has facilitated progress toward the goal of structure determination. Here we describe exploiting immunoaffinity purification of Cytb for initial examination of its size and shape by a combination of classical and cryoelectron microscopic (EM) methods. For these evaluations, we used conventional negative-stain transmission electron microscopy (TEM) to examine both detergent-solubilized Cytb as single particles and Cytb in phosphatidylcholine reconstituted membrane vesicles as densely packed random, partially ordered, and subcrystalline arrays. In preliminary trials, we also examined single particles by cryoelectron microscopy (cryoEM) methods. We conclude that Cytb in detergent and reconstituted in membrane is a relatively compact, symmetrical protein of about 100 Å in maximum dimension. The negative stain, preliminary cryoEM, and crude molecular models suggest that the protein is probably a heterotetramer of two p22^phox and gp91^phox subunits in both detergent micelles and membrane vesicles. This exploratory study also suggests that high-resolution 2D electron microscopic approaches may be accessible to human material collected from single donors.

An evaluation of ELISA using recombinant Brucella abortus bacterioferritin (Bfr) for bovine brucellosis

To date, detection of antibodies against the lipopolysaccharide portion is the backbone of most serodiagnostic methods for brucellosis screening. However this pose a risk for false positive reactions related to other pathogens especially that of Yersinia enterocolitica O:9 which has the most prominent cross reactivity with Brucella spp. In this study, cloning and expression of Brucella abortus bacterioferritin (Bfr) was accomplished by PCR amplification into an expression vector system, and purification of a recombinant B. abortus Bfr (rBfr). The immunogenicity of rBfr was confirmed by Western blot with Brucella-positive bovine serum. To determine whether rBfr has a potential benefit for use in the serodiagnosis of bovine brucellosis, rBfr-based ELISA was performed. Interestingly, rBfr was able to detect anti-Brucella antibodies in positive sera in a dependent manner of TAT values but did not show an immunoreaction with negative samples. Particularly, average OD492 values at the lowest, medium and highest TAT titer levels were 1.4, 2.2 and 2.6-fold increase compared with the cutoff value, respectively. The accuracy, specificity and sensitivity of rBfr showed 89.09%, 93.6% and 85.33%, respectively. These findings suggest that rBfr might be a good candidate for serological diagnosis development of bovine brucellosis.

Affinity purification and reconstitution of human phagocyte flavocytochrome B for detection of conformational dynamics in the membrane

Human flavocytochrome b (Cyt b) is the core electron transferase of the NADPH oxidase in phagocytes and a number of other cell types. The oxidase complex generates superoxide, initiating production of a cascade of reactive oxygen species critical for the killing of infectious agents. Many fundamental questions still remain concerning its structural dynamics and electron transfer mechanisms. In particular, Cyt b structure/function correlates in the membrane have been relatively unstudied. In order to facilitate the direct analysis of Cyt b structural dynamics in the membrane, the following method provides rapid and efficient procedures for the affinity purification of Cyt b from isolated neutrophil membrane fractions and its functional reconstitution in purified lipid preparations. The protocol presented here contains some new optimized procedures that will facilitate Cyt b isolation and reconstitution. Additional methods are presented that facilitate examination of conformational dynamics of the membrane reconstituted purified Cyt b by fluorescence resonance energy transfer (FRET) as measured by steady-state and lifetime fluorescence techniques.

Molecular interface of S100A8 with cytochrome b558 and NADPH oxidase activation

S100A8 and S100A9 are two calcium binding Myeloid Related Proteins, and important mediators of inflammatory diseases. They were recently introduced as partners for phagocyte NADPH oxidase regulation. However, the precise mechanism of their interaction remains elusive. We had for aim (i) to evaluate the impact of S100 proteins on NADPH oxidase activity; (ii) to characterize molecular interaction of either S100A8, S100A9, or S100A8/S100A9 heterocomplex with cytochrome b₅₅₈; and (iii) to determine the S100A8 consensus site involved in cytochrome b₅₅₈/S100 interface. Recombinant full length or S100A9-A8 truncated chimera proteins and ExoS-S100 fusion proteins were expressed in E. coli and in P. aeruginosa respectively. Our results showed that S100A8 is the functional partner for NADPH oxidase activation contrary to S100A9, however, the loading with calcium and a combination with phosphorylated S100A9 are essential in vivo. Endogenous S100A9 and S100A8 colocalize in differentiated and PMA stimulated PLB985 cells, with Nox2/gp91^phox and p22^phox. Recombinant S100A8, loaded with calcium and fused with the first 129 or 54 N-terminal amino acid residues of the P. aeruginosa ExoS toxin, induced a similar oxidase activation in vitro, to the one observed with S100A8 in the presence of S100A9 in vivo. This suggests that S100A8 is the essential component of the S100A9/S100A8 heterocomplex for oxidase activation. In this context, recombinant full-length rS100A9-A8 and rS100A9-A8 truncated 90 chimera proteins as opposed to rS100A9-A8 truncated 86 and rS100A9-A8 truncated 57 chimeras, activate the NADPH oxidase function of purified cytochrome b₅₅₈ suggesting that the C-terminal region of S100A8 is directly involved in the molecular interface with the hemoprotein. The data point to four strategic ⁸⁷HEES⁹⁰ amino acid residues of the S100A8 C-terminal sequence that are involved directly in the molecular interaction with cytochrome b₅₅₈ and then in the phagocyte NADPH oxidase activation.

Ferritin structure from Mycobacterium tuberculosis: comparative study with homologues identifies extended C-terminus involved in ferroxidase activity

Ferritins are recognized as key players in the iron storage and detoxification processes. Iron acquisition in the case of pathogenic bacteria has long been established as an important virulence mechanism. Here, we report a 3.0 Å crystal structure of a ferritin, annotated as Bacterioferritin B (BfrB), from Mycobacterium tuberculosis (Mtb), the causative agent of tuberculosis that continues to be one of the world's deadliest diseases. Similar to the other members of ferritin family, the Mtb BfrB subunit exhibits the characteristic fold of a four-helical bundle that possesses the ferroxidase catalytic centre. We compare the structure of Mtb BfrB with representatives of the ferritin family belonging to the archaea, eubacteria and eukarya. Unlike most other ferritins, Mtb BfrB has an extended C-terminus. To dissect the role of this extended C-terminus, truncated Mtb BfrB was purified and biochemical studies implicate this region in ferroxidase activity and iron release in addition to providing stability to the protein. Functionally important regions in a protein of known 3D-structure can be determined by estimating the degree of conservation of the amino-acid sites with its close homologues. Based on the comparative studies, we identify the slowly evolving conserved sites as well as the rapidly evolving variable sites and analyze their role in relation to structure and function of Mtb BfrB. Further, electrostatic computations demonstrate that although the electrostatic environment of catalytic residues is preserved within the family, extensive variability is exhibited by residues defining the channels and pores, in all likelihood keeping up with the diverse functions executed by these ferritins in varied environments.

High-yield production, purification and characterization of functional human duodenal cytochrome b in an Escherichia coli system

Human duodenal cytochrome b (Dcytb) is a transmembrane hemoprotein found in the duodenal brush border membrane and in erythrocytes. Dcytb has been linked to uptake of dietary iron and to ascorbate recycling in erythrocytes. Detailed biophysical and biochemical characterization of Dcytb has been limited by difficulties in expressing sufficient amounts of functional recombinant protein in yeast and insect cell systems. We have developed an Escherichia coli Rosetta-gami B(DE3) cell system for production of recombinant His-tagged human Dcytb with a yield of ∼26 mg of purified, ascorbate-reducible cytochrome per liter of culture. The recombinant protein is readily solubilized with n-dodecyl-β-D-maltoside and purified to electrophoretic homogeneity by one-step chromatography on cobalt affinity resin. The purified recombinant Dcytb has a heme to protein ratio very close to the theoretical value of 2 and retains functional reactivity with ascorbate, as assessed by spectroscopic and kinetic measurements. Ascorbate showed a marked kinetic selectivity for the high-potential heme center over the low-potential heme center in purified Dcytb. This new E. coli expression system for Dcytb offers ∼7-fold improvement in yield and other substantial advantages over existing expression systems for reliable production of functional Dcytb at levels suitable for biochemical, biophysical and structural characterization.

Auxin-responsive genes AIR12 code for a new family of plasma membrane b-type cytochromes specific to flowering plants

We report here on the identification of the major plasma membrane (PM) ascorbate-reducible b-type cytochrome of bean (Phaseolus vulgaris) and soybean (Glycine max) hypocotyls as orthologs of Arabidopsis (Arabidopsis thaliana) AIR12 (for auxin induced in root cultures). Soybean AIR12, which is glycosylated and glycosylphosphatidylinositol-anchored to the external side of the PM in vivo, was expressed in Pichia pastoris in a recombinant form, lacking the glycosylphosphatidylinositol modification signal and purified from the culture medium. Recombinant AIR12 is a soluble protein predicted to fold into a beta-sandwich domain and belonging to the DOMON (for dopamine beta-monooxygenase N terminus) domain superfamily. It is shown to be a b-type cytochrome with a symmetrical alpha-band at 561 nm, fully reduced by ascorbate, and fully oxidized by monodehydroascorbate radical. AIR12 is a high-potential cytochrome b showing a wide bimodal dependence from the redox potential between +80 mV and +300 mV. Optical absorption and electron paramagnetic resonance analysis indicate that AIR12 binds a single, highly axial low-spin heme, likely coordinated by methionine-91 and histidine-76, which are strongly conserved in AIR12 sequences. Phylogenetic analyses reveal that the auxin-responsive genes AIR12 represent a new family of PM b-type cytochromes specific to flowering plants. Circumstantial evidence suggests that AIR12 may interact with other redox partners within the PM to constitute a redox link between cytoplasm and apoplast.

Isolation of highly active photosystem II core complexes with a His-tagged Cyt b559 subunit from transplastomic tobacco plants

Photosystem II (PSII) is a huge multi-protein-complex consisting, in higher plants and green algae, of the PS II core and the adjacent light harvesting proteins. In the study reported here, N-terminal His-tags were added to the plastome-encoded alpha-subunit of cytochrome b559, PsbE, in tobacco plants, thus facilitating rapid, mild purification of higher plant PSII. Biolistic chloroplast transformation was used to replace the wildtype psbE gene by His-tagged counterparts. Transgenic plants did not exhibit an obvious phenotype. However, the oxygen evolution capacity of thylakoids prepared from the mutants compared to the wildtype was reduced by 10-30% depending on the length of the His-tag, although Fv/Fm values differed only slightly. Homoplasmic F1 plants were used to isolate PSII cores complexes. The cores contained no detectable traces of LHC or PsaA/B polypeptides, but the main core subunits of PSII could be identified using immunodetection and mass spectroscopy. In addition, Psb27 and PsbS were detected. The presence of the former was presumably due to the preparation method, since PSII complexes located in the stroma are also isolated. In contrast to previous reports, PsbS was solely found as a monomer on SDS-PAGE in the PSII core complexes of tobacco.

Cloning, expression, purification, crystallization and preliminary X-ray crystallographic analysis of bacterioferritin A from Mycobacterium tuberculosis

Bacterioferritins (Bfrs) comprise a subfamily of the ferritin superfamily of proteins that play an important role in bacterial iron storage and homeostasis. Bacterioferritins differ from ferritins in that they have additional noncovalently bound haem groups. To assess the physiological role of this subfamily of ferritins, a greater understanding of the structural details of bacterioferritins from various sources is required. The gene encoding bacterioferritin A (BfrA) from Mycobacterium tuberculosis was cloned and expressed in Escherichia coli. The recombinant protein product was purified by affinity chromatography on a Strep-Tactin column and crystallized with sodium chloride as a precipitant at pH 8.0 using the vapour-diffusion technique. The crystals diffracted to 2.1 A resolution and belonged to space group P4(2), with unit-cell parameters a = 123.0, b = 123.0, c = 174.6 A.

Immunoaffinity purification of human phagocyte flavocytochrome b and analysis of conformational dynamics

The heterodimeric integral membrane protein flavocytochrome b (Cyt b) is the catalytic core of the phagocyte NADPH oxidase, an enzyme complex that initiates a cascade of reactive oxygen species critical for the elimination of infectious agents. Many fundamental questions remain concerning the structure and catalytic mechanism of Cyt b, largely because of the inability to isolate this protein in quantities required for both biochemical analysis and meaningful attempts at high-resolution structure determination. In order to facilitate the direct analysis of Cyt b, the following method describes a rapid and efficient procedure for the immunoaffinity purification of Cyt b (under nondenaturing conditions) from neutrophil membrane fractions. The protocol presented here contains a number of steps that have been optimized and improved since the original description of this Cyt b isolation method. In order to address questions concerning the mechanism of superoxide generation by the NADPH oxidase complex, methods are additionally presented for analysis of conformational dynamics of immunoaffinity-purified Cyt b by resonance energy transfer.

Fortuitous structure determination of 'as-isolated' Escherichia coli bacterioferritin in a novel crystal form

Escherichia coli bacterioferritin was serendipitously crystallized in a novel cubic crystal form and its structure could be determined to 2.5 A resolution despite a high degree of merohedral twinning. This is the first report of crystallographic data on 'as-isolated' E. coli bacterioferritin. The ferroxidase active site contains positive difference density consistent with two metal ions that had co-purified with the protein. X-ray fluorescence studies suggest that the metal composition is different from that of previous structures and is a mix of zinc and native iron ions. The ferroxidase-centre configuration displays a similar flexibility as previously noted for other bacterioferritins.

Haloarcula marismortui cytochrome b-561 is encoded by the narC gene in the dissimilatory nitrate reductase operon

The composition of membrane-bound electron-transferring proteins from denitrifying cells of Haloarcula marismortui was compared with that from the aerobic cells. Accompanying nitrate reductase catalytic NarGH subcomplex, cytochrome b-561, cytochrome b-552, and halocyanin-like blue copper protein were induced under denitrifying conditions. Cytochrome b-561 was purified to homogeneity and was shown to be composed of a polypeptide with a molecular mass of 40 kDa. The cytochrome was autooxidizable and its redox potential was -27 mV. The N-terminal sequence of the cytochrome was identical to the deduced amino acid sequence of the narC gene product encoded in the third ORF of the nitrate reductase operon with a unique arrangement of ORFs. The sequence of the cytochrome was homologous with that of the cytochrome b subunit of respiratory cytochrome bc. A possibility that the cytochrome bc and the NarGH constructed a supercomplex was discussed.

Heme A synthase enzyme functions dissected by mutagenesis of Bacillus subtilis CtaA

Heme A, as a prosthetic group, is found exclusively in respiratory oxidases of mitochondria and aerobic bacteria. Bacillus subtilis CtaA and other heme A synthases catalyze the conversion of a methyl side group on heme O into a formyl group. The catalytic mechanism of heme A synthase is not understood, and little is known about the composition and structure of the enzyme. In this work, we have: (i) constructed a ctaA deletion mutant and a system for overproduction of mutant variants of the CtaA protein in B. subtilis, (ii) developed anaffinity purification procedure for isolation of preparative amounts of CtaA, and (iii) investigated the functional roles of four invariant histidine residues in heme A synthase by in vivo and in vitro analyses of the properties of mutant variants of CtaA. Our results show an important function of three histidine residues for heme A synthase activity. Several of the purified mutant enzyme proteins contained tightly bound heme O. One variant also contained trapped hydroxylated heme O, which is a postulated enzyme reaction intermediate. The findings indicate functional roles for the invariant histidine residues and provide strong evidence that the heme A synthase enzyme reaction includes two consecutive monooxygenations.

Purification and characterization of bovine adrenal cytochrome b561 expressed in insect and yeast cell systems

Bovine adrenal chromaffin granule cytochrome (cyt) b561 is a transmembrane hemoprotein that plays a key role in transporting reducing equivalents from ascorbate to dopamine-beta-hydroxylase for catecholamine synthesis. We have developed procedures for expression and purification of functional bovine adrenal cyt b561 in insect and yeast cell systems. The bovine cyt b561 coding sequence, with or without a hexahistidine-tag sequence at the C-terminus, was cloned into the pVL1392 transfer vector under the control of the polyhedrin promoter to generate recombinant baculovirus for protein expression in Sf9 insect cells (approximately 0.5 mg detergent-solubilized cyt b561/L culture). For the yeast system, the cyt b561 cDNA was modified with a hexahistidine-tag sequence at the C-terminus, and inserted into the pPICZB vector under the control of the alcohol oxidase promoter. The recombinant plasmid was transformed into Pichia pastoris GS115 competent cells to give methanol-inducible cyt b561 expression (approximately 0.7 mg detergent-solubilized cyt b561/L culture). Recombinant His-tagged cyt b561 expressed in Sf9 or Pichia cells was readily solubilized from membrane fractions with dodecyl maltoside and purified to electrophoretic homogeneity by one-step chromatography on Ni-NTA affinity resin. The purified recombinant cytochrome from both systems had a heme to protein ratio close to two and was fully functional, as judged by comparison with the spectroscopic and kinetic parameters of the endogenous cytochrome from chromaffin granules. A novel procedure for isolation of chromaffin granule membranes was developed to utilize frozen adrenal glands instead of fresh tissue.

Identification of an ascorbate-dependent cytochrome b of the tonoplast membrane sharing biochemical features with members of the cytochrome b561 family

Two membrane-bound, ascorbate-dependent b-type cytochromes were identified in etiolated bean (Phaseolus vulgaris L.) hypocotyls. Following solubilization of microsomal membranes and anion-exchange chromatography at pH 8.0, two major cytochrome peaks (P-I and P-II) were separated. Both cytochromes were reduced by ascorbate and re-oxidized by monodehydroascorbate, but P-I reduction by ascorbate was higher and saturated at far lower concentrations of ascorbate with respect to P-II. The alpha-band was symmetrically centered at 561 nm in P-I, but it was asymmetric in P-II with a maximum at 562 nm and shoulder at 557 nm. Ascorbate reduction of P-II, but not P-I, was inhibited by diethyl pyrocarbonate. Reduced P-II but not P-I was readily oxidized by certain ferric chelates, including FeEDTA and Fe-nitrilotriacetic acid. Purified P-I, associated with the plasma membrane, showed up as a 63-kDa glycosylated protein during sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and behaved as a monomer of about 70 kDa during size-exclusion chromatography. P-I identified with a previously purified ascorbate-dependent b-type cytochrome of bean hypocotyl plasma membranes. Partially purified P-II, on the other hand, correlated with a heme-protein of 27 kDa in SDS-PAGE gels, was dimeric (60 kDa) during size-exclusion chromatography, and was associated with the tonoplast marker V-ATPase in sucrose gradients. The sequence of a peptide of 11 residues obtained by tryptic digestion of P-II was found to be identical to a segment of a putative cytochrome b561 of Zea mays and highly conserved in other related plant sequences, including that of Arabidopsis thaliana cytochrome b561-1 (CAA18169). The biochemical features fully support the assignment of P-II cytochrome to the family of cytochrome b561, ascorbate-dependent (CYBASC) cytochromes, which also includes cytochrome b561 of animal chromaffin granules. The presence of a cytochrome reducing ferric chelates on the tonoplast is consistent with the role of plant vacuoles in iron homeostasis.

Heme O Synthase and Heme A Synthase from Bacillus subtilis and Rhodobacter sphaeroides Interact in Escherichia coli

Cytochrome c oxidase requires multiple heme and copper cofactors to catalyze the reduction of molecular oxygen to water. Although significant progress has been made in understanding the transport and incorporation of the copper ions, considerably less is known about the trafficking and insertion of the heme cofactors. Heme O synthase (HOS) and heme A synthase (HAS) from Rhodobacter sphaeroides (Cox10 and Cox15, respectively) and Bacillus subtilis (CtaB and CtaA, respectively) have been cloned and expressed in Escherichia coli. Our results demonstrate that HOS copurifies with HAS and that HAS copurifies with HOS, indicating that HOS and HAS interact and may form a physiologically relevant complex in vivo. Consistent with this hypothesis, the presence of HAS alters the total level of farnesylated hemes, providing further evidence that HOS and HAS interact. Our current working model is that HOS and HAS form a complex and that heme O is transferred directly from HOS to HAS. Because of the strong sequence similarity and evolutionary relationship between R. sphaeroides and mitochondria, our data suggest that this complex may form in eukaryotes as well.

Critical roles of bacterioferritins in iron storage and proliferation of cyanobacteria

Cyanobacteria are key contributors to global photosynthetic productivity, and iron availability is essential for cyanobacterial proliferation. While iron is abundant in the earth's crust, its unique chemical properties render it a limiting factor for photoautotrophic growth. As compared to other nonphotosynthetic organisms, oxygenic photosynthetic organisms such as cyanobacteria, algae, and green plants need large amounts of iron to maintain functional PSI complexes in their photosynthetic apparatus. Ferritins and bacterioferritins are ubiquitously present iron-storage proteins. We have found that in the cyanobacterium Synechocystis sp. PCC 6803 (Synechocystis 6803), bacterioferritins are responsible for the storage of as much as 50% of cellular iron. Synechocystis 6803, as well as many other cyanobacterial species, have two bacterioferritins, BfrA and BfrB, in which either the heme binding or di-iron center ligating residues are absent. Purified bacterioferritin complex from Synechocystis 6803 has both BfrA and BfrB proteins. Targeted mutagenesis of each of the two bacterioferritin genes resulted in poor growth under iron-deprived conditions. Inactivation of both genes did not result in a more severe phenotype. These results support the presence of a heteromultimeric structure of Synechocystis bacterioferritin, in which one subunit ligates a di-iron center while the other accommodates heme binding. Notably, the reduced internal iron concentrations in the mutant cells resulted in a lower content of PSI. In addition, they triggered iron starvation responses even in the presence of normal levels of external iron, thus demonstrating a central role of bacterioferritins in iron homeostasis in these photosynthetic organisms.

Properties of Two Distinct Heme Centers of Cytochrome b561 from Bovine Chromaffin Vesicles Studied by EPR, Resonance Raman, and Ascorbate Reduction Assay

Cytochrome b(561) from bovine adrenal chromaffin vesicles contains two hemes b with different midpoint potentials (+150 and +60 mV) and participates in transmembrane electron transport from extravesicular ascorbate to an intravesicular monooxygenase, dopamine beta-hydroxylase. Treatment of oxidized cytochrome b(561) with diethylpyrocarbonate caused a downshift of midpoint potential for the lower component, and this shift was prevented by the presence of ascorbate during the treatment. Present EPR analyses showed that, upon the treatment, the g(z) = 3.69 heme species was converted to a non-ascorbate-reducible form, although its g(z)-value showed no appreciable change. The treatment had no effect on the other heme (the g(z) = 3.13 species). Raman data indicated that the two heme b centers adopt a six-coordinated low-spin state, in both the reduced and oxidized forms. There was no significant effect of diethylpyrocarbonate-treatment on the Raman spectra of either form, but the reducibility by ascorbate differed significantly between the two hemes upon the treatment. The addition of ferrocyanide enhanced both the reduction rate and final reduction level of the diethylpyrocarbonate-treated cytochrome b(561) when ascorbate was used as a reductant. This observation suggests that ferrocyanide scavenges monodehydroascorbate radicals produced by the univalent oxidation of ascorbate and, thereby, increases both the reduction rate and the final reduction level of the heme center on the intravesicular side of the diethylpyrocarbonate-treated cytochrome. These results further clarify the physiological role of this heme center as the electron donor to the monodehydroascorbate radical.

Evidence that the PsbK polypeptide is associated with the photosystem II core antenna complex CP43

PsbK is encoded by the chloroplast psbK gene and is one of the small polypeptides of photosystem II (PSII). This polypeptide is required for accumulation of the PSII complex. In the present study, we generated an antibody against recombinant mature PsbK of Chlamydomonas and used it in Western blots to localize PsbK in the PSII core complex. PsbK was found in the thylakoid membranes, and purification of the PSII core complex from detergent-solubilized thylakoid membranes showed that PsbK is tightly associated with the PSII core complex. We used potassium thiocyanate to separate PSII into subcore complexes, including the D1/D2/cytochrome b559 reaction center complex, CP47, and CP43, and we found that PsbK co-purifies with one of the core antenna complexes, CP43, during ion exchange chromatography. Subsequent gel filtration chromatography of the purified CP43 confirmed that PsbK is tightly associated with CP43. Steady-state levels of PsbK were also determined in Chlamydomonas mutants expressing various levels of PSII. Quantitative Western blotting revealed that the levels of PsbK in these mutants are approximately equal to those of CP43, suggesting that PsbK is stable only when associated with CP43 in the chloroplast. Together, our results indicate that PsbK is an integral part of the PSII complex and may participate in the assembly and stability of the PSII complex.

Stopped-flow analyses on the reaction of ascorbate with cytochrome b561 purified from bovine chromaffin vesicle membranes

Cytochrome b(561) in adrenal chromaffin vesicle membranes conveys electron equivalents from extravesicular ascorbate to the intravesicular monodehydroascorbate radical. We conducted a stopped-flow study on the reaction of ascorbate with purified cytochrome b(561) in the detergent-solubilized state for the first time. The time course of the reduction of oxidized cytochrome b(561) with ascorbate could not be fitted with a single exponential but with a linear combination of at least four exponential functions. This result is consistent with the notion that cytochrome b(561) contains two hemes b, each having a distinct redox potential and a function upon reactions with ascorbate and monodehydroascorbate radical. The fastest phase, which was assigned to the first one-electron donation from ascorbate to heme b on the extravesicular side, was further analyzed by transient phase kinetics employing a two-step bi-uni sequential ordered mechanism. The result showed K(s) = 2.2 mM for ascorbate at pH6.0. At a region below pH5.5, there was a significant lag before the reduction of hemes b occurred. This time lag was interpreted as due to a pH-dependent transient state before the first electron transfer to take place. The fastest phase was completely lost by N-carbethoxylation of heme-coordinating histidyl residues (His88 and His161) and Lys85 upon treatment with diethylpyrocarbonate. The presence of ascorbate during the treatment inhibited the N-carbethoxylation of the histidyl residues and, thereby, restored the final reduction level of hemes b. But the reduction rate was still only one-twentieth of the native form. This result suggested an important role of the conserved Lys85 for the interaction with ascorbate.

Structural changes upon excitation of D1-D2-Cyt b559 photosystem II reaction centers depend on the beta-carotene content

Different preparations of D1-D2-Cyt b559 complexes from spinach with different beta-carotene (Car) content [on average from <0.5 to 2 per reaction center (RC)] were studied by means of laser-induced optoacoustic spectroscopy. phiP680(+)Pheo(-) does not depend on the preparation (or on the Car content) inasmuch as the magnitude of the prompt heat (produced within 20 ns) does not vary for the different samples upon excitation at 675 and 620 nm. The energy level of the primary charge-separated state, P680(+)Pheo(-), was determined as EP680(+)Pheo(-) = 1.55 eV. Thus, an enthalpy change accompanying charge separation from excited P680 of deltaH*P680Pheo-->P680(+)Pheo(-) = -0.27 eV is obtained. Calculations using the heat evolved during the time-resolved decay of P680(+)Pheo(-) (< or = 100 ns) affords a triplet (3[P680Pheo]) quantum yield phi3[P680Pheo] = 0.5 +/- 0.14. The structural volume change, deltaV1, corresponding to the formation of P680(+)Pheo(-), strongly depends on the Car content; it is ca. -2.5 A3 molecule(-1) for samples with <0.5 Car on average, decreases (in absolute value) to -0.5 +/- 0.2 A3 for samples with an average of 1 Car, and remains the same for samples with two Cars per RC. This suggests that the Car molecules induce changes in the ground-state RC conformation, an idea which was confirmed by preferential excitation of Car with blue light, which produced different carotene triplet lifetimes in samples with 2 Car compared to those containing less carotene. We conclude that the two beta-carotenes are not structurally equivalent. Upon blue-light excitation (480 nm, preferential carotene absorption) the fraction of energy stored is ca. 60% for the 9Chl-2Car sample, whereas it is 40% for the preparations with one or less Cars on average, indicating different paths of energy distribution after Car excitation in these RCs with remaining chlorophyll antennae.

Isolation and characterization of the stage-specific cytochrome b small subunit (CybS) of Ascaris suum complex II from the aerobic respiratory chain of larval mitochondria

We recently reported that Ascaris suum mitochondria express stage-specific isoforms of complex II: the flavoprotein subunit and the small subunit of cytochrome b (CybS) of the larval complex II differ from those of adult enzyme, while two complex IIs share a common iron-sulfur cluster subunit (Ip). In the present study, A. suum larval complex II was highly purified to characterize the larval cytochrome b subunits in more detail. Peptide mass fingerprinting and N-terminal amino acid sequencing showed that the larval and adult cytochrome b (CybL) proteins are identical. In contrast, cDNA sequences revealed that the small subunit of larval cytochrome b (CybS(L)) is distinct from the adult CybS (CybS(A)). Furthermore, Northern analysis and immunoblotting showed stage-specific expression of CybS(L) and CybS(A) in larval and adult mitochondria, respectively. Enzymatic assays revealed that the ratio of rhodoquinol-fumarate reductase (RQFR) to succinate-ubiquinone reductase (SQR) activities and the K(m) values for quinones are almost identical for the adult and larval complex IIs, but that the fumarate reductase (FRD) activity is higher for the adult form than for the larval form. These results indicate that the adult and larval A. suum complex IIs have different properties than the complex II of the mammalian host and that the larval complex II is able to function as a RQFR. Such RQFR activity of the larval complex II would be essential for rapid adaptation to the dramatic change of oxygen availability during infection of the host.

Site-directed photochemical coupling of cytochrome b6f-associated chlorophyll

Cytochrome b(6)f complexes contain a molecule of chlorophyll a (Chla), which, in Chlamydomonas reinhardtii, can be exchanged for extraneous chlorophyll during protracted incubation of the purified complex in detergent solution. The specificity of the site and its location in the complex have been studied by photochemical coupling and circular dichroism spectroscopy. Following substitution of the original chlorophyll with [(3)H]Chla, the complex was irradiated in the Soret absorption band of Chla to complete bleaching and the amount of radioactivity covalently bound to each b(6)f subunit determined. Strong labeling was found to be associated with cytochrome f. The labeling originates from [(3)H]Chla molecules bound to a slowly exchanging site and showing the properties of the endogenous Chl, not from molecules dissolved in the detergent belt surrounding the complex. Chlorophyll b (Chlb) can compete with Chla, albeit with a lower affinity. Irradiation of [(3)H]Chlb introduced into the slowly exchanging site yielded the same labeling pattern that was observed with [(3)H]Chla. Proteolytic cleavage showed [(3)H]Chla labeling to be strictly restricted to the C-terminal region of cytochrome f. Circular dichroism spectra of the native complex revealed a bilobed signal characteristic of excitonic interaction between chlorophylls. The structural and evolutionary implications of these findings are discussed.

Hyperconservation of the putative antigen recognition site of the MHC class I-b molecule TL in the subfamily Murinae: evidence that thymus leukemia antigen is an ancient mammalian gene

"Classical" MHC class I (I-a) genes are extraordinarily polymorphic, but "nonclassical" MHC class I (I-b) genes are monomorphic or oligomorphic. Although diversifying (positive) Darwinian selection is thought to explain the origin and maintenance of MHC class I-a polymorphisms, genetic mechanisms underlying MHC class I-b evolution are uncertain. In one extreme model, MHC class I-b loci are derived by gene duplication from MHC class I-a alleles but rapidly drift into functional obsolescence and are eventually deleted. In this model, extant MHC class I-b genes are relatively young, tend to be dysfunctional or pseudogenic, and orthologies are restricted to close taxa. An alternative model proposed that the mouse MHC class I-b gene thymus leukemia Ag (TL) arose approximately 100 million years ago, near the time of the mammalian radiation. To determine the mode of evolution of TL, we cloned TL from genomic DNA of 11 species of subfamily Murinae: Every sample we tested contained TL, suggesting this molecule has been maintained throughout murine evolution. The sequence similarity of TL orthologs ranged from 85-99% and was inversely proportional to taxonomic distance. The sequences showed high conservation throughout the entire extracellular domains with exceptional conservation in the putative Ag recognition site. Our results strengthen the hypotheses that TL has evolved a specialized function and represents an ancient MHC class I-b gene.

Aeropyrum pernix K1, a strictly aerobic and hyperthermophilic archaeon, has two terminal oxidases, cytochrome ba3 and cytochrome aa3

Aeropyrum pernix K1 is a strictly aerobic and hyperthermophilic archaeon that thrives even at 100 degrees C. The archaeon is quite interesting with respect to the evolution of aerobic electron transport systems and the thermal stability of the respiratory components. An isolated membrane fraction was found to oxidize bovine cytochrome c. The activity was solubilized in the presence of detergents and separated into two fractions by successive chromatography. Two cytochrome oxidases, designated as CO-1 and CO-2, were further purified. CO-1 was a ba(3)-type cytochrome containing at least two subunits. Chemically digested fragments of CO-1 revealed a peptide with a sequence identical to a part of a putative cytochrome oxidase subunit I encoded by the gene ape1623. CO-2, an aa(3)-type cytochrome, was present in lower amounts than CO-1 and was immunologically identified as a product of aoxABC gene (DDBJ accession no. AB020482). Both cytochromes reacted with carbon monoxide. The apparent K(m) values of CO-1 and CO-2 for oxygen were 5.5 and 32 micro M, respectively, at 25 degrees C. The terminal oxidases CO-1 and CO-2 phylogenetically correspond to the SoxB and SoxM branches, respectively, of the heme-copper oxidase tree.

An archaeal b-type cytochrome containing a nonfunctional carbonic anhydrase-like domain

A new type of cytochrome b was isolated from the cytoplasmatic fraction of the archaeon Acidianus ambivalens, which is the first soluble cytochrome found in this member of the thermoacidophilic order of the Sulfolobales. The protein is a monomeric and monohemic cytochrome b with a molecular mass of 22 kDa. Visible spectroscopy of the as-purified protein shows a Soret peak at 405 nm and a broad band at 625 nm, indicating the presence of a high-spin ferric heme. Upon reduction, the Soret band shifts to 422 nm and a broad band at 560 nm develops, again characteristic of high-spin ferrous heme. The reduced form can bind carbon monoxide, with visible absorption bands arising at 411 and 566 nm. EPR spectroscopy of the oxidized protein shows a spectrum typical of a high-spin heme, with major g values at 6.56 and 5.85. The reduction potential of the heme cofactor was determined to be -16+/-10 mV, at pH 6.5. Analysis of the protein amino acid sequence shows that it consists of a novel arrangement of domains. The first domain, at the N-terminus, has a remarkable similarity towards beta class carbonic anhydrases, whereas the second region comprises a putative cytochrome domain. The latter presumably consists of a novel fold, as it bears no sequence similarities towards other known cytochromes, or towards known domains. Strikingly, the first module contains the C-X (n)-H-X(2)-C motif that accounts for the binding of the catalytic zinc in carbonic anhydrases, but lacks several other critical residues required for substrate binding and proper active site geometry. In agreement with this finding, the isolated cytochrome contains one bound zinc atom, but has no carbonic anhydrase activity. Inspection of the sequences available from the genomic sequencing project of the close relative archaeon Sulfolobus solfataricus shows the presence of an identical protein, suggesting its dissemination among the Sulfolobales. The role of zinc as a key element for the intrinsic thermal stability of these proteins is discussed.

Characterization of a cytochrome b(558) ferric/cupric reductase from rabbit duodenal brush border membranes

Iron and probably also copper are absorbed by the intestine in their reduced form. A b-type cytochrome, Dcytb, has recently been cloned from mouse and has been proposed to be the corresponding reductase. However, the nature of the cytochrome and the reduction reaction remain unknown. Here we describe the isolation and functional characterization of a novel b-type cytochrome from rabbit enterocytes. The 33 kDa heme protein was solubilized from brush border membranes with Triton X-100 and purified by successive ion exchange chromatography and hydrophobic interaction chromatography. Spectroscopic analysis of the heme revealed a b(558) cytochrome. The purified hemoprotein exhibited ascorbate-stimulated reduction of iron(III) and copper(II). The rate constants, k(1), for these reactions were 1.38 +/- 0.12 and 0.64 +/- 0.16 min(-1), respectively. Cytochrome b(558) may be the rabbit Dcytb homologue. A novel mechanism of how cytochrome b(558) could shuttle electrons from cytoplasmic ascorbate to luminal dehydroascorbate is proposed.

Fractionation of cytochromes of phototrophically grown Chloroflexus aurantiacus. Is there a cytochrome bc complex among them?

The cytochrome-containing membrane complexes of the phototrophically grown green non-sulfur bacterium Chloroflexus aurantiacus were fractionated by anion exchange chromatography. Three cytochrome b and four cytochrome c peaks were observed. None of the separated complexes met the features of the cytochrome bc complex. Two main cytochrome b-containing complexes were further purified: a dimer of identical subunits with unknown function and a succinate:quinone oxidoreductase containing three subunit species. Two novel multisubunit complexes, similar to each other, with two heme c-bearing subunits were also purified.

The presence of 9-cis-beta-carotene in cytochrome b(6)f complex from spinach

Cytochrome b(6)f complex with stoichiometrically bound beta-carotene molecule was purified from spinach chloroplasts. The configuration of this beta-carotene was studied by reversed-phase HPLC and resonance Raman spectroscopy. Both the absorption spectrum of this beta-carotene in dissociated state and the Raman spectrum in native state can be unambiguously assigned to a 9-cis configuration. This finding is in contrast to the predominantly all-trans isomers commonly found in membranes and protein-pigment complexes of chloroplasts, suggesting that the 9-cis-beta-carotene is an authentic component and may have a unique structural and functional role in cytochrome b(6)f complex.

Determination of the animal origin of raw food by species-specific PCR

Specific PCR, amplifying a fragment of the mitochondrially encoded cytochrome b gene, are developed for discrimination of chicken, turkey, pig, cow and sheep. These PCR tests could be applied for detection and discrimination of animal food ingredients. For origin identification of milk, the bovine PCR is applied on DNA extracted with the DNeasy Tissue kit. A PCR of 30 cycles reached a sensitivity of 720 somatic cells in the PCR reaction corresponding to 29 x 10(4) ells per ml milk. A PCR of 40 cycles could detect 0.72 to 0.144 somatic cells in the PCR reaction corresponding to 29 to 2.9 cells per ml milk.

Protection of BALB/c mice against Brucella abortus 544 challenge by vaccination with bacterioferritin or P39 recombinant proteins with CpG oligodeoxynucleotides as adjuvant

The P39 and the bacterioferrin (BFR) antigens of Brucella melitensis 16M were previously identified as T dominant antigens able to induce both delayed-type hypersensivity in sensitized guinea pigs and in vitro gamma interferon (IFN-gamma) production by peripheral blood mononuclear cells from infected cattle. Here, we analyzed the potential for these antigens to function as a subunitary vaccine against Brucella abortus infection in BALB/c mice, and we characterized the humoral and cellular immune responses induced. Mice were injected with each of the recombinant proteins alone or adjuvanted with either CpG oligodeoxynucleotides (CpG ODN) or non-CpG ODN. Mice immunized with the recombinant antigens with CpG ODN were the only group demonstrating both significant IFN-gamma production and T-cell proliferation in response to either Brucella extract or to the respective antigen. The same conclusion holds true for the antibody response, which was only demonstrated in mice immunized with recombinant antigens mixed with CpG ODN. The antibody titers (both immunoglobulin G1 [IgG1] and IgG2a) induced by P39 immunization were higher than the titers induced by BFR (only IgG2a). Using a B. abortus 544 challenge, the level of protection was analyzed and compared to the protection conferred by one immunization with the vaccine strain B19. Immunization with P39 and CpG ODN gave a level of protection comparable to the one conferred by B19 at 4 weeks postchallenge, and the mice were still significantly protected at 8 weeks postchallenge, although to a lesser extent than the B19-vaccinated group. Intriguingly, no protection was detected after BFR vaccination. All other groups did not demonstrate any protection.

Soret-excited Raman spectroscopy of the spinach cytochrome b6f complex. Structures of the b- and c-type hemes, chlorophyll a, and beta-carotene

Soret-excited resonance Raman (RR) spectra of the spinach cytochrome b6f complex (cyt b6f) are reported for the oxidized, native, ascorbate-reduced, and dithionite-reduced forms. Using excitations at 441.6, 413.1, and 406.7 nm, RR contributions of chlorophyll a, beta-carotene, the c-type heme of cytochrome f, and the b-type hemes of cytochrome b6 of the b6f complex were identified and the data compared to those previously obtained for the Rhodospirillum rubrum bc1 complex [Le Moigne, C., Schoepp, B., Othman, S., Verméglio, A., and Desbois, A. (1999) Biochemistry 38, 1066-1076]. RR bands arising from the b(6)f-associated chlorophyll a and beta-carotene pigments were found to be particularly intense in the spectra excited at 441.6 nm. The frequencies of the phorbin skeleton of chlorophyll a at 1606, 1552, and 1525 cm(-1) are typical of a Mg atom with a single axial ligand. Strong RR bands corresponding to stretching or deformation modes of beta-carotene were detected at 1137, 1157, 1191, 1216, and 1531 cm(-1) in the different forms of cyt b6f. This set of frequencies is assigned to an all-trans configuration of the polyene chain. The redox titrations of the b(6)f complex allow the characterization of RR bands of the three hemes. The nu10, nu2, nu3, and nu8 modes of reduced cyt f are detected at 1619, 1591, 1492, and 356 cm(-1), respectively. From this set of frequencies, one can conclude that the particular histidine/amine heme coordination found in the truncated soluble domain of cyt f is a specific feature of the entire cyt f included in the b6f complex. The frequencies of the nu2, nu8, and nu10 marker modes are consistent with different conformations for the two b-type hemes of cyt b6f. One of these hemes is strongly distorted (nu2, nu8, and nu10 at 1581, 351, and 1610 cm(-1), respectively), while the other one is planar (1586, 345, and 1618 cm(-1), respectively). Largely different structures for the b-type hemes appear to be a common property for the bc1/b6f complexes.

Total synthesis of cytochrome b562 by native chemical ligation using a removable auxiliary

We have completed the total chemical synthesis of cytochrome b562 and an axial ligand analogue, [SeMet(7)]cyt b562, by thioester-mediated chemical ligation of unprotected peptide segments. A novel auxiliary-mediated native chemical ligation that enables peptide ligation to be applied to protein sequences lacking cysteine was used. A cleavable thiol-containing auxiliary group, 1-phenyl-2-mercaptoethyl, was added to the alpha-amino group of one peptide segment to facilitate amide bond-forming ligation. The amine-linked 1-phenyl-2-mercaptoethyl auxiliary was stable to anhydrous hydrogen fluoride used to cleave and deprotect peptides after solid-phase peptide synthesis. Following native chemical ligation with a thioester-containing segment, the auxiliary group was cleanly removed from the newly formed amide bond by treatment with anhydrous hydrogen fluoride, yielding a full-length unmodified polypeptide product. The resulting polypeptide was reconstituted with heme and folded to form the functional protein molecule. Synthetic wild-type cyt b562 exhibited spectroscopic and electrochemical properties identical to the recombinant protein, whereas the engineered [SeMet(7)]cyt b562 analogue protein was spectroscopically and functionally distinct, with a reduction potential shifted by approximately 45 mV. The use of the 1-phenyl-2-mercaptoethyl removable auxiliary reported here will greatly expand the applicability of total protein synthesis by native chemical ligation of unprotected peptide segments.

High-level production of heme-containing holoproteins in Escherichia coli

The expression of recombinant protein is essential for the investigation of the functions and properties of heme-containing protein as an electron carrier. For the expression of fully active recombinant protein, conversion of the expressed apoprotein into holoprotein is the most important and difficult problem. In this study, a system was developed for the production of heme-containing protein in a pure, recombinant holoprotein form, using the bovine cytochrome b5 tryptic fragment and Escherichia coli bacterioferritin as heterologous and homologous heme-containing model proteins, respectively. This system is based on the slow synthesis of recombinant apoprotein, which can maintain the balanced consumption of amino acids between protein synthesis and heme synthesis, so that the synthesized apoprotein continues to act as a heme sink. From a 1-1 culture, 15 mg of cytochrome b5 and 40 mg of bacterioferritin were purified as pure holoprotein forms. Our expression system provides a rapid and simple method for obtaining large quantities of the active holo-form of heme-containing proteins.

Structure determination of bacterioferritin from Desulfovibrio desulfuricans by the MAD method at the Fe K-edge

Bacterioferritins constitute a subfamily of heme ferritins, proteins involved in iron storage and homeostasis. The protein isolated from Desulfovibrio desulfuricans ATCC 27774 is a homodimer of mass 52 kDa. The monomers are linked by an iron-coproporphyrin group and each monomer contains a diferric center. The 24-monomer clusters found in the crystal are probably the functional particles. MAD data from cubic bacterioferritin crystals were collected at the K-shell iron edge. Preliminary phasing was performed using the positions of 23 of the 40 Fe atoms expected in the asymmetric unit. Further MAD phasing allowed the identification of individual iron sites. Clear and interpretable electron-density maps were obtained after density modification.

Higher-plant plasma membrane cytochrome b561: a protein in search of a function

During the past twenty years evidence has accumulated on the presence of a specific high-potential, ascorbate-reducible b-type cytochrome in the plasma membrane (PM) of higher plants. This cytochrome is named cytochrome b561 (cyt b561) according to the wavelength maximum of its alpha-band in the reduced form. More recent evidence suggests that this protein is homologous to a b-type cytochrome present in chromaffin granules of animal cells. The plant and animal cytochromes share a number of strikingly similar features, including the high redox potential, the ascorbate reducibility, and most importantly the capacity to transport electrons across the membrane they are located in. The PM cyt b561 is found in all plant species and in a variety of tissues tested so far. It thus appears to be a ubiquitous electron transport component of the PM. The cytochromes b561 probably constitute a novel class of transmembrane electron transport proteins present in a large variety of eukaryotic cells. Of particular interest is the recent discovery of a number of plant genes that show striking homologies to the genes coding for the mammalian cytochromes b561. A number of highly relevant structural features, including hydrophobic domains, heme ligation sites, and possible ascorbate and monodehydroascorbate binding sites are almost perfectly conserved in all these proteins. At the same time the plant gene products show interesting differences related to their specific location at the PM, such as potentially N-linked glycosylation sites. It is also clear that at least in several plants cyt b561 is represented by a multigene family. The current paper presents the first overview focusing exclusively on the plant PM cyt b561, compares it to the animal cyt b561, and discusses the possible physiological function of these proteins in plants.

b-type cytochromes in plasma membranes of Phaseolus vulgaris hypocotyls, Arabidopsis thaliana leaves, and Zea mays roots

The plasma membrane of higher plants contains more than one kind of b-type cytochromes. One of these has a high redox potential and can be fully reduced by ascorbate. This component, the cytochrome b561 (cyt b561), has its characteristic alpha-band absorbance close to 561 nm wavelength at room temperature. Cyt b561 was first isolated from etiolated bean hook plasma membranes by two consecutive anion exchange chromatography steps. During the first step performed at pH 8, cyt b561 did not bind to the anion exchange column, but other b-type cytochromes did. In the second step performed at pH 9.9, cyt b561 was bound to the column and was eluted from the column at an ionic strength of about 100 mM KCl. However, when the same protocol was applied to the solubilized plasma membrane proteins from Arabidopsis thaliana leaves and maize roots, the ascorbate-reducible cyt b561 bound already to the first anion exchange column at pH 8 and was eluted also at an ionic strength of about 100 mM KCl. Other b-type cytochromes than the ascorbate-reducible cyt b561 from the plasma membranes of Arabidopsis leaves and maize roots showed similar chromatographic characteristics to that of bean hypocotyls. These results demonstrate particular differences in the chromatographic behavior of cyt b561 from different sources.

Purification of cytochrome b-561 from bean hypocotyls plasma membrane. Evidence for the presence of two heme centers

The high potential, ascorbate-reducible b-type cytochrome of plant plasma membranes, named cytochrome b-561, has been purified to homogeneity from etiolated bean hypocotyls. The pure protein migrated in denaturing electrophoresis as a broad band of approximately 55 kDa, and was found to be glycosylated. Optical redox titrations of partially purified cytochrome b-561 indicated that it contains two hemes with similar spectral features, but distinct midpoint redox potentials (E(m7)+135 mV and +206 mV, respectively). The presence of two heme centers in cytochrome b-561 is consistent with its role in electron transfer across plant plasma membranes.

Inhibitory copper binding site on the spinach cytochrome b6f complex: implications for Qo site catalysis

The isolated cytochrome (cyt) b(6)f complex from spinach is inhibited by Cu(2+) with a K(D) of about 1 microM at pH 7.6 in the presence of 1.6 microM decyl-plastoquinol (C(10)-PQH(2)) as a substrate. Inhibition was competitive with respect to C(10)-PQH(2) but noncompetitive with respect to horse heart cyt c or plastocyanin (PC). Inhibition was also pH-sensitive, with an apparent pK at about 7, above which inhibition was stronger, suggesting that binding occurred at or near a protonatable amino acid residue. Equilibrium binding titrations revealed ca. 1.4 tight Cu(2+) binding sites with a K(D) of about 0.5 microM and multiple (>8) weak (K(D) > 50 microM) binding sites per complex. Pulsed electron paramagnetic resonance (EPR) techniques were used to identify probable binding sites for inhibitory Cu(2+). A distinct enhancement of the relaxation time constant for the EPR signal from bound Cu(2+) was observed when the cyt f was paramagnetic. The magnitude and temperature-dependence of this relaxation enhancement were consistent with a dipole interaction between Cu(2+) and the cyt f (Fe(3+)) heme at a distance of between 30 and 54 A, depending upon the relative orientations of Cu(2+) and cyt f heme g-tensors. Two-pulse electron spin-echo envelope modulation (ESEEM) and 4-pulse 2-dimensional hyperfine sublevel correlation (2D HYSCORE) measurements of Cu(2+) bound to isolated cyt b(6)f complex indicated the presence of a weakly coupled nitrogen nucleus. The nuclear quadrupole interaction (NQI) and the hyperfine interaction (HFI) parameters identified one Cu(2+) ligand as an imidazole nitrogen of a His residue, and electron-nuclear double resonance (ENDOR) confirmed the presence of a directly coordinated nitrogen. A model of the 3-dimensional structure of the cytochrome b(6)f complex was constructed on the basis of sequences and structural similarities with the mitochondrial cyt bc(1) complex, for which X-ray structures have been solved. This model indicated three possible His residues as ligands to inhibitory Cu(2+). Two of these are located on the "Rieske" iron-sulfur protein protein (ISP) while the third is found on the cyt f protein. None of these potential ligands appear to interact directly with the quinol oxidase (Q(o)) binding pocket. A model is thus proposed wherein Cu(2+) interferes with the interaction of the ISP protein with the Q(o) site, preventing the binding and subsequent oxidation of plastoquinonol. Implications for the involvement of ISP "domain movement" in Q(o) site catalysis are discussed.

Complementation of NADPH oxidase in p67-phox-deficient CGD patients p67-phox/p40-phox interaction

Chronic granulomatous disease (CGD) is due to a functional defect of the O2- generating NADPH oxidase of phagocytes. Epstein-Barr-virus-immortalized B lymphocytes express all the constituents of oxidase with activity 100 times less than that of neutrophils. As in neutrophils, oxidase activity of Epstein-Barr-virus-immortalized B lymphocytes was shown to be defective in the different forms of CGD; these cells were used as a model for the complementation studies of two p67-phox-deficient CGD patients. Reconstitution of oxidase activity was performed in vitro by using a heterologous cell-free assay consisting of membrane-suspended or solubilized and purified cytochrome b558 that was associated with cytosol or with the isolated cytosolic-activating factors (p67-phox, p47-phox, p40-phox) from healthy or CGD patients. In p67-phox-deficient CGD patients, two cytosolic factors are deficient or missing: p67-phox and p40-phox. Not more than 20% of oxidase activity was recovered by complementing the cytosol of p67-phox-deficient patients with recombinant p67-phox. On the contrary, a complete restoration of oxidase activity was observed when, instead of cytosol, the cytosolic factors were added in the cell-free assay after isolation in combination with cytochrome b558 purified from neutrophil membrane. Moreover, the simultaneous addition of recombinant p67-phox and recombinant p40-phox reversed the previous complementation in a p40-phox dose-dependent process. These results suggest that in the reconstitution of oxidase activity, p67-phox is the limiting factor; the efficiency of complementation depends on the membrane tissue and the cytosolic environment. In vitro, the transition from the resting to the activated state of oxidase, which results from assembling, requires the dissociation of p40-phox from p67-phox for efficient oxidase activity. In the process, p40-phox could function as a negative regulatory factor and stabilize the resting state.

Structure and characterization of Ectothiorhodospira vacuolata cytochrome b(558), a prokaryotic homologue of cytochrome b(5)

A soluble cytochrome b(558) from the purple phototropic bacterium Ectothiorhodospira vacuolata was completely sequenced by a combination of automated Edman degradation and mass spectrometry. The protein, with a measured mass of 10,094.7 Da, contains 90 residues and binds a single protoheme. Unexpectedly, the sequence shows homology to eukaryotic cytochromes b(5). As no prokaryotic homologue had been reported so far, we developed a protocol for the expression, purification, and crystallization of recombinant cytochrome b(558). The structure was solved by molecular replacement to a resolution of 1.65 A. It shows that cytochrome b(558) is indeed the first bacterial cytochrome b(5) to be characterized and differs from its eukaryotic counterparts by the presence of a disulfide bridge and a four-residue insertion in front of the sixth ligand (histidine). Eukaryotes contain a variety of b(5) homologues, including soluble and membrane-bound multifunctional proteins as well as multidomain enzymes such as sulfite oxidase, fatty-acid desaturase, nitrate reductase, and lactate dehydrogenase. A search of the Mycobacterium tuberculosis genome showed that a previously unidentified gene encodes a fatty-acid desaturase with an N-terminal b(5) domain. Thus, it may provide another example of a bacterial b(5) homologue.

Purification and two-dimensional crystallization of highly active cytochrome b(6)f complex from spinach

The purification and two-dimensional crystallization of highly active cytochrome b(6)f complex from spinach is described. The preparation shows all spectroscopic characteristics of the pure complex. The electron transfer activity of 450+/-60 electrons per s is the highest in vitro activity reported to date. Using dimethyl sulfoxide (DMSO) as a solvent for the electron donor enhanced the performance and reproducibility of the assay. The high yield and the high activity of the protein make it an ideal candidate for biophysical and structural studies. Preliminary two-dimensional crystallization experiments yielded several different forms of two-dimensional and thin three-dimensional crystals, exhibiting varying degrees of order.

The active site structure of ba3 oxidase from Thermus thermophilus studied by resonance raman spectroscopy

The ba3 cytochrome oxidase from Thermus thermophilus was studied by resonance Raman spectroscopy. The component spectra of both heme groups were determined by using different excitation wavelengths. In the ferric state the heme a3 group reveals resonance Raman marker bands characteristic for two high spin species with the heme iron in an in-plane and an out-of-plane configuration that reflects a coordination equilibrium. This equilibrium obviously results from protonation of one of the axial ligands that is ascribed to a hydroxide. Coordination by its protonated form, a water molecule, may be too weak to keep the heme iron in the porphyrin plane. The corresponding Fe-OH2 stretching mode was attributed to a weak H/D-sensitive band at 464 cm(-1). The coordination equilibrium not only depends on the pH but is also affected by the buffer, the salt concentration, and the binding of the natural redox partner cytochrome c552. These changes of the coordination equilibrium are attributed to the perturbation of the hydrogen bonding network at the catalytic center that is connected to the protein surface via a relay of hydrogen bonds. Environmental changes at the catalytic site are sensitively reflected by the formyl stretching of heme a3. The unique structural properties of the ba3 oxidase may be related to the unusual proton pump efficiency and heme a3 redox potential.

Mössbauer and electron paramagnetic resonance studies of the cytochrome bf complex

The (57)Fe-enriched cytochrome bf complex has been isolated from hydrocultures of spinach. It has been studied at different redox states by optical, EPR, and Mössbauer spectroscopy. The Mössbauer spectrum of the native complex at 190 K with all iron centers in the oxidized state reveals the presence of four different iron sites: low-spin ferric iron in cytochrome b [with an isomer shift (delta) of 0.20 mm/s, a quadrupole splitting (DeltaE(Q)) of 1.77 mm/s, and a relative area of 40%], low-spin ferric iron of cytochrome f (delta = 0.26 mm/s, DeltaE(Q) = 1.90 mm/s, and a relative area of 20%), and two high-spin ferric iron sites of the Rieske iron-sulfur protein (ISP) with a bis-cysteine and a bis-histidine ligated iron (delta(1) = 0.15 mm/s, DeltaE(Q1) = 0.70 mm/s, and a relative area of 20%, and delta(2) = 0.25 mm/s, DeltaE(Q2) = 0.90 mm/s, and a relative area of 20%, respectively). EPR and magnetic Mössbauer measurements at low temperatures corroborate these results. A crystal-field analysis of the EPR data and of the magnetic Mössbauer data yields estimates for the g-tensors (g(z)(), g(y)(), and g(x)()) of cytochrome b (3.60, 1.35, and 1.1) and of cytochrome f (3.51, 1.69, and 0.9). Addition of ascorbate reduces not only the iron of cytochrome f to the ferrous low-spin state (delta = 0.43 mm/s, DeltaE(Q) = 1.12 mm/s at 4.2 K) but also the bis-histidine coordinated iron of the Rieske 2Fe-2S center to the ferrous high-spin state (delta(2) = 0.73 mm/s, DeltaE(Q2) = -2.95 mm/s at 4.2 K). At this redox step, the Mössbauer parameters of cytochrome b have not changed, indicating that the redox changes of cytochrome f and the Rieske protein did not change the first ligand sphere of the low-spin ferric iron in cytochrome b. Reduction with dithionite further reduces the two hemes of cytochrome b to the ferrous low-spin state (delta = 0.49 mm/s, DeltaE(Q) = 1.08 mm/s at 4.2 K). The spin Hamiltonian analysis of the magnetic Mössbauer spectra at 4.2 K yields hyperfine parameters of the reduced Rieske 2Fe-2S center in the cytochrome bf complex which are very similar to those reported for the Rieske center from Thermus thermophilus [Fee, J. A., Findling, K. L., Yoshida, T., et al. (1984) J. Biol. Chem. 259, 124-133].

Spontaneous activation of NADPH oxidase in a cell-free system: unexpected multiple effects of magnesium ion concentrations

The role of magnesium ions in the activation of NADPH oxidase has been investigated using flavocytochrome b-245 and either neutrophil cytosol or mixtures of recombinant p40phox, p47phox, p67phox and Rac2. Purified flavocytochrome b-245 is highly active (turnover number 120-150 mol of O2(-)/s per mol of cytochrome haem) in the absence of Mg2+, in marked contrast to neutrophil membranes or detergent-solubilized membranes, which have an absolute requirement for Mg2+ for NADPH oxidase activity. It was also found that Mg2+ affected the anionic amphiphile requirement for oxidase activation, and this was dependent on whether neutrophil cytosol or mixtures of recombinant cytosolic proteins were used in the assay. Unexpectedly we found that, using purified flavocytochrome b-245 and recombinant cytosolic proteins, NADPH oxidase undergoes spontaneous activation in the absence of anionic amphiphiles under Mg2+-free conditions. The results suggest that Mg2+ ions play an important role in NADPH oxidase function, perhaps stabilizing the 260 kDa complex of cytosolic phox proteins or the regulation of a guanine nucleotide-binding protein. We provide evidence that if the latter explanation is correct, the identity of the guanine nucleotide-binding protein is unlikely to be Rap1a.

Membrane-bound electron transfer chain of the thermohalophilic bacterium Rhodothermus marinus: a novel multihemic cytochrome bc, a new complex III

A novel multihemic cytochrome bc complex was isolated from the membranes of Rhodothermus marinus. It is a complex with a minimum of three subunits (43, 27, and 18 kDa), containing five low-spin heme centers of the B and C types, in a 1:4 ratio. All the C-type hemes are in the same subunit (27 kDa). Three distinct redox transitions, at 235, 80, and -45 mV, were observed by visible redox titrations. The first involves one B- and one C-type hemes, and in the other two transitions one and two C-type hemes are involved, respectively. Spectroscopic data strongly suggest that the two hemes intervening in the last transition are in van der Waals contact, yielding a split Soret band. Electron paramagnetic resonance spectra of the oxidized complex show resonances of five low-spin ferric heme centers. Upon reduction with ascorbate, all these resonances vanish and a new one attributed to the last pair of hemes appears. A [3Fe-4S]1+/0 center copurifies with this complex, having a high reduction potential of +140 mV. No Rieske-type centers are detected in R. marinus and no effect is observed in the respiratory rates when the typical bc1 complex inhibitors are present, suggesting that such a complex is absent in R. marinus [Pereira et al. (1994) FEBS Lett. 352, 327-330]. The newly isolated cytochrome bc complex has quinol:cytochrome c or high-potential iron-sulfur protein (HiPIP) oxidoreductase activity, being a functional analogue of the canonical bc1 complexes; i.e., it is the complex III in R. marinus. This complex plays a central role in this bacterium's electron-transfer chain, coupling the electron transfer between the quinols reduced by the dehydrogenases and the HiPIP, the final electron donor to the terminal oxidases [Pereira, M. M., Carita, J. N., and Teixeira, M. (1999) Biochemistry 38, 1276-1283].

Biochemical and immunochemical properties of B lymphocyte cytochrome b558

Like neutrophils, Epstein-Barr virus (EBV)-immortalized B lymphocytes express all constituents of the NADPH oxidase complex necessary to generate superoxide anion O2-. The NADPH oxidase activity in EBV-B lymphocytes is only 5% of that measured in neutrophils upon PMA stimulation. Cytochrome b558 is the sole redox membrane component of NADPH oxidase; it is the protein core around which cytosolic factors assemble in order to mediate oxidase activity. In the present study, we have compared the structural and functional properties of cytochrome b558 from EBV-B lymphocytes and neutrophils. Cytochrome b558 from EBV-B lymphocyte plasma membrane, like that from neutrophils, is characterized by a heterodimeric structure with a highly glycosylated beta subunit, known as gp91-phox. While the amount of cytochrome b558 recovered after purification from EBV-B lymphocytes (approximately 0.24 nmol from 1010 cells) was low compared to that recovered from neutrophils (approximately 10 nmol), the biochemical properties of purified cytochrome b558 from both EBV-B lymphocytes and neutrophils were quite similar with respect to their differential spectra, redox potential, and FAD binding site. Once cytochrome b558 was extracted from the EBV-B lymphocyte membrane, it was able to mediate, in a reconstituted system of O2- production the same oxidase turnover as that found for cytochrome b558 extracted from neutrophils. A comparison between membrane bound and soluble cytochrome b558 suggested that the weak oxidase activity measured in intact EBV-B cells might be the result not only of the small amount of expressed cytochrome b558, but also of a defect of the activation process in lymphocyte membrane.