Characterization of an LM unit purified by affinity chromatography from Rhodobacter sphaeroides reaction centers and interactions with the H subunit

Removal of the H subunit results in enhanced exposure of the semiquinone sites in the LM dimer from Rhodobacter sphaeroides to oxidation by ferricyanide and by O2

Bacterial reaction centers (RC) from Rhodobacter sphaeroides have been widely used to functionalize electrodes and to generate photocurrent. However, in most studies, direct electron transfer from the semiquinone to the electrode was not observed because the H subunit of the RC shields the semiquinone. It is demonstrated in the current work that removal of the H subunit effectively exposes the semiquinone sites in the LM dimer. This is demonstrated by measuring the second-order rate constant for the reaction between ferricyanide and the anionic semiquinone Q _A^- formed by an actinic flash. The rate constant increases 1000-fold for Q _A^- oxidation by ferricyanide in the LM dimer compared to the intact RC. The second-order rate constant approaches the diffusion limit of 6 × 10⁹ M^-1·s^-1 at low pH, but it decreases steadily when the pH is above 6.5. This pH dependence suggests that the protonation state of the LM dimer plays an important role in controlling the electron transfer kinetics. It is also shown that the addition of exogenous ubiquinone to replenish the Q_B site, which is mostly empty in the LM dimer, leads to oxidation of Q _A^- by O₂ following an actinic flash. It is concluded that removal of the H subunit results in exposure of the semiquinone sites of the LM dimer to externally added oxidants and may provide a strategy for enhancing direct electron transfer from the RC to an electrode.

Ultrafast Electron Transfer Kinetics in the LM Dimer of Bacterial Photosynthetic Reaction Center from Rhodobacter sphaeroides

It has become increasingly clear that dynamics plays a major role in the function of many protein systems. One system that has proven particularly facile for studying the effects of dynamics on protein-mediated chemistry is the bacterial photosynthetic reaction center from Rhodobacter sphaeroides. Previous experimental and computational analysis have suggested that the dynamics of the protein matrix surrounding the primary quinone acceptor, QA, may be particularly important in electron transfer involving this cofactor. One can substantially increase the flexibility of this region by removing one of the reaction center subunits, the H-subunit. Even with this large change in structure, photoinduced electron transfer to the quinone still takes place. To evaluate the effect of H-subunit removal on electron transfer to QA, we have compared the kinetics of electron transfer and associated spectral evolution for the LM dimer with that of the intact reaction center complex on picosecond to millisecond time scales. The transient absorption spectra associated with all measured electron transfer reactions are similar, with the exception of a broadening in the QX transition and a blue-shift in the QY transition bands of the special pair of bacteriochlorophylls (P) in the LM dimer. The kinetics of the electron transfer reactions not involving quinones are unaffected. There is, however, a 4-fold decrease in the electron transfer rate from the reduced bacteriopheophytin to QA in the LM dimer compared to the intact reaction center and a similar decrease in the recombination rate of the resulting charge-separated state (P(+)QA(-)). These results are consistent with the concept that the removal of the H-subunit results in increased flexibility in the region around the quinone and an associated shift in the reorganization energy associated with charge separation and recombination.

Regulation of the primary quinone binding conformation by the H subunit in reaction centers from Rhodobacter sphaeroides

Unlike photosystem II (PSII) in higher plants, bacterial photosynthetic reaction centers (bRCs) from Proteobacteria have an additional peripheral membrane subunit "H". The H subunit is necessary for photosynthetic growth, but can be removed chemically in vitro. The remaining LM dimer retains its activity to perform light-induced charge separation. Here we investigate the influence of the H subunit on interactions between the primary semiquinone and the protein matrix, using a combination of site-specific isotope labeling, pulsed electron paramagnetic resonance (EPR), and density functional theory (DFT) calculations. The data reveal substantially weaker binding interactions between the primary semiquinone and the LM dimer than observed for the intact bRC; the amount of electron spin transferred to the nitrogen hydrogen bond donors is significantly reduced, the methoxy groups are more free to rotate, and the spectra indicate a heterogeneous mixture of bound semiquinone states. These results are consistent with a loosening of the primary quinone binding pocket in the absence of the H subunit.

Mössbauer studies of the non-heme iron and cytochrome b559 in a Chlamydomonas reinhardtii PSI- mutant and their interactions with alpha-tocopherol quinone

Spin and valence states of the non-heme iron and the heme iron of cytochrome b559, as well as their interactions with alpha-tocopherol quinone (alpha-TQ) in photosystem II (PSII) thylakoid membranes prepared from the Chlamydomonas reinhardtii PSI- mutant have been studied using Mössbauer spectroscopy. Both of the iron atoms are in low spin ferrous states. The Debye temperature of the non-heme is 194 K and of the heme iron is 182 K. The treatment of alpha-TQ does not change the spin and the valence states of the non-heme iron but enhances the covalence of its bonds. alpha-TQ oxidizes the heme iron into the high spin Fe3+ state. A possible role of the non-heme iron and alpha-TQ in electron flow through the PSII is discussed.

Sodium alkyl ether sulfate preparative electrophoresis for the preparation of reaction centers without H-subunit from Rhodopseudomonas viridis

Sodium alkyl ether sulfate (AES), an analog of sodium dodecyl sulfate (SDS) was used in polyacrylamide gel electrophoresis (PAGE) for the partial decomposition of the photosynthetic reaction center (RC) of Rhodopseudomonas viridis. Unlike SDS, AES did not completely dissociate RC into its subunits but selectively detached H-subunit from RC to give RC(-H) without losing the spectroscopic nature of RC. For the denaturation of RC(-H), AES was found to be as mild as 3-[3-cholamidopropyl)dimethylammonio]-l-propanesulfonate (CHAPS).

Flow-force relationships in lettuce thylakoids. 2. Effect of the uncoupler FCCP on local proton resistances at the ATPase level

The relationship between the steady-state proton gradient (delta pH) and the rate of phosphorylation was investigated in thylakoids under various conditions. Under partial uncoupling by carbonyl cyanide p-(trifluoromethoxy)phenylhydrazone (FCCP), the rate of ATP synthesis was reduced by less than expected from the decrease of delta pH. This was observed in the case of the pyocyanine-mediated cyclic electron flow around photosystem 1, but not with the H2O-->photosystem 2-->cytochrome b6f-->photosystem 1-->methyl viologen system. In state 4, a unique relation was found between delta pH and the "phosphate potential", delta Gp, regardless of whether the energy level was controlled by light input or FCCP. The anomalous effect of FCCP on the rate of ATP synthesis disappeared when the ATPase was partially blocked by the reversible inhibitor venturicidin, but not in the presence of tentoxin, an irreversible inhibitor. These results are consistent with the existence of a small kinetic barrier for protons, limiting their access to the ATPase. This resistance would be collapsed by FCCP.

Influence of the H-subunit and Fe2+ on electron transport from I- to QA in Fe(2+)-free and/or H-free reaction centers from Rhodobacter sphaeroides R-26

From reaction centres (RC) of Rhodobacter sphaeroides R-26 two LM preparations with 0.90 Fe2+/RC (LM) and 0.10 Fe2+/RC (LM/dFe) were prepared. Reconstitution of LM/dFe with the H-subunit and subsequently with Zn2+ yielded LMH/dFe and LMH/dFe and LMH/dFe + Zn preparations, respectively. In these four samples the decay of the primary radical pair P+I- was studied by means of transient absorption spectroscopy and compared with that in native RC. In LMH/dFe the reduction of QA by Bpheo a occurred in 5 ns, with concomitant increase in the yield of PT, the triplet state of the primary donor. In the LM/dFe, LM and LMH/dFe + Zn preparations the decay of I- had the same rate (200 ps)-1 as in native RC. Thus, neither the H-subunit in the RC nor a divalent metal as Fe2+ or Zn2+ are necessary per se for fast reduction of QA. Only demetallation in the presence of the H-subunit slows down the reduction of QA.

On the depletion and reconstitution of both QA and metal in reaction centers of the photosynthetic bacterium Rb. sphaeroides R-26

Four possible ways to prepare QA-depleted, Fe-depleted and QA-reconstituted RCs were studied: (1) first depleting the Fe, then depleting QA and finally reconstituting QA (D-Fe, D-Q, R-Q), (2) first depleting QA, then depleting the Fe and finally reconstituting QA (D-Q, D-Fe, R-Q), (3) first depleting QA, then reconstituting QA and finally depleting Fe (D-Q, R-Q, D-Fe), (4) first depleting QA, then depleting the Fe and reconstituting QA in the same step (D-Q, D-Fe-R-Q). Our results showed that: method (1) results in the irreversible loss of photochemical activity; method (2) and (3) result in low recovery of the photochemical activity and poor yield of Fe-depleted, QA-reconstituted RCs; method (4) gives surprisingly good results. This method allows for the first time to prepare the QA-depleted, Fe-depleted, QA-reconstituted RCs with high recovery of the photochemical activity and good yield. The sample has 98% of photochemical activity (yield of P(+) QA (-)) compared with that of the native RCs and shows strong polarization of the EPR signal of QA (-) under continuous illumination at 5K. The decay halftime of I(-) is slow (∼5 ns) compared with that of the native RCs, but it is the same as that measured for the RCs from which only iron is removed. These results indicate that the depletion of iron and the reconstitution of QA have been successful. Reconstitution of the QA-depleted, Fe-depleted and QA-reconstituted RCs with Zn(2+) gives also the spin-polarized QA (-), and yields the same decay of I(-) (halftime 200 ps) as that of the native RCs.

Reaction center light harvesting B875 complexes from Rhodocyclus gelatinosus: characterization and identification of quinones

Reaction center-B875 pigment-protein complexes were purified from Rhodocyclus gelatinosus. The proteic components consist of 7-8 polypeptides among which some were identified by their apparent molecular weights: the light harvesting B875 polypeptides α and β of 8 and 6 kDa, reaction center L (23 kDa), M (28 kDa) and H (34 kDa), cytochrome c (43 kDa). Four c-type hemes were found per reaction center. Flash-induced absorbance changes showed the presence of both QA and QB in the complex. Charge recombination times were determined to be: 1.16±0.2 (n=30) for P(+)QAQB (-) and 7-10 ms for P(+)QA (-) in presence of herbicides. From quinone analysis on one hand and kinetics of charge recombination on the other hand, we proposed that in the reaction center of Rhodocyclus gelatinosus QA is menaquinone 8 and QB is ubiquinone 8.

Formation of functional inter-species hybrid photosynthetic complexes in Rhodobacter capsulatus

A Rhodobacter capsulatus mutant strain deficient in all pigment-binding peptides and hence incapable of photosynthetic growth was genetically complemented with a plasmid-borne copy of the Rhodobacter sphaeroides puf operon. Hybrid reaction centers composed of R. sphaeroides L and M and R. capsulatus H subunits assembled in vivo, and host cells were photosynthetically competent. Light-harvesting complex B875, also encoded by the R. sphaeroides puf operon, was present along with the hybrid reaction center. These cells emitted fluorescence, however, indicating an impairment in energy transduction.

Control of photosynthetic membrane assembly in Rhodobacter sphaeroides mediated by puhA and flanking sequences

A reaction center H- strain (RCH-) of Rhodobacter sphaeroides, PUHA1, was made by in vitro deletion of an XhoI restriction endonuclease fragment from the puhA gene coupled with insertion of a kanamycin resistance gene cartridge. The resulting construct was delivered to R. sphaeroides wild-type 2.4.1, with the defective puhA gene replacing the wild-type copy by recombination, followed by selection for kanamycin resistance. When grown under conditions known to induce intracytoplasmic membrane development, PUHA1 synthesized a pigmented intracytoplasmic membrane. Spectral analysis of this membrane showed that it was deficient in B875 spectral complexes as well as functional reaction centers and that the level of B800-850 spectral complexes was greater than in the wild type. The RCH- strain was photosythetically incompetent, but photosynthetic growth was restored by complementation with a 1.45-kilobase (kb) BamHI restriction endonuclease fragment containing the puhA gene carried in trans on plasmid pRK404. B875 spectral complexes were not restored by complementation with the 1.45-kb BamHI restriction endonuclease fragment containing the puhA gene but were restored along with photosynthetic competence by complementation with DNA from a cosmid carrying the puhA gene, as well as a flanking DNA sequence. Interestingly, B875 spectral complexes, but not photosynthetic competence, were restored to PUHA1 by introduction in trans of a 13-kb BamHI restriction endonuclease fragment carrying genes encoding the puf operon region of the DNA. The effect of the puhA deletion was further investigated by an examination of the levels of specific mRNA species derived from the puf and puc operons, as well as by determinations of the relative abundances of polypeptides associated with various spectral complexes by immunological methods. The roles of puhA and other genetic components in photosynthetic gene expression and membrane assembly are discussed.

Structural determination of the photosystem II core complex from spinach

A photosystem II core complex was purified with high yield from spinach by solubilization with beta-dodecylmaltoside. The complex consisted of polypeptides with molecular mass 47, 43, 34, 31, 9 and 4 kDa and some minor components, as detected by silver-staining of polyacrylamide gels. There was no indication for the chlorophyll-a/b-binding, light-harvesting complex polypeptides. The core complex revealed electron-transfer activity (1,5-diphenylcarbazide----2,6-dichloroindophenol) of about 30 mumol reduced 2,6-dichloroindophenol/mg chlorophyll/h. The structural integrity was analyzed by electron microscopy. The detergent-solubilized protein complex has the shape of a triangular disk with a maximum diameter of 13 nm and a maximum height of 6.8 nm. The shape of this core complex differs considerably from that of cyanobacterial photosystem II membrane fragments, which are elongated particles. The structural differences between both the complexes of higher plants and cyanobacteria are discussed with special emphasis on their association with the antenna apparatus in the photosynthetic membranes.

Primary photochemistry of reaction centers from the photosynthetic purple bacteria

Photosynthetic organisms transform the energy of sunlight into chemical potential in a specialized membrane-bound pigment-protein complex called the reaction center. Following light activation, the reaction center produces a charge-separated state consisting of an oxidized electron donor molecule and a reduced electron acceptor molecule. This primary photochemical process, which occurs via a series of rapid electron transfer steps, is complete within a nanosecond of photon absorption. Recent structural data on reaction centers of photosynthetic bacteria, combined with results from a large variety of photochemical measurements have expanded our understanding of how efficient charge separation occurs in the reaction center, and have changed many of the outstanding questions.

Photochemical electron transfer reactions in the acceptor complex of reaction centers of Rhodopseudomonas spheroides treated with sodium dodecyl sulfate

We have compared some photochemical properties of the reaction-center complex of Rhodopseudomonas spheroides (wild-type) treated with various amounts of either sodium dodecyl sulfate (SDS) or dodecyl dimethylamine N-oxide. In the presence of the latter, the native structure and activity of the reaction center are preserved even at high concentrations of detergent. In contrast, SDS denatures the protein. It does this by a cooperative process, as shown by the sigmoidal relationship between primary photochemical activity and SDS concentration. SDS binds to the reaction center at up to about 6 g SDS/g protein. The most notable modifications brought about by low SDS concentrations are: a slowing down of rereduction of P+ by Q-B from 1 s-1 to 0.25 s-1 and, to a lesser extent, of P+ by Q-A; the reoxidation of Q-A and Q-B by N,N,N',N'-tetramethyl-1,4-phenylenediamine is accelerated as an exponential function of SDS concentration. In this case Q-A become more accessible to external acceptors but not Q-B. It is thought that these changes are mainly due to progressive binding of SDS resulting in the unfolding of the protein, probably accompanied by the loss of the metal. Herbicides partly protect the reaction center against SDS denaturation, but their efficiency to block electron transfer between QA and QB is greatly reduced in the altered reaction centers present in the transition region of the denaturation curve. It is concluded that in the early region of the SDS denaturation curve, no dissociation of the reaction center in subunits occurs. In the transition region, it appears that the reaction center reaches the critical state in which mainly dissociation of polypeptide H from the complex of chains L and M and loss of QA take place.

Kinetic study of P(F) and Car (T) states in the LM subunit purified from the wild-type Rhodobacter sphaeroides reaction centers

The kinetics of absorbance changes related to the charge-separated state, P(F), and to the formation and decay of the carotenoid triplet state (Car(T)) were studied in the LM reaction center subunit isolated from a wild-type strain of the purple bacterium Rhodobacter sphaeroides (strain Y). The P(F) lifetime is lengthened (20±1.5 ns) in the LM complex as compared to the intact reaction centers (11±1 ns). The yield of the carotenoid triplet formation is higher (0.28±0.01) in the LM complex than in native reaction centers. We interpret our results in terms of perturbations of a first-order reaction connecting the singlet and the triplet state of the radical-pair state. Our results, together with those of a recent work (Agalidis, I., Nuijs, A.M. and Reiss-Husson, F. (1987) Biochim. Biophys. Acta (in press)) are consistent with a high I to QA electron transfer rate in this LM subunit, which is metal-depleted.The LM complex is considerably more sensitive than the reaction centers to photooxidative damage in the presence of oxygen. This is not readily accounted for simply by the higher carotenoid triplet yield, and may suggest a greater accessibility of the internal structures in the absence of the H-subunit.The lifetime of the carotenoid triplet decay (6.4±0.3 μs) in the LM subunit is unchanged compared to the native reaction centers.