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

Influence of Polar Mutations on the Electronic and Structural Properties of QA-· in Bacterial Reaction Centers

Reaction centers from Rhodobacter sphaeroides with residue M265 mutated from isoleucine to threonine, serine, and asparagine (M265IT, M265IS, and M265IN, respectively) in the Q_A^-· state are studied by high-resolution electron spin echo envelope modulation (ESEEM) and electron nuclear double resonance spectroscopy methods to investigate the structural characteristics of these mutants influencing the redox properties of the Q_A site. All three mutants decrease the redox midpoint potential (E_m) of Q_A by ∼0.1 V, yet the mechanism for this drop in E_m is unclear. In this work, we examine (i) the hydrogen bonding interactions between Q_A^-· and residues histidine M219 and alanine M260, (ii) the electron spin density distribution of the semiquinone, and (iii) the orientations of the ubiquinone methoxy substituents. ¹³C measurements show no significant contribution of methoxy dihedral angles to the observed decrease in E_m for the Q_A mutants. Instead, ¹⁴N three-pulse ESEEM data suggest that electrostatic or hydrogen bond formation between the mutated M265 side chain and His-M219 N_δ may be involved in the observed lowering of the Q_A midpoint potential. For mutant M265IN, analysis of the proton hyperfine couplings reveals a weakened hydrogen bond network, resulting in an altered Q_A^-· spin density distribution. The magnetic resonance study presented here is most consistent with an electrostatic or structural perturbation of the His-M219 N_δ hydrogen bond in these mutants as a mechanism for the ∼0.1 V decrease in Q_A E_m.

Affinity and activity of non-native quinones at the Q(B) site of bacterial photosynthetic reaction centers

Purple, photosynthetic reaction centers from Rhodobacter sphaeroides bacteria use ubiquinone (UQ10) as both primary (Q(A)) and secondary (Q(B)) electron acceptors. Many quinones reconstitute Q(A) function, while a few will act as Q(B). Nine quinones were tested for their ability to bind and reconstitute Q(A) and Q(B) functions. Only ubiquinone (UQ) reconstitutes both functions in the same protein. The affinities of the non-native quinones for the Q(B) site were determined by a competitive inhibition assay. The affinities of benzoquinones, naphthoquinone (NQ), and 2-methyl-NQ for the Q(B) site are 7 ± 3 times weaker than that at Q(A) site. However, di-ortho-substituted NQs and anthraquinone bind tightly to the Q(A) site (K d ≤ 200 nM), and ≥1,000 times more weakly to the Q(B) site, perhaps setting a limit on the size of the site. With a low-potential electron donor, 2-methyl, 3-dimethylamino-1,4-NQ, (Me-diMeAm-NQ) at Q(A), Q(B) reduction is 260 meV, more favorable than with UQ as Q(A). Electron transfer from Me-diMeAm-NQ at the Q(A) site to NQ at the Q(B) site can be detected. In the Q(B) site, the NQ semiquinone is estimated to be ≈60-100 meV higher in energy than the UQ semiquinone, while in the Q(A) site, the semiquinone energy level is similar or lower with NQ than with UQ. Thus, the NQ semiquinone is more stable in the Q(A) than in the Q(B) site. In contrast, the native UQ semiquinone is ≈60 meV lower in energy in the Q(B) than in the Q(A) site, stabilizing forward electron transfer from Q(A) to Q(B).

Purification, crystallization and preliminary X-ray crystallographic studies of FMN-bound and FMN-free forms of aromatic acid decarboxylase (CpsUbiX) from the psychrophilic bacterium Colwellia psychrerythraea 34H

The ubiX gene (UniProtKB code Q489U8) of Colwellia psychrerythraea strain 34H has been annotated as a putative flavin mononucleotide (FMN)-dependent aromatic acid decarboxylase. Based on previous studies of homologous proteins, CpsUbiX is thought to catalyze the decarboxylation of 3-octaprenyl-4-hydroxybenzoate to produce 2-polyprenylphenol in the ubiquinone-biosynthesis pathway using a noncovalently bound FMN molecule as a cofactor. However, the detailed mechanisms of this important enzyme are not yet clear and need to be further elucidated. In this study, it was found that the V47S single mutation resulted in a loss of FMN binding, resulting in the production of FMN-free CpsUbiX protein. This mutation is likely to destabilize FMN-protein interactions without affecting the overall structural folding. To fully characterize the conformational changes upon FMN binding and the enzymatic mechanism at the molecular level, the wild-type (FMN-bound) and V47S mutant (FMN-free) CpsUbiX proteins were purified and crystallized using the sitting-drop vapour-diffusion method. Furthermore, complete diffraction data sets for FMN-bound (space group C222(1)) and FMN-free (space group P23) forms were obtained to 2.0 and 1.76 Å resolution, respectively.

Conformational differences between the methoxy groups of QA and QB site ubisemiquinones in bacterial reaction centers: a key role for methoxy group orientation in modulating ubiquinone redox potential

Ubiquinone is an almost universal, membrane-associated redox mediator. Its ability to accept either one or two electrons allows it to function in critical roles in biological electron transport. The redox properties of ubiquinone in vivo are determined by its environment in the binding sites of proteins and by the dihedral angle of each methoxy group relative to the ring plane. This is an attribute unique to ubiquinone among natural quinones and could account for its widespread function with many different redox complexes. In this work, we use the photosynthetic reaction center as a model system for understanding the role of methoxy conformations in determining the redox potential of the ubiquinone/semiquinone couple. Despite the abundance of X-ray crystal structures for the reaction center, quinone site resolution has thus far been too low to provide a reliable measure of the methoxy dihedral angles of the primary and secondary quinones, QA and QB. We performed 2D ESEEM (HYSCORE) on isolated reaction centers with ubiquinones (13)C-labeled at the headgroup methyl and methoxy substituents, and have measured the (13)C isotropic and anisotropic components of the hyperfine tensors. Hyperfine couplings were compared to those derived by DFT calculations as a function of methoxy torsional angle allowing estimation of the methoxy dihedral angles for the semiquinones in the QA and QB sites. Based on this analysis, the orientation of the 2-methoxy groups are distinct in the two sites, with QB more out of plane by 20-25°. This corresponds to an ≈50 meV larger electron affinity for the QB quinone, indicating a substantial contribution to the experimental difference in redox potentials (60-75 mV) of the two quinones. The methods developed here can be readily extended to ubiquinone-binding sites in other protein complexes.

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].

Interaction between the intermediary electron acceptor (pheophytin) and a possible plastoquinone-iron complex in photosystem II reaction centers

Photoreduction of the intermediary electron acceptor, pheophytin (Pheo), in photosystem II reaction centers of spinach chloroplasts or subchloroplast particles (TSF-II and TSF-IIa) at 220 K and redox potential E(h) = -450 mV produces an EPR doublet centered at g = 2.00 with a splitting of 52 G at 7 K in addition to a narrow signal attributed to Pheo([unk]) (g = 2.0033, DeltaH approximately 13 G). The doublet is eliminated after extraction of lyophilized TSF-II with hexane containing 0.13-0.16% methanol but is restored by reconstitution with plastoquinone A (alone or with beta-carotene) although not with vitamin K(1). TSF-II and TSF-IIa are found to contain approximately 2 nonheme Fe atoms per reaction center. Incubation with 0.55 M LiClO(4) plus 2.5 mM o-phenanthroline (but not with 0.55 M LiClO(4) alone) decreases this value to approximately 0.6 and completely eliminates the EPR doublet, but photoreduction of Pheo is not significantly affected. Partial restoration of the doublet (about 25%) was achieved by subsequent incubation with 0.2 mM Fe(2+), but not with either Mn(2+) or Mg(2+). The Fe removal results in the development of a photoinduced EPR signal (g = 2.0044 +/- 0.0003, DeltaH = 9.2 +/- 0.5 G) at E(h) = 50 mV, which is not observed after extraction with 0.16% methanol in hexane. It is ascribed to plastosemiquinone no longer coupled to Fe in photosystem II reaction centers. The results show that a complex of plastoquinone and Fe can act as the stable "primary" electron acceptor in photosystem II reaction centers and that the interaction of its singly reduced form with the reduced intermediary acceptor, Pheo([unk]), is responsible for the EPR doublet.

Energies and kinetics of radical pairs involving bacteriochlorophyll and bacteriopheophytin in bacterial reaction centers

Absorbance changes reflecting the formation of a transient radical-pair state, P(F), were measured in reaction centers from Rhodopseudomonas sphaeroides under conditions that blocked electron transfer to a later carrier (a quinone, Q). The temperature dependence of the absorbance changes suggests that P(F) is an equilibrium mixture of two states, which appear to be mainly (1)[P([unk])B([unk])] and (1)[P([unk])H([unk])]. P is a bacteriochlorophyll dimer, B is a bacteriochlorophyll absorbing at 800 nm, and H is a bacteriopheophytin. In the presence of Q([unk]), the energy of (1)[P([unk])B([unk])] is about 0.025 eV above that of (1)[P([unk])H([unk])], (1)[P([unk])H([unk])] can decay to a triplet state, P(R), which also is an equilibrium mixture of two states, separated by about 0.03 eV. The lower of these appears to be mainly a locally excited triplet state of P, (3)P; the upper state contains a major contribution from a triplet charge-transfer state, (3)[P([unk])B([unk])]. The temperature dependence of delayed fluorescence from P(R) indicates that (3)P lies 0.40 eV below the excited singlet state, P(*), which is about 0.05 eV above (1)[P([unk])H([unk])]. The (1,3)[P([unk])B([unk])] charge-transfer states thus appear to interact with the locally excited states of P and B to give singlet and triplet states that are separated in energy by about 0.35 eV. This is 10(6) times larger than the splitting between (1)[P([unk])H([unk])] and (3)[P([unk])H([unk])] and implies strong orbital overlap between P([unk]) and B([unk]). This is consistent with recent picosecond studies which suggest that electron transfer from P(*) to B occurs within 1 ps and is followed in 4 to 10 ps by electron transfer from B([unk]) to H.

Formation and geminate quenching of singlet oxygen in purple bacterial reaction center

The phosphorescence of singlet oxygen ((1)X( *)) photosensitized by the carotenoidless reaction center (RC) of Rhodobacter sphaeroides R26.1 has been investigated, using H(2)O and D(2)O as the suspending media. To enhance (under neutral conditions) the triplet quantum yield of the special pair P(870) (P) by the radical pair mechanism, the quinone acceptor Q(A) was removed by means of a chemical treatment. The phosphorescence signal fits the functional form P(0)[exp (-t/tau)-exp(-t/zeta)], regardless of whether (1)X( *) is sensitized by P(dagger) or M(dagger) (where the dagger denotes triplet excitation and M is a water-soluble molecule). The time constant zeta was identified with the decay time of (1)X( *); when P(dagger) is the sensitizer, one finds zeta(P)((1))=3.3+/-0.3 micros, and zeta(P)((2))=34+/-3 micros, where the superscripts 1 and 2 refer to H(2)O and D(2)O, respectively; the corresponding values for sensitization by M(dagger) (in the absence of RC) are zeta(M)((1))=3.7+/-0.4 micros, and zeta(M)((2))=75+/-5 micros. The addition of RC's to the solution of M in D(2)O reveals that the RC is a quencher of (1)X( *); however, for equal concentrations of the RC, zeta(P)((2))<zeta(M)((2)), showing that (1)X( *) is deactivated, after its entry into the suspending medium, mainly by the solvent or the same RC which acts as the sensitizer. The values of tau(P) are similar in both solvents, ca. 2 micros, but this time constant does not figure in the disappearance of P(dagger), which follows a bi-exponential course, alpha(1)exp(-t/tau(1))+alpha(2)exp(-t/tau(2)). The time constants tau(1) and tau(2) (72+/-5 micros and 12+/-1 micros, respectively) as well as the factor alpha(2) are insensitive to the oxygen content, and quenching of P(dagger) is manifested only through a threefold reduction in the magnitude of alpha(1); these data imply the absence of dynamic quenching and heterogeneity of the RC. The mean lifetime of (1)X( *) inside the protein matrix is identified with tau(P), and the absence of a prompt component in the phosphorescence signal rationalized by proposing that the radiative decay of (1)X( *) within the RC is much slower than that in an aqueous environment.

Investigation on the detergent role in the function of secondary quinone in bacterial reaction centers

In this paper are reported studies on the detergent role in isolated reaction centers (RC) from Rhodobacter sphaeroides, over a large range of lauryldimethylamino-N-oxide (LDAO) concentrations, in influencing the thermodynamics of the quinone exchange reaction as well as the protein aggregation. The occurrence of the quinone exchange reaction between the QB-binding site (where QB is the second quinone molecule of two in the RC) and the ubiquinone 0 dissolved in the different environments (water, LDAO micelles and detergent phase of the protein-detergent complex) has also been analyzed. Measurements carried out in QB-depleted RC to which exogenous quinone has been added show that the relative amplitudes of the slow and fast phase of the recombination reaction depend on this parameter. The overall amount of the restored QB-functionality is affected by the concentration of the LDAO in solution. Interpolation of the titration curves with a quadratic function obtained by simple considerations allowed the binding constant of UQ0 to the QB-binding site to be calculated. From the fitting procedure, the distribution of the quinone in the different environments present in solution was evaluated, indicating that the exchange reaction can take place only between the QB-site and the detergent phase. The dependence of the quinone pool size upon the volume of the phase in which the interacting quinone is solubilized is also discussed. The increasing difficulty in saturating the QB-pocket above the LDAO critical micellar concentration is finally related to the association of protein-detergent complexes to form large protein clusters.

Kinetics and free energy gaps of electron-transfer reactions in Rhodobacter sphaeroides reaction centers

The rates of the light-driven, electron-transfer reactions in the photosynthetic reaction center (RC) of Rhodobacter sphaeroides are examined in mutant strains in which tyrosine (M)210 is replaced by phenylalanine, isoleucine, or tryptophan. The spectra of the absorbance changes between 700 and 975 nm, following excitation by 0.6-ps pulses at 605 nm, are analyzed globally by singular value decomposition. The spectra measured at room temperature are interpreted in terms of a model in which the excited bacteriochlorophyll dimer (P*) transfers an electron to a bacteriopheophytin (HL) with time constants of 3.5 +/- 0.3, 10.5 +/- 1.0, 16 +/- 2, and 41 +/- 4 ps in wild-type RCs and the Phe, Ile, and Trp mutants, respectively, and an electron then moves from HL- to a quinone (QA) with a time constant of 0.16 ns in wild-type RCs, 0.24 ns in the Phe mutant, and 0.20 ns in the Ile and Trp mutants. The first step speeds up with decreasing temperature in wild-type RCs, remains virtually unchanged in the Phe mutant, and slows down in the Ile and Trp mutants. At 80 K, the signals in the 850-975-nm region include an apparent shift of the stimulated emission or absorption spectrum of P*, with a time constant of 5 ps in the Ile mutant and 13 pcs in the Trp mutant. Most of the electron transfer to HL occurs with time constants of 55 and 155 ps in the Ile and Trp mutants, respectively, and probably occurs from the relaxed form of P*. Electron transfer from the initial state cannot be ruled out, however. Relaxations of P* are not resolved in wild-type RCs or the Phe mutant. The midpoint potential (Em) of the P/P+ redox couple is measured by an electrochemical technique; the Em values are 500 +/- 5, 530 +/- 6, 533 +/- 3, and 552 +/- 10 mV for the wild-type and the Phe, Ile, and Trp mutant RCs, respectively. These values are corroborated by chemical titrations. The free energy change (delta G degrees) associated with formation of the P+HL-radical pair from P* also is determined by measuring the amplitude of fluorescence on the nanosecond time scale after blocking electron transfer from HL- to QA. The free energy of P+HL- is elevated by an amount comparable to that calculated from the increase in the Em of P in the Ile mutant and by about 16 meV more than this in the Phe and Trp mutants.(ABSTRACT TRUNCATED AT 400 WORDS)

Temperature dependence of charge recombination from the P+QA- and P+QB- states in photosynthetic reaction centers isolated from the thermophilic bacterium Chloroflexus aurantiacus

The temperature dependence of charge recombination from the P+QA- and from the P+QB- states produced by a flash was studied in reaction centers isolated from the photosynthetic thermophilic bacterium Chloroflexus aurantiacus. P designates the primary electron donor; QA and QB the primary and secondary quinone electron acceptors respectively. In QB-depleted reaction centers the rate constant (kAP) for P+QA- recombination was temperature independent between 0-50 degrees C (17.6 +/- 0.7 s-1 at pH 8 and pH 10). The same value was obtained in intact membranes in the presence of o-phenanthroline. Upon lowering the temperature from 250 K to 160 K, kAP increased by a factor of two and remained constant down to 80 K. The overall temperature dependence of kAP was consistent with an activationless process. Ubiquinone (UQ-3) and different types of menaquinone were used for QB reconstitution. In UQ-3 reconstituted reaction centers charge recombination was monoexponential (rate constant k = 0.18 +/- 0.03 s-1) and temperature independent between 5-40 degrees C. In contrast, in menaquinone-3- and menaquinone-4-reconstituted reaction centers P+ rereduction following a flash was markedly biphasic and temperature dependent. In menaquinone-6-reconstituted reaction centers a minor contribution from a third kinetic phase corresponding to P+QA- charge recombination was detected. Analysis of these kinetics and of the effects of the inhibitor o-phenanthroline at high temperature suggest that in detergent suspensions of menaquinone-reconstituted reaction centers a redox reaction removing electrons from the quinone acceptor complex competes with charge recombination. Instability of the semiquinone anions is more pronounced when QB is a short-chain menaquinone. From the temperature dependence of P+ decay the activation parameters for the P+QB- recombination and for the competing side oxidation of the reduced menaquinone acceptor have been derived. For both reactions the activation enthalpies and entropies change markedly with menaquinone chain length but counterbalance each other, resulting in activation free energies at ambient temperature independent of the menaquinone tail. When reaction centers are incorporated into phospholipid vesicles containing menaquinone-8 a temperature-dependent, monophasic, o-phenanthroline-sensitive recombination from the P+QB- state is observed, which is consistent with the formation of stable semiquinone anions. This result seems to indicate a proper QB functioning in the two-subunit reaction center isolated from Chlorflexus aurantiacus when the complex is inserted into a lipid bilayer.

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.

Kinetics of photosynthetic electron transfer in artificial vesicles reconstituted with purified complexes from Rhodobacter capsulatus. I. The interaction of cytochrome c2 with the reaction center

1. The kinetics of the interaction of cytochrome c2 and photosynthetic reaction centers purified from Rhodobacter capsulatus were studied in proteoliposomes reconstituted with a mixture of phospholipids simulating the native membrane (i.e. containing 25% L-alpha-phosphatidylglycerol). 2. At low ionic strength, the kinetics of cytochrome-c2 oxidation induced by a single turnover flash was very different, depending on the concentration of cytochrome c2: at concentrations lower than 1 microM, the process was strictly bimolecular (second-order rate constant, k = 1.7 x 10(9) M-1 s-1), while at higher concentrations a fast oxidation process (half-time lower than 20 microseconds) became increasingly dominant and encompassed the total process at a cytochrome c2 concentration around 10 microM. From the concentration dependence of the amplitude of this fast phase an association constant for a reaction-center--cytochrome-c2 complex of about 10(5) M-1 was evaluated. From the fraction of photo-oxidized reaction centers promptly re-reduced in the presence of saturating concentrations of externally added cytochrome c2, it was found that in approximately 60% of the centers the cytochrome-c2 site was exposed to the external compartment. 3. Both the second-order oxidation reaction and the formation of the reaction-center--cytochrome-c2 complex were very sensitive to ionic strength. In the presence of 180 mM KCl, the value of the second-order rate constant was decreased to 7.0 x 10(7) M-1 s-1 and no fast oxidation of cytochrome c2 could be observed at 10 microM cytochrome c2. 4. The kinetics of exchange of oxidized cytochrome c2 bound to the reaction center with the reduced form of the same carrier, following a single turnover flash, was studied in double-flash experiments, varying the dark time between photoactivations over the range 30 microseconds to 5ms. The experimental results were analyzed according to aminimal kinetic model relating the amounts of oxidized cytochrome c2 and reaction centers observable after the second flash to the dark time between flashes. This model included the rate constants for the electron transfer between the primary and secondary ubiquinone acceptors of the complex (k1) and for the exchange of cytochrome c2 (k2). Fitting to the experimental results indicated a value of k1 equal to 2.4 x 10(3) s-1 and a lower limit for k2 of approximately 2 x 10(4) s-1 (corresponding to a second-order rate constant of approximately 3 x 10(9) M-1 s-1).

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.

Transient optical spectroscopy of single crystals of the reaction center from Rhodobacter sphaeroides wild-type 2.4.1

The photoactivity of the crystallized reaction centers from Rhodobacter sphaeroides wild-type strain 2.4.1 has been examined by light-induced absorption spectral changes associated with charge separation and triplet state formation in the reaction center. Upon excitation of a crystal at ambient redox potential, the primary donor 865 nm band bleaches reversibly. The kinetics of its recovery were found to be biphasic with rate constants 11.5 +/- 1.3 s-1 and 0.9 +/- 0.4 s-1 which correspond to lifetimes of 87.0 +/- 9.0 ms and 1.0 +/- 0.7 s, respectively. The ratio of the fast-to-slow component preexponential terms was 3.5 +/- 1.1 suggesting that the majority (78.9 +/- 13.0%) of the reaction centers in the crystals lack the secondary quinone, QB. The addition of sodium ascorbate to the crystals attenuates the 865 nm absorption change, and gives rise to strong carotenoid triplet-triplet absorption changes at 547 nm. These data indicate that the reaction center-bound carotenoid in the crystals is capable of accepting triplet energy from the primary donor triplet.

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.

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.

Structure of the reaction center from Rhodobacter sphaeroides R-26: the cofactors

The three-dimensional structure of the cofactors of the reaction center of Rhodobacter sphaeroides R-26 has been determined by x-ray diffraction and refined at a resolution of 2.8 A with an R value of 26%. The main features of the structure are similar to the ones determined for Rhodopseudomonas viridis [Michel, H., Epp, O. & Deisenhofer, J. (1986) EMBO J. 5, 2445-2451]. The cofactors are arranged along two branches, which are approximately related to each other by a 2-fold symmetry axis. The structure is well suited to produce light-induced charge separation across the membrane. Most of the structural features predicted from physical and biochemical measurements are confirmed by the x-ray structure.

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.

Demonstration of a collisional interaction of ubiquinol with the ubiquinol-cytochrome c2 oxidoreductase complex in chromatophores from Rhodobacter sphaeroides

Ubiquinone-10 can be extracted from lyophilized chromatophores of Rhodobacter sphaeroides (previously called Rhodopseudomonas sphaeroides) without significant losses in other components of the electron-transfer chain or irreversible damages in the membrane structure. The pool of ubiquinone can be restored with exogenous UQ-10 to sizes larger than the ones in unextracted membranes. The decrease in the pool size has marked effects on the kinetics of reduction of cytochrome b-561 induced by a single flash of light and measured in the presence of antimycin. The initial rate of reduction, which in unextracted preparations increases on reduction of the suspension over the Eh range between 170 and 100 mV (pH 7), is also stimulated in partially UQ-depleted membranes, although at more negative Eh's. When the UQ pool is completely extracted the rate of cytochrome (Cyt) b-561 reduction is low and unaffected by the redox potential. In membranes enriched in UQ-10 above the physiological level the titration curve of the rate of Cyt b-561 reduction is displaced to Eh values more positive than in controls. This effect is saturated when the size of the UQ pool is about 2-3 times larger than the native one. The reduction of Cyt b-561 always occurs a short time after the flash is fired; also the duration of this lag is dependent on Eh and on the size of the UQ pool. A decrease or an increase in the pool size causes a displacement of the titration curve of the lag to more negative or to more positive Eh's, respectively. Similarly, the lag becomes Eh independent and markedly longer than in controls when the pool is completely extracted. These results demonstrate that the rate of turnover of the ubiquinol oxidizing site in the b-c1 complex depends on the actual concentration of ubiquinol present in the membrane and that ubiquinol from the pool is oxidized at this site with a collisional mechanism. Kinetic analysis of the data indicates that this reaction obeys a Michaelis-Menten type equation, with a Km of 3-5 ubiquinol molecules per reaction center.

Vibronic coupling to electron transfer and the structure of the R. Viridis reaction center

This paper points out that the orientations of the porphyrins, bacteriochlorophyll and bacteriopheophytin, in the reaction centers of Rhodopseudomonas viridis, as shown by the new X-ray determined structure, have a peculiar orientation towards each other: electron donors are broadside toward the acceptors and acceptors are edgeon toward donors. Vibronic coupling which is the mechanism of converting free-energy loss in electron transport to vibrational energy is examined as a possible explanation. Preliminary calculations do not support this as an explanation of the orientations but suggest strongly that the non-heme iron atom has the function of promoting vibronic coupling in the electron transfer from bacteriopheophytin to menaquinone. It is further suggested that the system of electron transport from the special pair of bacteriochlorophyll to the bacteriopheophytin is arranged to keep virbonic coupling to a minimum to match the very small electronic free-energy loss in this region.

The acceptor quinone complex of Rhodopseudomonas viridis reaction centers

The acceptor complex of isolated reaction centers from Rhodopseudomonas viridis contains both menaquinone and ubiquinone. In a series of flashes the ubiquinone was observed to undergo binary oscillations in the formation and disappearance of a semiquinone, indicative of secondary acceptor (QB) activity. The oscillating signal, Q-B, was typical of a ubisemiquinone anion with a peak at 450 nm (delta epsilon = 6 mM-1 X cm-1) and a shoulder at 430 nm. Weak electrochromic bandshifts in the infrared were also evident. The spectrum of the reduced primary acceptor (Q-A) exhibited a major peak at 412 nm (delta epsilon = 10 mM-1 X cm-1) consistent with the assignment of menaquinone as QA. The Q-A spectrum also had minor peaks at 385 and 455 nm in the blue region. The same spectrum was recorded after quantitative removal of the secondary acceptor, when only menaquinone was present in the reaction centers. Spectral features in the near-infrared due to Q-A were attributed to electrochromic effects on bacteriochlorophyll (BChl) b and bacteriopheophytin (BPh) b pigments resulting in a distinctive split peak at 810 and 830 nm (delta epsilon = 8 mM-1 X cm-1). The menaquinone was identified as 2-methyl-3-nonylisoprenyl-1,4-naphthoquinone (menaquinone-9). The native QA activity was uniquely provided by this menaquinone and ubiquinone was not involved. QB activity, on the other hand, displayed at least a 40-fold preference for ubiquinone (Q-10) as compared to menaquinone. Thus, both quinone-binding sites display remarkable specificity for their respective quinones. In the absence of donors to P+, charge recombination of the P+Q-A and P+Q-B pairs had half-times of 1.1 +/- 0.2 and 110 +/- 20 ms, respectively, at pH 9.0, indicating an electron-transfer equilibrium constant (Kapp2) of at least 100 for Q-AQB in equilibrium QAQ-B. Also observed was a slow recombination of the cytochrome c-558+ Q-A pair, with t 1/2 = 2 +/- 0.5 s at pH 6.

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: (Formula: see text). We found that the charge recombination pathway of D+QAQ(-)B proceeds via the intermediate state D+Q(-)AQB, 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 kAD-times the fraction of reaction centers in the Q-AQB state) with the observed recombination rate, kobsD+----D. The kinetic measurements were used to obtain the pH dependence (6.1 smaller than or equal to pH smaller than or equal to 11.7) of the free energy difference between the states Q(-)AQB and QAQ(-)B. At low pH (less than 9) QAQ(-)B is stabilized relative to Q(-)AQB by 67 meV, whereas at high pH Q(-)AQB 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.

On the role of Fe2+ in bacterial photosynthesis. The effect of biosynthetic substitution of Fe2+ by Mn2+ on the electron transfer step Q-1Q2----Q1Q-2 in reaction centers

A test of the 'iron-wire' hypothesis for the role of Fe2+ in promoting the electron transfer between the primary (Q1) and secondary (Q2) quinones in bacterial reaction centers of Rhodopseudomonas sphaeroides strain R-26.1 has been conducted. Kinetics of this step, P+Q-1Q2----P+Q1Q-2, and of recombination with the oxidized donor, P+Q-1----PQ1 and P+Q-2----PQ2, were followed optically at 4 degrees C in normal iron-containing reaction centers and in reaction centers having 58% Mn2+, replacing Fe2+. This significant replacement is accomplished biosynthetically by control of the growth conditions, and so should preserve the native interactions between the cofactors. There are no significant differences observed in the recombination kinetics of the two types of reaction centers. The electron transfer between the quinones was observed to show apparent biphasic kinetics with major components of approx. 170 microseconds and 1.5 ms at 4 degrees C and pH = 7.5. There is no statistically significant difference observed between the two types of reaction centers. This major change in the electronic structure of the metal and the unaltered kinetics discount the likelihood of any direct orbital participation of the metal in the electron transfer between the quinones.

Photoelectric currents across planar bilayer membranes containing bacterial reaction centers. Response under conditions of single electron turnover

Light-induced electric current and potential responses have been measured across planar phospholipid membranes containing reaction centers from the photosynthetic bacterium Rhodopseudomonas sphaeroides. Under conditions in which the reaction centers are restricted to a single electron turnover, the responses can be correlated with the light-induced electron transfer reactions associated with the reaction center. The results indicate that electron transfer from the bacteriochlorophyll dimer to the primary ubiquinone molecule, and from ferrocytochrome c to the oxidized dimer occur in series across the planar membrane. Electron transfer from the primary to secondary ubiquinone molecule is not electrogenic.