Structure and function of the tetraheme cytochrome associated to the reaction center of Roseobacter denitrificans

Peroxidase activity and involvement in the oxidative stress response of roseobacter denitrificans truncated hemoglobin

Roseobacter denitrificans is a member of the widespread marine Roseobacter genus. We report the first characterization of a truncated hemoglobin from R. denitrificans (Rd. trHb) that was purified in the heme-bound form from heterologous expression of the protein in Escherichia coli. Rd. trHb exhibits predominantly alpha-helical secondary structure and absorbs light at 412, 538 and 572 nm. The phylogenetic classification suggests that Rd. trHb falls into group II trHbs, whereas sequence alignments indicate that it shares certain important heme pocket residues with group I trHbs in addition to those of group II trHbs. The resonance Raman spectra indicate that the isolated Rd. trHb contains a ferric heme that is mostly 6-coordinate low-spin and that the heme of the ferrous form displays a mixture of 5- and 6-coordinate states. Two Fe-His stretching modes were detected, notably one at 248 cm^-1, which has been reported in peroxidases and some flavohemoglobins that contain an Fe-His-Asp (or Glu) catalytic triad, but was never reported before in a trHb. We show that Rd. trHb exhibits a significant peroxidase activity with a (k_cat/K_m) value three orders of magnitude higher than that of bovine Hb and only one order lower than that of horseradish peroxidase. This enzymatic activity is pH-dependent with a pK_a value ~6.8. Homology modeling suggests that residues known to be important for interactions with heme-bound ligands in group II trHbs from Mycobacterium tuberculosis and Bacillus subtilis are pointing toward to heme in Rd. trHb. Genomic organization and gene expression profiles imply possible functions for detoxification of reactive oxygen and nitrogen species in vivo. Altogether, Rd. trHb exhibits some distinctive features and appears equipped to help the bacterium to cope with reactive oxygen/nitrogen species and/or to operate redox biochemistry.

Photo-induced dynamics of the heme centers in cytochrome bc1

The ultrafast response of cytochromebc₁is investigated for the first time,viatransient absorption spectroscopy.

The photosynthetic apparatus and photoinduced electron transfer in the aerobic phototrophic bacteria Roseicyclus mahoneyensis and Porphyrobacter meromictius

Photosynthetic electron transfer has been examined in whole cells, isolated membranes and in partially purified reaction centers (RCs) of Roseicyclus mahoneyensis, strain ML6 and Porphyrobacter meromictius, strain ML31, two species of obligate aerobic anoxygenic phototrophic bacteria. Photochemical activity in strain ML31 was observed aerobically, but the photosynthetic apparatus was not functional under anaerobic conditions. In strain ML6 low levels of photochemistry were measured anaerobically, possibly due to incomplete reduction of the primary electron acceptor (Q(A)) prior to light excitation, however, electron transfer occurred optimally under low oxygen conditions. Photoinduced electron transfer involves a soluble cytochrome c in both strains, and an additional reaction center (RC)-bound cytochrome c in ML6. The redox properties of the primary electron donor (P) and Q(A) of ML31 are similar to those previously determined for other aerobic phototrophs, with midpoint redox potentials of +463 mV and -25 mV, respectively. Strain ML6 showed a very narrow range of ambient redox potentials appropriate for photosynthesis, with midpoint redox potentials of +415 mV for P and +94 mV for Q(A). Cytoplasm soluble and photosynthetic complex bound cytochromes were characterized in terms of apparent molecular mass. Fluorescence excitation spectra revealed that abundant carotenoids not intimately associated with the RC are not involved in photosynthetic energy conservation.

Influence of light and anoxia on chemiosmotic energy conservation in Dinoroseobacter shibae

In the present study we have investigated the influence of light and anoxia on the energetic state of the aerobic anoxygenic phototroph (AAP) Dinoroseobacter shibae. Respiration, chemiosmotic proton translocation and the adenylate energy charge (AEC) of the cells were measured comparing light versus dark and oxic versus anoxic conditions. Light caused a decrease of the respiration rates of washed cells. This might be a substitution rather than a direct inhibitory effect, because both photosynthesis and respiration contribute to the proton-motive force. As known from other AAPs, light alone did not induce proton translocation if applied to anoxic cell suspensions. However, additions of small oxygen pulses to anoxic cell suspensions caused two times more proton translocation in the light than in the dark. The AEC of the cells was measured by means of a modified luciferin-luciferase method. Growing cells of D. shibae kept an AEC of 0.93, indicating that the adenylate pool was highly phosphorylated. After harvesting and storing the cells under anoxic conditions for 2 h, the AEC dropped to 0.12. However, the cells remained reactive. Upon addition of oxygen, the AEC increased to its original value within 40 s by the formation of about 12 mM of intracellular ATP. There were no differences whether this recovery experiment was carried out in the dark or in the light. We conclude that D. shibae is able to change its energetic state not only in response to the light regime but also during oxic-anoxic transitions. Both responses appear suited to save in situ organic substrates and endogenous electron donors, thus enhancing the role of photosynthetic energy conservation.

Isolation of aerobic anoxygenic photosynthetic bacteria from black smoker plume waters of the juan de fuca ridge in the pacific ocean

A strain of the aerobic anoxygenic photosynthetic bacteria was isolated from a deep-ocean hydrothermal vent plume environment. The in vivo absorption spectra of cells indicate the presence of bacteriochlorophyll a incorporated into light-harvesting complex I and a reaction center. The general morphological and physiological characteristics of this new isolate are described.

Relative orientation of the hemes of the cytochrome bc(1) complexes from Rhodobacter sphaeroides, Rhodospirillum rubrum, and beef heart mitochondria. A linear dichroism study

The orientation of the chromophores in the cytochrome bc(1) of Rhodospirillum rubrum, Rhodobacter sphaeroides, and beef heart mitochondria is reported. The combination of redox-resolved absorption spectrophotometry and linear dichroism experiments at low temperature allows the determination of the orientation of the three hemes with respect to the membrane plane. The orientations of the b(H)-and b(L)-hemes of the R. sphaeroides and beef heart mitochondrial complexes are similar to those determined by crystallographic studies of the mitochondrial cytochrome bc(1). On the other hand the orientations of the b-hemes of the R. rubrum complex lead to the conclusion that the b(H)-heme is more perpendicular to the membrane plane than the b(L)-heme. This could be explained by a specific organization of the b-hemes due to subunit composition of the complex or, alternatively, to a different spatial position of the heme transitions with respect to the porphyrin macrocycle compared with the other complexes. Moreover, our results demonstrate a different orientation of the heme c(1) of the three studied complexes in comparison to crystallographic studies. This difference may arise from the above hypothesis on the transitions of the heme or from flexibility of this subunit in function of its redox state.

Photo-induced cyclic electron transfer involving cytochrome bc1 complex and reaction center in the obligate aerobic phototroph Roseobacter denitrificans

Flash-induced redox changes of b-type and c-type cytochromes have been studied in chromatophores from the aerobic photosynthetic bacterium Roseobacter denitrificans under redox-controlled conditions. The flash-oxidized primary donor P+ of the reaction center (RC) is rapidly re-reduced by heme H1 (Em,7 = 290 mV), heme H2 (Em,7 = 240 mV) or low-potential hemes L1/L2 (Em,7 = 90 mV) of the RC-bound tetraheme, depending on their redox state before photoexcitation. By titrating the extent of flash-induced low-potential heme oxidation, a midpoint potential equal to -50 mV has been determined for the primary quinone acceptor QA. Only the photo-oxidized heme H2 is re-reduced in tens of milliseconds, in a reaction sensitive to inhibitors of the bc1 complex, leading to the concomitant oxidation of a cytochrome c spectrally distinct from the RC-bound hemes. This reaction involves cytochrome c551 in a diffusional process. Participation of the bc1 complex in a cyclic electron transfer chain has been demonstrated by detection of flash-induced reduction of cytochrome b561, stimulated by antimycin and inhibited by myxothiazol. Cytochrome b561, reduced upon flash excitation, is re-oxidized slowly even in the absence of antimycin. The rate of reduction of cytochrome b561 in the presence of antimycin increases upon lowering the ambient redox potential, most likely reflecting the progressive prereduction of the ubiquinone pool. Chromatophores contain approximately 20 ubiquinone-10 molecules per RC. At the optimal redox poise, approximately 0.3 cytochrome b molecules per RC are reduced following flash excitation. Cytochrome b reduction titrates out at Eh < 100 mV, when low-potential heme(s) rapidly re-reduce P+ preventing cyclic electron transfer. Results can be rationalized in the framework of a Q-cycle-type model.

Purification, redox and spectroscopic properties of the tetraheme cytochrome c isolated from Rubrivivax gelatinosus

The tetraheme cytochrome c subunit of the Rubrivivax gelatinosus reaction center was isolated in the presence of octyl beta-D-thioglucoside by ammonium sulfate precipitation and solubilization at pH 9 in a solution of Deriphat 160. Several biochemical properties of this purified cytochrome were characterized. In particular, it forms small oligomers and its N-terminal amino acid is blocked. In the presence or absence of diaminodurene, ascorbate and dithionite, different oxidation/reduction states of the isolated cytochrome were studied by absorption, EPR and resonance Raman spectroscopies. All the data show two hemes quickly reduced by ascorbate, one heme slowly reduced by ascorbate and one heme only reduced by dithionite. The quickly ascorbate-reduced hemes have paramagnetic properties very similar to those of the two low-potential hemes of the reaction center-bound cytochrome (gz = 3.34), but their alpha band is split with two components peaking at 552 nm and 554 nm in the reduced state. Their axial ligands did not change, being His/Met and His/His, as indicated by the resonance Raman spectra. The slowly ascorbate-reduced heme and the dithionite-reduced heme are assigned to the two high-potential hemes of the bound cytochrome. Their alpha band was blue-shifted at 551 nm and the gz values decreased to 2.96, although the axial ligations (His/Met) were conserved. It was concluded that the estimated 300 mV potential drop of these hemes reflected changes in their solvent accessibility, while the reduction in gz indicates an increased symmetry of their cooordination spheres. These structural modifications impaired the cytochrome's essential function as the electron donor to the photooxidized bacteriochlorophyll dimer of the reaction center. In contrast to its native state, the isolated cytochrome was unable to reduce efficiently the reaction center purified from a Rubrivivax gelatinosus mutant in which the tetraheme was absent. Despite the conformational changes of the cytochrome, its four hemes are still divided into two groups with a pair of low-potential hemes and a pair of high-potential hemes.

Aerobic anoxygenic phototrophic bacteria

The aerobic anoxygenic phototrophic bacteria are a relatively recently discovered bacterial group. Although taxonomically and phylogenetically heterogeneous, these bacteria share the following distinguishing features: the presence of bacteriochlorophyll a incorporated into reaction center and light-harvesting complexes, low levels of the photosynthetic unit in cells, an abundance of carotenoids, a strong inhibition by light of bacteriochlorophyll synthesis, and the inability to grow photosynthetically under anaerobic conditions. Aerobic anoxygenic phototrophic bacteria are classified in two marine (Erythrobacter and Roseobacter) and six freshwater (Acidiphilium, Erythromicrobium, Erythromonas, Porphyrobacter, Roseococcus, and Sandaracinobacter) genera, which phylogenetically belong to the alpha-1, alpha-3, and alpha-4 subclasses of the class Proteobacteria. Despite this phylogenetic information, the evolution and ancestry of their photosynthetic properties are unclear. We discuss several current proposals for the evolutionary origin of aerobic phototrophic bacteria. The closest phylogenetic relatives of aerobic phototrophic bacteria include facultatively anaerobic purple nonsulfur phototrophic bacteria. Since these two bacterial groups share many properties, yet have significant differences, we compare and contrast their physiology, with an emphasis on morphology and photosynthetic and other metabolic processes.

Structure of the puf operon of the obligately aerobic, bacteriochlorophyll alpha-containing bacterium Roseobacter denitrificans OCh114 and its expression in a Rhodobacter capsulatus puf puc deletion mutant

Roseobacter denitrificans (Erythrobacter species strain OCh114) synthesizes bacteriochlorophyll a (BChl) and the photosynthetic apparatus only in the presence of oxygen and is unable to carry out primary photosynthetic reactions and to grow photosynthetically under anoxic conditions. The puf operon of R. denitrificans has the same five genes in the same order as in many photosynthetic bacteria, i.e., pufBALMC. PufC, the tetraheme subunit of the reaction center (RC), consists of 352 amino acids (Mr, 39,043); 20 and 34% of the total amino acids are identical to those of PufC of Chloroflexus aurantiacus and Rubrivivax gelatinosus, respectively. The N-terminal hydrophobic domain is probably responsible for anchoring the subunit in the membrane. Four heme-binding domains are homologous to those of PufC in several purple bacteria. Sequences similar to pufQ and pufX of Rhodobacter capsulatus were not detected on the chromosome of R. denitrificans. The puf operon of R. denitrificans was expressed in trans in Escherichia coli, and all gene products were synthesized. The Roseobacter puf operon was also expressed in R. capsulatus CK11, a puf puc double-deletion mutant. For the first time, an RC/light-harvesting complex I core complex was heterologously synthesized. The strongest expression of the R. denitrificans puf operon was observed under the control of the R. capsulatus puf promoter, in the presence of pufQ and pufX and in the absence of pufC. Charge recombination between the primary donor P+ and the primary ubiquinone Q(A)- was observed in the transconjugant, showing that the M and L subunits of the RC were correctly assembled. The transconjugants did not grow photosynthetically under anoxic conditions.

Interaction between cytochrome c and the photosynthetic reaction center of purple bacteria: behaviour at low temperature

In purple photosynthetic bacteria the electron donor to the special pair, after its oxidation by a light-induced reaction, is a c-type cytochrome: either a soluble monoheme cytochrome which forms a transitory complex with the reaction center, or a tetraheme cytochrome which remains permanently bound to the reaction center. The effects of low temperatures on electron transfer in the complex are presented and discussed. They provide estimates for the reorganization energy. The most prominent effect of low temperature is that a dominant fast phase of electron transfer becomes impossible at a temperature of around 250 K (monoheme cytochrome) or located between 250 K and 80 K according to the redox state (tetraheme cytochrome). This inhibition is attributed to a freezing-like transition of pools of water molecules which blocks structural changes of the protein which are normally associated with the cytochrome oxidation.