Electron transfer in reaction centers of Rhodopseudomonas sphaeroides. I. Determination of the charge recombination pathway of D+QAQ−B and free energy and kinetic relations between Q−AQB and QAQ−B

The electron-transfer reactions and thermodynamic equilibria involving the quinone acceptor complex in bacterial reaction centers from R. sphaeroides were investigated. The reactions are described by the scheme: We found that the charge recombination pathway of D⁺Q_AQ^?_B proceeds via the intermediate state D⁺Q^?_AQ_B, the direct pathway contributing less than approx. 5% to the observed recombination rate. The method used to obtain this result was based on a comparison of the kinetics predicted for the indirect pathway (given by the product k_AD-times the fraction of reaction centers in the Q^?_AQ_B state) with the observed recombination rate, k^obs_{D⁺ →D}. The kinetic measurements were used to obtain the pH dependence (6.1 ? pH ? 11.7) of the free energy difference between the states Q^?_AQ_B and Q_AQ^?_B. At low pH (less than 9) Q_AQ^?_B is stabilized relative to Q^?_AQ_B by 67 meV, whereas at high pH Q^?_AQ_B is energetically favored. Both Q^?_A and Q^?_B associate with a proton, with pK values of 9.8 and 11.3, respectively. The stronger interaction of the proton with Q^?_B provides the driving force for the forward electron transfer.     

Studies on well-coupled Photosystem-I-enriched subchloroplast vesicles. Discrimination of flash-induced fast and slow electric potential components

This work aimed at the resolution of the multi-component electric potential changes induced by single-turnover flash illumination of Photosystem-I-enriched subchloroplast vesicles. If supplemented with ferredoxin and under carefully adjusted redox poising, these vesicles show a pronounced slow-rising and -decaying electric potential component, as monitored by endogenous and exogenous field-sensitive probes, carotenoids and oxonol VI, respectively. The fast and slow potential components can be easily discriminated without the need for computer-assisted deconvolution after selective presaturation of the slow component by preillumination or a transmembrane ΔpH, after selective suppression of the slow component by low valinomycin or uncoupler concentrations or in the absence of ferredoxin. The slow electric potential component, as compared to the fast one, is relatively sensitive to low concentrations of ionophores and uncouplers, detergent, ageing and lower temperatures (4–12°C), is associated with electrogenic proton displacements and is interpreted to respond to a field that is more located on the membrane-bulk interface. Temperature effects show transition temperatures around 20°C for both the rise and decay of the slow potential component. The results provide further evidence that the carotenoids and oxonol VI sense the same (slow) electric field, but may be differently located in the thylakoid membrane.     

Studies on well-coupled Photosystem-I-enriched subchloroplast vesicles. Electron transfer by b- and c-type cytochromes in relation to the origin of the ‘slow’ electric potential component

Cytochrome redox changes and electric potential generation are kinetically compared during cyclic electron transfer in Photosystem-I-enriched and Photosystem-II-depleted subchloroplast vesicles (i.e., stroma lamellae membrane vesicles) supplemented with ferredoxin using a suitable electron donating system. In response to a single-turnover flash, the sequence of events is: (1) fast reduction of cytochrome b-563 (t_0.5 ≈ 0.5 ms) (2) oxidation of cytochrome c-554 (t_0.5 ≈ 2 ms), (3) slower reduction of cytochrome b-563 (t_0.5 ≈ 4 ms), (4) generation of the ‘slow’ electric potential component (t_0.5 ≈ 15–20 ms), (5) re-reduction of cytochrome c-554 (t_0.5 ≈ 30 ms) and (6) reoxidation of cytochrome b-563t_0.5 ≈ 90 ms). Per flash two cytochrome b-563 species turn over for one cytochrome c-554. These b-563 cytochromes are reduced with different kinetics via different pathways. The fast reductive pathway proceeds probably via ferredoxin, is insensitive to DNP-INT, DBMIB and HQNO and is independent on the dark redox state of the electron transfer chain. In contrast, the slow reductive pathway is sensitive to DNP-INT and DBMIB, is strongly delayed at suboptimal redox poising (i.e., low $\text{NADPH}\text{NADP}{}^{\mathrm{+}}$ ratio) and is possibly coupled to the reduction of cytochrome c-554. Each reductive pathway seems obligatory for the generation of about 50% of the slow electric potential component. Also cytochrome c-559_LP (LP, low potential) is involved in Photosystem-I-associated cyclic electron flow, but its flash-induced turnover is only observed at low preestablished electron pressure on the electron-transfer chain. Data suggest that cyclic electron flow around Photosystem I only proceeds if cytochrome b-559_LP is in the reduced state before the flash, and a tentative model is presented for electron transfer through the cyclic system.     

Interaction of α-cyano[14C]cinnamate with the mitochondrial pyruvate translocator

The binding of α-cyanocinnamate to rat-heart mitochondrial membrane was investigated using α-cyano[¹⁴C]cinnamate. The binding was correlated to the inhibition of pyruvate transport. The results obtained demonstrate that both these functions reach saturation at the same titre of the inhibitor. Quantitative parameters of α-cyano[¹⁴C]cinnamate binding have been determined. The binding can be prevented by pyruvate and other substrates of the carrier but not by acetate. Pyruvate decreases the affinity of α-cyanocinnamate binding, leaving the maximum number of binding unchanged. It is concluded that rat-heart mitochondria contain a specific site at which α-cyanocinnamate binds which is directly involved in the inhibition of pyruvate transport.     

Excitation-energy distribution in green algae. The existence of two independent light-driven control mechanisms

Three distinct states can be identified for cells of the green alga Chlorella vulgaris; State 1 and State 2 obtained by preillumination in far-red and red light, respectively, and the dark state obtained by dark-adaptation. Addition of the inhibitor DCMU to algal cells leads to an initial rapid increase in chlorophyll-a fluorescence reflecting the closure of Photosystem II traps. This, in the case of dark and state-2-adapted algae is followed by a slow light-dependent increase to a fluorescence yield typical of State-1-adapted cells. Measurements of low temperature (77 K) emission spectra indicate that the low fluorescence yields of dark and State-2-adapted algae reflect similar balances in excitation-energy distribution between the two photosystems. In both cases, the balance favours PS I and the slow fluorescence increase seen in the poisoned algae reflects a redressing of this balance in favour of PS II. The low fluorescence yield of State-2-adapted algae is thought to be associated with the phosphorylation of chlorophyll a/b light-harvesting protein (Biochim. Biophys. Acta (1983) 724, 94–103). Measurements of the uncoupler and ATPase sensitivity of the light-dependent increases seen in DCMU-poisoned cells indicate that the low fluorescence yield of dark-adapted algae is of different origin. Evidence is presented showing that the light-driven changes in excitation-energy distribution seen in green algae involve two distinct processes; a low-intensity, wavelenght-independent change reflecting simple light/dark changes and a higher intensity, wavelength-dependent change reflecting State 1/State 2 adaptation. The former changes appear to be associated with changes in the local ionic environment within the algal chloroplast, whilst the latter appear to reflect changes in the phosphorylation state of chlorophyll a/b light-harvesting protein.     

Isolation and amino-terminal sequences of subunits from the photosynthetic reaction center of Rhodopseudomonas capsulata

The amino-terminal sequences have been determined by Edman degradation for the reaction center polypeptides from a carotenoidless mutant of Rhodopseudomonas capsulata. Individual polypeptides were isolated by preparative electrophoresis and electroelution. By comparison with the sequences deduced from the DNA (Youvan, D.C., Alberti, M., Begush, H., Bylina, E.J. and Hearst, J.E. (1984) Proc. Natl. Acad. Sci. USA 81, 189–192) we conclude that the M and L subunits are processed so as to remove the amino-terminal methionine, whereas the H subunit is not processed at the amino-terminus after translation. None of the subunits is synthesized with a significant amino-terminal extension peptide.     

An analysis of proton fluxes coupled to electron transport and ATP synthesis in chloroplast thylakoids

Electron transport, phosphorylation and internal proton concentration were measured in illuminated spinach chloroplast thylakoid membranes under a number of conditions. Regardless of the procedure used to vary these parameters, the data fit a simple chemiosmotic model. Protons from Photosystem II did not appear to be utilized differently from those derived from Photosystem I. The maximal phosphorylation efficiency ($\text{P}\text{e}{}_2$) for photophosphorylation in washed thylakoids under oxidizing conditions is likely to be $\text{4}\text{3}$. This value is consistent with a proton-to-electron-pair ratio of 4 for electron flow through both photosystems and a proton-to-ATP ratio of 3 for the chloroplast proton-ATPase.     

EPR detection of a cryogenically photogenerated intermediate in photosynthetic oxygen evolution

In Photosystem II preparations at low temperature we were able to generate and trap an intermediate state between the S₁ and S₂ states of the Kok scheme for photosynthetic oxygen evolution. Illumination of dark-adapted, oxygen-evolving Photosystem II preparations at 140 K produces a 320-G-wide EPR signal centered near g = 4.1 when observed at 10 K. This signal is superimposed on a 5-fold larger and somewhat narrower background signal; hence, it is best observed in difference spectra. Warming of illuminated samples to 190 K in the dark results in the disappearance of the light-induced g = 4.1 feature and the appearance of the multiline EPR signal associated with the S₂ state. Low-temperature illumination of samples prepared in the S₂ state does not produce the g = 4.1 signal. Inhibition of oxygen evolution by incubation of PS II preparations in 0.8 M NaCl buffer or by the addition of 400 μM NH₂OH prevents the formation of the g = 4.1 signal. Samples in which oxygen evolution is inhibited by replacement of Cl^? with F^? exhibit the g = 4.1 signal when illuminated at 140 K, but subsequent warming to 190 K neither depletes the amplitude of this signal nor produces the multiline signal. The broad signal at g = 4.1 is typical for a $\text{S =}\text{5}\text{2}$ spin system in a rhombic environment, suggesting the involvement of non-heme Fe in photosynthetic oxygen evolution.     

The chloride requirement for photosynthetic oxygen evolution. Analysis of the effects of chloride and other anions on amine inhibition of the oxygen-evolving complex

In the presence of Cl^?, the severity of ammonia-induced inhibition of photosynthetic oxygen evolution is attenuated in spinach thylakoid membranes (Sandusky, P.O. and Yocum, C.F. (1983) FEBS Lett. 162, 339–343). A further examination of this phenomenon using steady-state kinetic analysis suggests that there are two sites of ammonia attack, only one of which is protected by the presence of Cl^?. In the case of Tris-induced inhibition of oxygen evolution only the Cl^? protected site is evident. In both cases the mechanism of Cl^? protection involves the binding of Cl^? in competition with the inhibitory amine. Anions (Br^? and NO^?₃) known to reactive oxygen evolution in Cl^?-depleted membranes also protect against Tris-induced inhibition, and reactivation of Cl^?-depleted membranes by Cl^? is competitively inhibited by ammonia. Inactivation of the oxygen-evolving complex by NH₂OH is impeded by Cl^?, whereas Cl^? does not affect the inhibition induced by so-called ADRY reagents. We propose that Cl^? functions in the oxygen-evolving complex as a ligand bridging manganese atoms to mediate electron transfer. This model accounts both for the well known Cl^? requirement of oxygen evolution, and for the inhibitory effects of amines on this reaction.     

Resonance Raman resolution of a-, b- and c-type cytochromes in membrane vesicles of alkalophilic bateria

Resonance Raman spectroscopy has been used to obtain complete spectra of each individual cytochrome type — a, b and c — in the reduced state within membrane vesicle preparations from two species of obligately alkalophilic bacteria: Bacillus alcalophilus and Bacillus firmus RAB. The vibrational spectra, in the range 250–1700 cm^?1, were obtained with tunable dye laser excitation in the wavelength range 550–600 nm tuned to resonance with the appropriate reduced alpha band maximum for the cytochrome type of interest. The spectra reveal details which serve to characterize the specific type of cytochrome as well as to confirm the similarity of the heme prosthetic group to previously well-characterized cytochromes of the the a- b- or c-type. Preliminary evidence in support of heterogeneity of b-type, and possibly a-type cytochromes, or of heme-heme interaction within the membrane is presented.