Menaquinone as pool quinone in a purple bacterium

Biochimica et Biophysica Acta (BBA) - Bioenergetics, 1993

Reaction centers (RCs)with one antenna complex attached (RC/LH~ complexes)were isolated from the alkaliphilic and moderately halophilic, phototrophic purple sulfur bacterium Ectothiorhodospira (E.) mobilis and analyzed with respect to quinone composition and charge recombination kinetics, using time-resolved (low-and room-temperature) spectroscopy and thin-layer chromatography. The RCs contain menaquinone as primary and ubiquinone as secondary quinone electron acceptor, QA and Q a, respectively. Q B is lost during isolation of the RC/LH I complexes. P+ QA charge recombination kinetics, which were shown to be pH independent, were only slightly temperature dependent, indicating that this recombination proceeds via the direct (electron tunnelling) route. At room temperature, its average lifetime was 34.5 ms. These decay kinetics were shown to be monophasic at room temperature and biphasic at low temperature. Addition of an excess of UQ 6 to RC/LH I complexes resulted in retardation of the P+ recovery, due to charge recombination of the state P+QAQa. Functional reconstitution of QB was also evident from flash-induced binary oscillations at 450 nm, which could be observed in RC/LH I complexes in the presence of an excess of UQ 6. This reconstitution of QB activity was, however, incomplete and decreased with increasing pH. The P+QAQB decay kinetics were pH independent; the average lifetime was about 6 s at room temperature. The apparent equilibrium constant g 2 between the states QAQB and QAQB, and consequently the free energy difference between these states, were relatively large and pH independent. K 2 was on average 167, whereas the mean value of the free energy difference between the two states was 130 meV. Finally, a scheme is presented for the kinetics and free energy changes of the electron transfer steps in isolated E. mobilis RCs.

Photosynthesis Research, 2018

Photosynthetic reaction centers (RCs) evolved > 3 billion years ago and have diverged into Type II RCs reducing quinones and Type I RCs reducing soluble acceptors via iron-sulfur clusters. Photosystem I (PSI), the exemplar Type I RC, uses modified menaquinones as intermediate electron transfer cofactors, but it has been controversial if the Type I RC of heliobacteria (HbRC) uses its two bound menaquinones in the same way. The sequence of the quinone-binding site in PSI is not conserved in the HbRC, and the recently solved crystal structure of the HbRC does not reveal a quinone in the analogous site. We found that illumination of heliobacterial membranes resulted in reduction of menaquinone to menaquinol, suggesting that the HbRC can perform a function thought restricted to Type II RCs. Experiments on membranes and live cells are consistent with the hypothesis that the HbRC preferentially reduces soluble electron acceptors (e.g., ferredoxins) in low light, but switches to reducing lipophilic quinones in high light, when the soluble acceptor pool becomes full. Thus, the HbRC may represent a functional evolutionary intermediate between PSI and the Type II RCs.

Photochemical & Photobiological Sciences, 2007

The cytochrome bc 1 complexes are proton-translocating, dimeric membrane ubiquinol:cytochrome c oxidoreductases that serve as "hubs" in the vast majority of electron transfer chains. After each ubiquinol molecule is oxidized in the catalytic center P at the positively charged membrane side, the two liberated electrons head out, according to the Mitchell's Q-cycle mechanism, to different acceptors. One is taken by the [2Fe-2S] iron-sulfur Rieske protein to be passed further to cytochrome c 1. The other electron goes across the membrane, via the low-and high-potential hemes of cytochrome b, to another ubiquinone-binding site N at the opposite membrane side. It has been assumed that two ubiquinol molecules have to be oxidized by center P to yield first a semiquinone in center N and then to reduce this semiquinone to ubiquinol. This review is focused on the operation of cytochrome bc 1 complexes in phototrophic purple bacteria. Their membranes provide a unique system where the generation of membrane voltage by light-driven, energy-converting enzymes can be traced via spectral shifts of native carotenoids and correlated with the electron and proton transfer reactions. An "activated Q-cycle" is proposed as a novel mechanism that is consistent with the available experimental data on the electron/proton coupling. Under physiological conditions, the dimeric cytochrome bc 1 complex is suggested to be continually primed by prompt oxidation of membrane ubiquinol via center N yielding a bound semiquinone in this center and a reduced, high-potential heme b in the other monomer of the enzyme. Then the oxidation of each ubiquinol molecule in center P is followed by ubiquinol formation in center N, proton translocation and generation of membrane voltage.

ChemPhysChem, 2004

1990

INTPODUCTlON chloroflexus aurantiacus is a membea of one of the earliest branchj.ng lioes of the eubacteria (Gj.bson et a1.,1985). The other gene.a in this g.ouping, the rgreen Erre Herpetosiphon and Thermomicrobiun , with which Chloroflexus shares litt1e appa.aent phenotypic rese,nblar:ce. Although contaios Bchls a aad c in its 1r6 Chloroflexus chloaosones, 165 rRNA sequencing studies have shown no phylogenetj.c relatedness to the green suÌphur bacte.ia (cibson et a1.,19a5). Indeed, in terms of .eaction center photochenistry (B.uce et a1.,1942), some aspects of electron iransport (Zannoni and Ingledew. 1945), the Ìi.ght-dependent energy transducing machinary of Chloroflexus most resenbles that of the purple non-sulphur bacteria (Ventuioli and Zannoni, 1987). Unlike the Rhodospili llaceae , hov/ever, the calvin cycle is apparently not used to fix COz into cell mateaial under autot.ophic growth conditions (uo1o and Siverag,1987) and a novel nechanism has been postulated. ll/hi.le Chlo.oflexus stains O.am negative, the co,ngosition of the cell u/a1Ì is more similar to that found in Crarn positive oaganisms (Jurgens et a1.,198"). Finally , the thernophilic character of 9IlgI9É19.199 (optinuD gro'rth at 55'c) is seemingly raÌ'e amongst phototrophi.c bacteria. These chaiacteristics indicate

FEBS Letters, 2003

The photoreduction of the quinone (Q) pool in the photosynthetic membrane of the purple bacterium Rhodobacter sphaeroides was investigated by steady-state and time-resolved Fourier transform infrared difference spectroscopy. The results are consistent with the existence of a homogeneous Q pool inside the chromatophore membrane, with a size of around 20 Q molecules per reaction center. IR marker bands for the quinone/quinol (Q/QH(2)) redox couple were recognized. QH(2) bands are identified at 1491, 1470, 1433 and 1388-1375 cm(-1). The 1491 cm(-1) band, which is sensitive to (1)H/(2)H exchange, is assigned to a C-C ring mode coupled to a C-OH mode. A feature at approximately 1743/1720 cm(-1) is tentatively related to a perturbation of the carbonyl modes of phospholipid head groups induced by QH(2) formation. Complex conformational changes of the protein in the amide I and II spectral ranges are also apparent during reduction and reoxidation of the Q pool.

Photosynthesis …, 1997

Site-specific mutations in the quinone binding sites of the photosynthetic reaction center (RC) protein complexes of Rhodobacter (R.) capsulatus caused pronounced effects on sequential electron transfer. Conserved residues that break the twofold symmetry in this region of the RC -M246Ala and M247Ala in the Q A binding pocket, and L212Glu and L213Asp in the Q B binding pocket -were targeted. We constructed a Q B -site mutant, L212Glu-L213Asp ! Ala-Ala, and a Q A -site mutant, M246Ala-M247Ala ! Glu-Asp, to partially balance the differences in charge distribution normally found between the two quinone binding sites. In addition, two photocompetent revertants were isolated from the photosynthetically-incompetent M246Glu-M247Asp mutant: M246Ala-M247Asp and M246Gly-M247Asp. Sequential electron transfer was investigated by continuous light excitation and time-resolved electron paramagnetic resonance (EPR), and time-resolved optical techniques. Several lines of EPR evidence suggested that the forward electron transfer rate to Q A , k Q , was slowed in those strains containing altered Q A sites. The slower rates of secondary electron transfer were confirmed by time-resolved optical results with the M246Glu-M247Asp mutations in the Q A site resulting in a dramatically lowered secondary electron transfer efficiency [k Q < (2 ns) 1 ] in comparison with either the native R. capsulatus RC or the Q B site mutant [k Q (200 ps) 1 ]. Secondary electron transfer in the two revertants was intermediate between that of the native RC and the Q A mutant. The P + Q A ! PQ A charge recombination rates were also changed in the strains that carried altered Q A sites. We show that local mutations in the Q A site, presumably through local electrostatic changes, significantly alter binding and electron transfer properties of Q A .

Journal of Biological Chemistry, 2000

Biochemical Society Transactions, 2005

This review is focused on reactions that gate (control) the electron transfer between the primary quinone Q A and secondary quinone Q B in the photosynthetic reaction centre of Rhodobacter sphaeroides. The results on electron and proton transfer are discussed in relation to structural information and to the steered molecular dynamics simulations of the Q B ring flip in its binding pocket. Depending on the initial position of Q B in the pocket and on certain conditions, the rate of electron transfer is suggested to be limited either by the quinone ring flip or by the charge-compensating proton equilibration between the surface and the buried Q B site.

Biochemistry, 2005

The secondary quinone acceptor, Q B , has been studied in photosystem II (PSII) isolated from Thermosynechococcus (T.) elongatus. Thermoluminescence indicated that Q B was present in this preparation. An EPR signal observed at low temperature at g ) 1.9 was attributed to Fe 2+ Q Bon the basis of the characteristic period-of-two variations in its intensity depending on the number of laser flashes given at 20°C. When samples showing the Fe 2+ Q Bsignal were illuminated at 77 K, an EPR signal at g ) 1.66 appeared with an amplitude proportional to that of the Fe 2+ Q Bsignal. This signal is attributed to the Q A -Fe 2+ Q Bstate. While these attributions have been made previously in PSII from other origins, they have remained relatively tentative since the characteristic period-of-two oscillations of Q B had not previously been observed. The flash experiments indicated that more than one exchangeable plastoquinone is associated with the isolated PSII. The g ) 1.66 signal from the Q A -Fe 2+ Q Bstate was used to study the temperature dependence of electron transfer between the two quinones. Electron transfer occurred in half of the centers (after 30 s incubation) at -28°C for Q Ato Q B but at -58°C for Q Ato Q B -. This marked difference for the two electron transfer reactions indicates different types of rate-limiting reactions. In the better studied but homologous system, the purple bacterial reaction center, the Q Ato Q B step is limited by a gating process, while the Q Ato Q Bstep is limited by protonation events. Similar reactions in PSII could give rise to the observed temperature dependence. 1 Abbreviations: PSII, photosystem II; QA, primary quinone acceptor; QB, secondary quinone acceptor; FCCP, carbonyl cyanide 4-(trifluoromethoxy)phenylhydrazone; cyt b559, cytochrome b559; ChlZ, the redoxactive chlorophyll; Car, redox-active -carotene; TyrZ, redox-active tyrosine Y161 of the D1 protein; TyrD, redox-active tyrosine Y160 of the D2 protein; S0, S1, S2, and S3, redox states of the water oxidizing enzyme.