Ubiquinone reduction and proton uptake by chromatophores of Rhodopseudomonas sphaeroides R-26: periodicity of two in consecutive light flashes

Mode of inhibition of electron transport by orthophenanthroline in chromatophores and reaction centers of Rhodopseudomonas sphaeroides

In Rhodopseudomonas sphaeroides, light causes the transfer of electrons from bacteriochlorophyll to ubiquinone in the photochemical reaction centers. Electrons from this "primary" ubiquinone move on to a secondary ubiquinone. We have studied the manner in which o-phenanthroline inhibits the transfer of electrons from primary to secondary ubiquinone in chromatophores (intracytoplasmic membrane fragments) and isolated reaction centers of Rp. sphaeroides. The formation of anionic semiquinones, both primary and secondary, is signaled by an absorption band at 450 nm and by band shifts of bacteriopheophytin and bacteriochlorophyll in the near infrared. The pattern of band shifts is different for primary and secondary semiquinone, allowing us to distinguish which quinone has become semireduced. This point was established for isolated reaction centers [Vermeglio, A. & Clayton, R. K. (1977) Biochim. Biophys. Acta 461, 159-165] and is confirmed here for chromatophores. By adding o-phenanthroline at various times during a sequence of actinic light flashes and monitoring the resulting optical absorbance changes, we have found that o-phenanthroline inhibits electron transfer from primary semiquinone to secondary quinone if the latter is in its oxidized form, but not if the latter is semireduced. Our findings can explain the decay kinetics of delayed fluorescence from bacteriochlorophyll in Rp. sphaeroides as measured by R. P. Carithers and W. W. Parson [(1976) Biochim. Biophys. Acta 440, 215-232].

Spatial correlation between primary redox components in reaction centers of Rhodopseudomonas sphaeroides measured by two electrical methods in the nanosecond range

Relative distances between the the primary donor P, the intermediary pheophytin acceptor H, and the iron-quinone acceptor Q of bacterial reaction centers were determined by recording laser flash-induced photovoltages in two experimental systems with nanosecond time resolution. In one system a suspension of chromatophores was subjected to a light gradient and in the other system chromatophores were spread at a heptane/water interface. The 10-ns back reaction occurring in reaction centers with reduced Q could be time resolved. The initial photovoltage amplitude under conditions in which the charge separation proceeded up to the state [P(+)H(-)] was about (2/3) of that when it proceeded up to the state [P(+)HQ(-)]. If the amplitude of the photovoltage is considered to be proportional to the spatial displacement of charges, this result means that pheophytin lies closer to Q than to P.

Photoreduction of the quinone pool in the bacterial photosynthetic membrane: identification of infrared marker bands for quinol formation

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.

The rates of proton uptake and electron transfer at the reducing side of photosystem II in thylakoids

Proton and electron transfer at the reducing side of photosystem II of green plants was studied under flashing light, the former at improved time resolution by using Neutral red. The rates of electron transfer within QAFeQB were determined by pump-probe flashes through electrochromic transients. The extent of proton binding was about 1 H+/e-. The rates of proton transfer were proportional to the concentration of Neutral red (collisional transfer), whereas the rates of electron transfer out of QA- and from QAFeQB- to the cytochrome b6f complex were constant. The half-rise times of electron transfer (tau e) and the apparent times of proton binding (tau h) at 30 microM Neutral red were: QA- --> FeIIIQB (tau c < or = 100 microseconds, tau h = 230 microseconds); QA- --> FeIIQB (tau c = 150 microseconds, tau h = 760 microseconds); and QA- --> FeIIQB (tau c = 150 microseconds, tau h = 760 microseconds); and QA- --> FeIIQB (tau c = 620 microseconds, tau h = 310 microseconds).

Enthalpy and volume changes accompanying electron transfer from P-870 to quinones in Rhodopseudomonas sphaeroides reaction centers

A capacitor microphone was used to measure the enthalpy and volume changes that accompany the electron transfer reactions, PQAhv leads to P+Q-A and PQAQBhv leads to P+QAQ-B, following flash excitation of photosynthetic reaction centers isolated from Rhodopseudomonas sphaeroides. P is a bacteriochlorophyll dimer (P-870), and QA and QB are ubiquinones. In reaction centers containing only QA, the enthalpy of P+Q-A is very close to that of the PQA ground state (delta Hr = 0.05 +/- 0.03 eV). The free energy of about 0.65 eV that is captured in the photochemical reaction evidently takes the form of a substantial entropy decrease. In contrast, the formation of P+QAQ-B in reaction centers containing both quinones has a delta Hr of 0.32 +/- 0.02 eV. The entropy change must be near zero in this case. In the presence of o-phenanthroline, which blocks electron transfer between Q-A and QB, delta Hr for forming P+Q-AQB is 0.13 +/- 0.03 eV. The influence of flash-induced proton uptake on the results was investigated, and the delta Hr values given above were measured under conditions that minimized this influence. Although the reductions of QA and QB involve very different changes in enthalpy and entropy, both reactions are accompanied by a similar volume decrease of about 20 ml/mol. The contraction probably reflects electrostriction caused by the charges on P+ and Q-A or Q-B.

Electron acceptors of bacterial photosynthetic reaction centers. II. H+ binding coupled to secondary electron transfer in the quinone acceptor complex

The photoreduction of ubiquinone in the electron acceptor complex (QIQII) of photosynthetic reaction centers from Rhodopseudomonas sphaeroides, R26, was studied in a series of short, saturating flashes. The specific involvement of H+ in the reduction was revealed by the pH dependence of the electron transfer events and by net H+ binding during the formation of ubiquinol, which requires two turnovers of the photochemical act. On the first flash QII receives an electron via QI to form a stable ubisemiquinone anion (QII-); the second flash generates QI-. At low pH the two semiquinones rapidly disproportionate with the uptake of 2 H+, to produce QIIH2. This yields out-of-phase binary oscillations for the formation of anionic semiquinone and for H+ uptake. Above pH 6 there is a progressive increase in H+ binding on the first flash and an equivalent decrease in binding on the second flash until, at about pH 9.5, the extent of H+ binding is the same on all flashes. The semiquinone oscillations, however, are undiminished up to pH 9. It is suggested that a non-chromophoric, acid-base group undergoes a pK shift in response to the appearance of the anionic semiquinone and that this group is the site of protonation on the first flash. The acid-base group, which may be in the reaction center protein, appears to be subsequently involved in the protonation events leading to fully reduced ubiquinol. The other proton in the two electron reduction of ubiquinone is always taken up on the second flash and is bound directly to QII-. At pH values above 8.0, it is rate limiting for the disproportionation and the kinetics, which are diffusion controlled, are properly responsive to the prevailing pH. Below pH 8, however, a further step in the reaction mechanism was shown to be rate limiting for both H+ binding electron transfer following the second flash.

Two regimens of electrogenic cyclic redox chain operation in chromatophores of non-sulfur purple bacteria. A study using antimycin A

Antimycin A causes a biphasic suppression of the light-induced membrane potential generation in Rhodospirillum rubrum and Rhodopseudomonas sphaeroides chromatophores incubated anerobically. The first phase is observed at low antibiotic concentrations and is apparently due to its action as a cyclic electron transfer inhibitor. The second phase is manifested at concentrations which are greater than 1--2 muM and is due to uncoupling that may be connected with an antibiotic-induced dissipation of the electrochemical H+ gradient across the chromatophore membrane. The inhibitory effect of antimycin added at low concentrations under aerobic conditions is removed by succinate to a large extent. It is expected that the electrogenic cyclic redox chain in the bacterial chromatophores incubed under conditions of continuous illumination may function at two regimes: (1) as a complete chain involving all the redox components, and (2) as a shortened chain involving only the P-870 photoreaction center, ubiquinone and cytochrome c2.

Factors controlling the binding of two protons per electron transferred through the ubiquinone and cytochrome b/c2 segment of Rhodopseudomonas sphaeroides chromatophores

1. On every turnover, 2.0 protons can be bound by the membrane for each single electron moving through the Q-b/c2 oxidoreductase. 2. One proton (H+II) binding reaction is, and one (H+I) is not, sensitive to antimycin. 3. The redox states of electron transfer components other than the proton binding agents can affect both the rate of proton uptake and the apparent pK values on the agents binding the protons. 4. The presence of valinomycin under certain well-defined conditions can strongly influence the value of the measured pK on the H+II binding agent.

The involvement of iron and ubiquinone in electron transfer reactions mediated by reaction centers from photosynthetic bacteria

Reaction centers from Rhodopseudomonas sphaeroides strain R-26 were prepared with varying Fe and ubiquinone (Q) contents. The photooxidation of P-870 to P-870+ was found to occur with the same quantum yield in Fe-depleted reaction centers as in control samples. The kinetics of electron transfer from the initial electron acceptor (I) to Q also were unchanged upon Fe removal. We conclude that Fe has no measurable role in the primary photochemical reaction. The extent of secondary reaction from the first quinone acceptor (QA) to the second quinone acceptor (QB) was monitored by the decay kinetics of P-870+ after excitation of reaction centers with single flashes in the absence of electron donors, and by the amount of P-870 photooxidation that occurred on the second flash in the presence of electron donors. In reaction centers with nearly one iron and between 1 and 2 ubiquinones per reaction center, the amount of secondary electron transfer is proportional to the ubiquinone content above one per reaction center. In reaction centers treated with LiClO4 and o-phenanthroline to remove Fe, the amount of secondary reaction is decreased and is proportional to Fe content. Fe seems to be required for the secondary reaction. In reaction centers depleted of Fe by treatment with SDS and EDTA, the correlation between Fe content and secondary activity is not as good as that found using LiClO4. This is probably due in part to a loss of primary photochemical activity in samples treated with SDS; but the correlation is still not perfect after correction for this effect. The nature of the back reaction between P-870+ and Q-B was investigated using stopped flow techniques. Reaction centers in the P-870+ Q-B state decay with a 1-s half-time in both the presence and absence of o-phenanthroline, an inhibitor of electron transfer between Q-B and QB. This indicates that the back reaction between P-870+ and Q-A is direct, rather than proceeding via thermal repopulation of Q-A. The P-870+ Q-B state is calculated to lie at least 100 mV in free energy below the P-870+ Q-A state.

The mechanism of reduction of the ubiquinone pool in photosynthetic bacteria at different redox potentials

(1) A flash number dependency of flash-induced absorbance changes was observed with whole cells of Rhodospirillum rubrum and chromatophores of R. rubrum and Rhodopseudomonas sphaeroides wild type and the G1C mutant. The oscillatory behavior was dependent on the redox potential; it was observed under oxidizing conditions only. Absorbance difference spectra measured after each flash in the 275--500 nm wavelength region showed that a molecule of ubiquinone, R, is reduced to the semiquinone (R-) after odd-numbered flashes and reoxidized after even-numbered flashes. The amount of R reduced was approximately one molecule per reaction center. (2) The flash number dependency of the electrochromic shift of the carotenoid spectrum was studied with chromatophores of Rps. sphaeroides wild type and the G1C mutant. At higher values of the ambient redox potential a relatively slow phase with a rise time of 30 ms was observed after even-numbered flashes, in addition to the fast phase (completed within 0.2 ms) occurring after each flash. Evidence was obtained that the slow phase represents the formation of an additional membrane potential during a dark reaction that occurs after flashes with an even number. This reaction is inhibited by antimycin A, whereas the oscillations of the R/R- absorbance changes remain unaffected. At low potentials (E = 100 mV) no oscillations of the carotenoid shift were observed: a fast phase was followed by a slow phase (antimycin-sensitive) with a half-time of 3 ms after each flash. (3) The results are discussed in terms of a model for the cyclic electron flow as described by Prince and Dutton (Prince, R.C. and Dutton, P.L. (1976) Bacterial Photosynthesis Conference, Brussels, Belgium, September 6--9, Abstr. TB4) employing the so-called Q-cycle.