Electron transfer in reaction centers of Rhodopseudomonas sphaeroides. II. Free energy and kinetic relations between the acceptor states QA−QB− and QAQ2−B

Mn-porphyrins in a four-helix bundle participate in photo-induced electron transfer with a bacterial reaction center

Hybrid complexes incorporating synthetic Mn-porphyrins into an artificial four-helix bundle domain of bacterial reaction centers created a system to investigate new electron transfer pathways. The reactions were initiated by illumination of the bacterial reaction centers, whose primary photochemistry involves electron transfer from the bacteriochlorophyll dimer through a series of electron acceptors to the quinone electron acceptors. Porphyrins with diphenyl, dimesityl, or fluorinated substituents were synthesized containing either Mn or Zn. Electrochemical measurements revealed potentials for Mn(III)/Mn(II) transitions that are ~ 0.4 V higher for the fluorinated Mn-porphyrins than the diphenyl and dimesityl Mn-porphyrins. The synthetic porphyrins were introduced into the proteins by binding to a four-helix bundle domain that was genetically fused to the reaction center. Light excitation of the bacteriochlorophyll dimer of the reaction center resulted in new derivative signals, in the 400 to 450 nm region of light-minus-dark spectra, that are consistent with oxidation of the fluorinated Mn(II) porphyrins and reduction of the diphenyl and dimesityl Mn(III) porphyrins. These features recovered in the dark and were not observed in the Zn(II) porphyrins. The amplitudes of the signals were dependent upon the oxidation/reduction midpoint potentials of the bacteriochlorophyll dimer. These results are interpreted as photo-induced charge-separation processes resulting in redox changes of the Mn-porphyrins, demonstrating the utility of the hybrid artificial reaction center system to establish design guidelines for novel electron transfer reactions.

Supramolecular Biohybrid Construct for Photoconversion Based on a Bacterial Reaction Center Covalently Bound to Cytochrome c by an Organic Light Harvesting Bridge

A supramolecular construct for solar energy conversion is developed by covalently bridging the reaction center (RC) from the photosynthetic bacterium Rhodobacter sphaeroides and cytochrome c (Cyt c) proteins with a tailored organic light harvesting antenna (hCy2). The RC-hCy2-Cyt c biohybrid mimics the working mechanism of biological assemblies located in the bacterial cell membrane to convert sunlight into metabolic energy. hCy2 collects visible light and transfers energy to the RC, increasing the rate of photocycle between a RC and Cyt c that are linked in such a way that enhances proximity without preventing protein mobility. The biohybrid obtained with average 1 RC/10 hCy2/1.5 Cyt c molar ratio features an almost doubled photoactivity versus the pristine RC upon illumination at 660 nm, and ∼10 times higher photocurrent versus an equimolar mixture of the unbound proteins. Our results represent an interesting insight into photoenzyme chemical manipulation, opening the way to new eco-sustainable systems for biophotovoltaics.

Protonated rhodosemiquinone at the Q(B) binding site of the M265IT mutant reaction center of photosynthetic bacterium Rhodobacter sphaeroides

The second electron transfer from primary ubiquinone Q(A) to secondary ubiquinone Q(B) in the reaction center (RC) from Rhodobacter sphaeroides involves a protonated Q(B)(-) intermediate state whose low pK(a) makes direct observation impossible. Here, we replaced the native ubiquinone with low-potential rhodoquinone at the Q(B) binding site of the M265IT mutant RC. Because the in situ midpoint redox potential of Q(A) of this mutant was lowered approximately the same extent (≈100 mV) as that of Q(B) upon exchange of ubiquinone with low-potential rhodoquinone, the inter-quinone (Q(A) → Q(B)) electron transfer became energetically favorable. After subsequent saturating flash excitations, a period of two damped oscillations of the protonated rhodosemiquinone was observed. The Q(B)H(•) was identified by (1) the characteristic band at 420 nm of the absorption spectrum after the second flash and (2) weaker damping of the oscillation at 420 nm (due to the neutral form) than at 460 nm (attributed to the anionic form). The appearance of the neutral semiquinone was restricted to the acidic pH range, indicating a functional pK(a) of <5.5, slightly higher than that of the native ubisemiquinone (pK(a) < 4.5) at pH 7. The analysis of the pH and temperature dependencies of the rates of the second electron transfer supports the concept of the pH-dependent pK(a) of the semiquinone at the Q(B) binding site. The local electrostatic potential is severely modified by the strongly interacting neighboring acidic cluster, and the pK(a) of the semiquinone is in the middle of the pH range of the complex titration. The kinetic and thermodynamic data are discussed according to the proton-activated electron transfer mechanism combined with the pH-dependent functional pK(a) of the semiquinone at the Q(B) site of the RC.

Molecular mechanisms for generating transmembrane proton gradients

Membrane proteins use the energy of light or high energy substrates to build a transmembrane proton gradient through a series of reactions leading to proton release into the lower pH compartment (P-side) and proton uptake from the higher pH compartment (N-side). This review considers how the proton affinity of the substrates, cofactors and amino acids are modified in four proteins to drive proton transfers. Bacterial reaction centers (RCs) and photosystem II (PSII) carry out redox chemistry with the species to be oxidized on the P-side while reduction occurs on the N-side of the membrane. Terminal redox cofactors are used which have pKas that are strongly dependent on their redox state, so that protons are lost on oxidation and gained on reduction. Bacteriorhodopsin is a true proton pump. Light activation triggers trans to cis isomerization of a bound retinal. Strong electrostatic interactions within clusters of amino acids are modified by the conformational changes initiated by retinal motion leading to changes in proton affinity, driving transmembrane proton transfer. Cytochrome c oxidase (CcO) catalyzes the reduction of O2 to water. The protons needed for chemistry are bound from the N-side. The reduction chemistry also drives proton pumping from N- to P-side. Overall, in CcO the uptake of 4 electrons to reduce O2 transports 8 charges across the membrane, with each reduction fully coupled to removal of two protons from the N-side, the delivery of one for chemistry and transport of the other to the P-side.

Proton release upon oxidation of tyrosine in reaction centers from Rhodobacter sphaeroides

Markedly different light-induced protonational changes were measured in two reaction center mutants of Rhodobacter sphaeroides. A quadruple mutant containing alterations, at residues L131, M160, M197, and M210, that elevate the midpoint potential of the bacteriochlorophyll dimer was compared to the Y(M) mutant, which contains these alterations plus a tyrosine at M164 serving as a secondary electron donor [Kálmán et al., Nature 402 (1999) 696]. In the quadruple mutant, a proton uptake of 0.1-0.3 H(+)/reaction center between pH 6 and 10 resulted from formation of the oxidized bacteriochlorophyll donor and reduced primary quinone. In the Y(M) mutant, a maximal proton release of -0.5 H(+)/reaction center at pH 8 was attributed to formation of the tyrosyl radical and modeled using electrostatic and direct proton-releasing mechanisms.

Identification of the proton pathway in bacterial reaction centers: both protons associated with reduction of QB to QBH2 share a common entry point

The reaction center from Rhodobacter sphaeroides uses light energy for the reduction and protonation of a quinone molecule, Q(B). This process involves the transfer of two protons from the aqueous solution to the protein-bound Q(B) molecule. The second proton, H(+)(2), is supplied to Q(B) by Glu-L212, an internal residue protonated in response to formation of Q(A)(-) and Q(B)(-). In this work, the pathway for H(+)(2) to Glu-L212 was studied by measuring the effects of divalent metal ion binding on the protonation of Glu-L212, which was assayed by two types of processes. One was proton uptake from solution after the one-electron reduction of Q(A) (DQ(A)-->D(+)Q(A)(-)) and Q(B) (DQ(B)-->D(+)Q(B)(-)), studied by using pH-sensitive dyes. The other was the electron transfer k(AB)((1)) (Q(A)(-)Q(B)-->Q(A)Q(B)(-)). At pH 8.5, binding of Zn(2+), Cd(2+), or Ni(2+) reduced the rates of proton uptake upon Q(A)(-) and Q(B)(-) formation as well as k(AB)((1)) by approximately an order of magnitude, resulting in similar final values, indicating that there is a common rate-limiting step. Because D(+)Q(A)(-) is formed 10(5)-fold faster than the induced proton uptake, the observed rate decrease must be caused by an inhibition of the proton transfer. The Glu-L212-->Gln mutant reaction centers displayed greatly reduced amplitudes of proton uptake and exhibited no changes in rates of proton uptake or electron transfer upon Zn(2+) binding. Therefore, metal binding specifically decreased the rate of proton transfer to Glu-L212, because the observed rates were decreased only when proton uptake by Glu-L212 was required. The entry point for the second proton H(+)(2) was thus identified to be the same as for the first proton H(+)(1), close to the metal binding region Asp-H124, His-H126, and His-H128.

A plausible mechanism of electron transfer between quinones in photosynthetic reaction centers

The mechanism of long-range electron transfer between the primary and the secondary quinone of photosynthetic reaction centers has been investigated, with particular attention on the role of the iron-histidine bridge. Computations suggest that in such a system, where the molecular subunits are packed together by H-bonds, a mobile electron, injected on one end of the chain, can be carried to the other end by switching the positions of the H-bonded hydrogens. Energy estimates would suggest that the proposed mechanism is plausible and worthy of further experimental investigations.

Cumulant analysis of charge recombination kinetics in bacterial reaction centers reconstituted into lipid vesicles

The kinetics of charge recombination between the primary photoxidized donor (P(+)) and the secondary reduced quinone acceptor (Q(B)(-)) have been studied in reaction centers (RCs) from the purple photosynthetic bacterium Rhodobacter sphaeroides incorporated into lecithin vesicles containing large ubiquinone pools over the temperature range 275 K </= T </= 307 K. To account for the non-exponential kinetics of P(+) re-reduction observed following a flash, a new approach has been developed, based on the following assumptions: 1) the exchange of quinone between different vesicles is negligible; 2) the exchange of quinone between the Q(B) site of the RC and the quinone pool within each single vesicle is faster than the return of the electron from the primary reduced acceptor Q(A)(-) to P(+); 3) the size polydispersity of proteoliposomes and the distribution of quinone molecules among them result in a quinone concentration distribution function, P(Q). The first and second moments of P(Q) have been evaluated from the size distribution of proteoliposomes probed by quasi-elastic light scattering (mean radius, <R> = (50 +/- 15) nm). Following these premises, we describe the kinetics of P(+)Q(B)(-) recombination with a truncated cumulant expansion and relate it to P(Q) and to the free energy changes for Q(A)(-)Q(B) --> Q(A)Q(B)(-) electron transfer (DeltaG(AB)(o)) and for quinone binding (DeltaG(bind)(o)) at Q(B). The model accounts well for the temperature and quinone dependence of the charge recombination kinetics, yielding DeltaG(AB)(o) = -7.67 +/- 0.05 kJ mol(-1) and DeltaG(bind)(o) = -14.6 +/- 0.6 kJ mol(-1) at 298 K.

Identification of the proton pathway in bacterial reaction centers: replacement of Asp-M17 and Asp-L210 with asn reduces the proton transfer rate in the presence of Cd2+

The reaction center (RC) from Rhodobacter sphaeroides converts light into chemical energy through the reduction and protonation of a bound quinone molecule Q(B) (the secondary quinone electron acceptor). We investigated the proton transfer pathway by measuring the proton-coupled electron transfer, k(AB)((2)) [Q(A)Q(B) + H(+) --> Q(A)(Q(B)H)(-)] in native and mutant RCs in the absence and presence of Cd(2+). Previous work has shown that the binding of Cd(2+) decreases k(AB)((2)) in native RCs approximately 100-fold. The preceding paper shows that bound Cd(2+) binds to Asp-H124, His-H126, and His-H128. This region represents the entry point for protons. In this work we investigated the proton transfer pathway connecting the entry point with Q(B) by searching for mutations that greatly affect k(AB)((2)) ( greater, similar10-fold) in the presence of Cd(2+), where k(AB)((2)) is limited by the proton transfer rate (k(H)). Upon mutation of Asp-L210 or Asp-M17 to Asn, k(H) decreased from approximately 60 s(-1) to approximately 7 s(-1), which shows the important role that Asp-L210 and Asp-M17 play in the proton transfer chain. By comparing the rate of proton transfer in the mutants (k(H) approximately 7 s(-1)) with that in native RCs in the absence of Cd(2+) (k(H) >/= 10(4) s(-1)), we conclude that alternate proton transfer pathways, which have been postulated, are at least 10(3)-fold less effective.

Identification of the proton pathway in bacterial reaction centers: inhibition of proton transfer by binding of Zn2+ or Cd2+

The reaction center (RC) from Rhodobacter sphaeroides converts light into chemical energy through the light induced two-electron, two-proton reduction of a bound quinone molecule QB (the secondary quinone acceptor). A unique pathway for proton transfer to the QB site had so far not been determined. To study the molecular basis for proton transfer, we investigated the effects of exogenous metal ion binding on the kinetics of the proton-assisted electron transfer kAB(2) (QA-*QB-* + H+ --> QA(QBH)-, where QA is the primary quinone acceptor). Zn2+ and Cd2+ bound stoichiometrically to the RC (KD /= 10(2)-fold) and has become the rate-limiting step. The lack of an effect of the metal binding on the charge recombination reaction D+*QAQB-* --> DQAQB suggests that the binding site is located far (>10 A) from QB. This hypothesis is confirmed by preliminary x-ray structure analysis. The large change in the rate of proton transfer caused by the stoichiometric binding of the metal ion shows that there is one dominant site of proton entry into the RC from which proton transfer to QB-* occurs.

Purification, redox and spectroscopic properties of the tetraheme cytochrome c isolated from Rubrivivax gelatinosus

The tetraheme cytochrome c subunit of the Rubrivivax gelatinosus reaction center was isolated in the presence of octyl beta-D-thioglucoside by ammonium sulfate precipitation and solubilization at pH 9 in a solution of Deriphat 160. Several biochemical properties of this purified cytochrome were characterized. In particular, it forms small oligomers and its N-terminal amino acid is blocked. In the presence or absence of diaminodurene, ascorbate and dithionite, different oxidation/reduction states of the isolated cytochrome were studied by absorption, EPR and resonance Raman spectroscopies. All the data show two hemes quickly reduced by ascorbate, one heme slowly reduced by ascorbate and one heme only reduced by dithionite. The quickly ascorbate-reduced hemes have paramagnetic properties very similar to those of the two low-potential hemes of the reaction center-bound cytochrome (gz = 3.34), but their alpha band is split with two components peaking at 552 nm and 554 nm in the reduced state. Their axial ligands did not change, being His/Met and His/His, as indicated by the resonance Raman spectra. The slowly ascorbate-reduced heme and the dithionite-reduced heme are assigned to the two high-potential hemes of the bound cytochrome. Their alpha band was blue-shifted at 551 nm and the gz values decreased to 2.96, although the axial ligations (His/Met) were conserved. It was concluded that the estimated 300 mV potential drop of these hemes reflected changes in their solvent accessibility, while the reduction in gz indicates an increased symmetry of their cooordination spheres. These structural modifications impaired the cytochrome's essential function as the electron donor to the photooxidized bacteriochlorophyll dimer of the reaction center. In contrast to its native state, the isolated cytochrome was unable to reduce efficiently the reaction center purified from a Rubrivivax gelatinosus mutant in which the tetraheme was absent. Despite the conformational changes of the cytochrome, its four hemes are still divided into two groups with a pair of low-potential hemes and a pair of high-potential hemes.

Fast reversible electron transfer for photosynthetic reaction center from wild type Rhodobacter sphaeroides re-constituted in polycation sandwiched monolayer film

Direct reversible electron transfer for photosynthetic reaction center from wild type Rhodobacter sphaeroides re-constituted in polycation sandwiched monolayer film was observed in this work. The redox potential E0' = 0.46 V vs. NHE for first primary donor redox couple P/P+ was accurately measured from reversible CV or SWV peaks, which were quite close to those obtained from optic redox titration method. Reaction center (RC) in film was found re-constituted in such an ordered way that the orientation of RC favored the electron transfer in film. Thus, the protein electroactivity seems to be turned on in this artificial biomimic thin film. Furthermore, RC in the film features a photo-induced redox-peak fluctuation, suggesting an intact and functional state for RC in such film. Redox peaks were also found dependent of pH, implying a proton-coupled electron transfer occurring in film. Charge recombination was observed accompanied with change of electrochemical driving force. Electrochemical model assuming several classes of electroactive sites in the films on the electrode with a dispersion of standard potentials successfully fits SWV experimental data at different pulse height and frequency.

Proton and electron transfer to the secondary quinone (QB) in bacterial reaction centers: the effect of changing the electrostatics in the vicinity of QB by interchanging asp and glu at the L212 and L213 sites

The bacterial reaction center (RC) plays a central role in photosynthetic energy conversion by facilitating the light induced double reduction and protonation of a bound quinone molecule, QB. Two carboxylic acid residues, Asp-L213 and Glu-L212, located near QB, were previously shown to be important for proton transfer to QB. In this work, the ability of Glu to substitute for Asp at L213 and Asp to substitute for Glu at L212 was tested by site-directed mutagenesis. Both single mutants and a double mutant in which Asp and Glu were exchanged between the two sites were constructed. The electron transfer rate constants kBD (D+QAQB- --> DQAQB), and kAB(2) (DQA-QB- + H+ --> DQA(QBH)-), that are known to be sensitive to the energy of the QB- state, were found to be altered by Asp/Glu substitutions. Both rates were fastest ( approximately 10-fold) in RCs with Asp at both sites, slowest with Glu at both sites ( approximately 50-fold) and relatively unchanged by the caboxylic acid exchange. These changes could be explained if Asp was predominantly ionized and Glu was predominantly protonated at both sites (pH 7.5). The charge recombination kBD suggests an observed approximately 5 pKa unit difference of Glu over Asp. Modeling of kBD by strong electrostatic interactions ( approximately 3-4 pKa units) among negatively charged acids and QB- indicated a lower intrinsic pKa for Asp compared to Glu at either site of approximately 2-3 units. The mechanism of the kAB(2) reaction was determined to be the same in all mutant RCs as for native RCs. A quantitative explanation of the effect of the electrostatic environment on kAB(2) was obtained using the two-step model proposed for native RCs [Graige, M. S., Paddock, M. L., Bruce, J. M., Feher, G., & Okamura, M. Y. (1996) J. Am. Chem. Soc. 118, 9005-9016] which involves fast protonation of the semiquinone followed by rate-limiting electron transfer. Using simple models for the quinone/quinol conversion rate, it is shown that the optimal electrostatic potential for the QB site is close to that found in native RCs.

The Ile(L229) → Met mutation impairs the quinone binding to the QB-pocket in reaction centers of Rhodobacter sphaeroides

A spontaneous mutant (R/89) of photosynthetic purple bacterium Rhodobacter sphaeroides R-26 was selected for resistance to 200 μM atrazin. It showed increased resistance to interquinone electron transfer inhibitors of o-phenanthroline (resistance factor, RF=20) in UQo reconstituted isolated reaction centers and terbutryne in reaction centers (RF=55) and in chromatophores (RF=85). The amino acid sequence of the QB binding protein of the photosynthetic reaction center (the L subunit) was determined by sequencing the corresponding pufL gene and a single mutation was found (Ile(L229) → Met). The changed amino acid of the mutant strain is in van der Waals contact with the secondary quinone QB. The binding and redox properties of QB in the mutant were characterized by kinetic (charge recombination) and multiple turnover (cytochrome oxidation and semiquinone oscillation) assays of the reaction center. The free energy for stabilization of QAQB (-) with respect to QA (-)QB was ΔGAB=-60 meV and 0 meV in reaction centers and ΔGAB=-85 meV and -46 meV in chromatophores of R-26 and R/89 strains at pH 8, respectively. The dissociation constants of the quinone UQo and semiquinone UQo (-) in reaction centers from R-26 and R/89 showed significant and different pH dependence. The observed changes in binding and redox properties of quinones are interpreted in terms of differential effects (electrostatics and mesomerism) of mutation on the oxidized and reduced states of QB.

Pathway of proton transfer in bacterial reaction centers: role of aspartate-L213 in proton transfers associated with reduction of quinoneto dihydroquinone

The role of Asp-L213 in proton transfer to reduced quinone QB in the reaction center (RC) from Rhodobacter sphaeroides was studied by site-directed replacement of Asp with residues having different proton donor properties. Reaction centers (RCs) with Asn, Leu, Thr, and Ser at L213 had greatly reduced (approximately 6000-fold) proton-coupled electron transfer [kAB(2)] and proton uptake rates associated with the second electron reduction of QB (QA- QB- + 2H(+)-->QAQBH2) compared to native RCs. RCs containing Glu at L213 showed faster (approximately 90-fold) electron and proton transfer rates than the other mutant RCs but were still reduced (approximately 70-fold) compared with native RCs. These results show that kAB(2) is larger when a carboxylic acid occupies the L213 site, consistent with the proposal that Asp-L213 is a component of a proton transfer chain. The reduced kAB(2) observed with Glu versus Asp at L213 suggests that Asp at L213 is important for proton transfer for some other reason in addition to its proton transfer capabilities. Glu-L213 is estimated to have a higher apparent pKa (pKa > or = 7) than Asp-L213 (pKa < or = 4), as indicated by the slower rate of charge recombination (D+QAQB(-)-->DQAQB) in the mutant RCs. The importance of the pKa and charge of the residue at L213 for proton transfer are discussed. Based on these studies, a model for proton transfer is proposed in which Asp-L213 contributes to proton transfer in native RCs in two ways: (1) it is a component of a proton transfer chain connecting the buried QB molecule with the solvent and/or (2) it provides a negative charge that stabilizes a proton on or near QB.

Light-induced proton uptake by photosynthetic reaction centers from Rhodobacter sphaeroides R-26.1. II. Protonation of the state DQAQB2-

Proton uptake associated with the two-electron reduction of QB was investigated in reaction centers (RCs) from Rhodobacter sphaeroides R-26.1 using pH-sensitive dyes. An uptake of two protons was observed at pH < or = 7.5, consistent with the formation of the dihydroquinone QBH2. At higher pH, the proton uptake decreased with an apparent pKa of approx. 8.5, i.e., to 1.5 H+/2 e- at pH 8.5. A molecular model is presented in which the apparent pKa is due to the protonation of either the carbonyl oxygen on QB or of an amino acid residue near QB (e.g., His-L190). Experimental evidence in favor of the protonation of the oxygen is discussed. The kinetics of the electron transfer from QA-QB- to QAQB2- and the associated proton uptake were compared at several pH values and temperatures. At pH 8.5 (21.5 degrees C) the rate constants for the proton uptake and electron transfer are the same within the precision of the measurement. At lower pH, the proton uptake rate constant is smaller than that for electron transfer. The difference between the rate constants is temperature dependent, i.e., it varies from 12 +/- 4% at 21.5 degrees C (pH 7.5) to 28 +/- 4% at 4.0 degrees C (pH 7.5). We show that the kinetics can be explained by a previously proposed model (Paddock, M. L., McPherson, P. H., Feher, G. and Okamura, M. Y. (1990) Proc. Natl. Acad. Sci. USA 87, 6803-6807) in which the uptake of two protons by doubly reduced QB occurs sequentially, one concomitant with and the other after electron transfer.

The relationship between the structure of plastoquinone derivatives and their biological activity in Photosystem II of spinach chloroplasts

The relationship between the structure of reconstituted plastoquinone derivatives and their ability to recover the Hill reaction was investigated by extraction and reconstitution of lyophilized chloroplasts from spinach, followed by monitoring DCIP photoreduction at 600 nm. The results show that: It is not essential that the plastoquinone side chain be an isoprenoid or a phytol; the activity increases with increasing length of the side chain up to 13-15 carbon atoms; for chains longer than 15 carbon atoms, the activity is practically constant. Lipophilic groups (such as -Br) in the side chain increased the activity, hydrophilic groups (such as -OH) decreased the activity. Conjugated double bonds in the side chain decreased the activity greatly, but non-conjugated double bonds had almost no effect on the activity, indicating a requirement of flexibility of the side chain. The activity is decreased in the order of PQ, UbiQ and MQ, showing a large effect of the ring structure.

Partial reversion of the electrogenic reaction in the ubiquinol: cytochrome c2-oxidoreductase of Rhodobacter sphaeroides chromatophores under neutral and alkaline conditions

The interaction of the photosynthetic reaction center (RC)-generated ubiquinol with the ubiquinone-reducing center C of ubiquinol:cytochrome c2-oxidoreductase (bc1-complex) has been studied electrometrically in Rhodobacter sphaeroides chromatophores. The addition of myxothiazol inhibited the ubiquinol-oxidizing center Z, suppressing the phases of membrane potential generation by the bc1-complex, but at the same time induced an electrogenic phase of opposite polarity, sensitive to antimycin A, the inhibitor of center C. The rise time of this reverse phase varied from 3 ms at pH 6.0 to 1 ms at pH 9.5. At pH greater than 9.5 the reverse phase was limited by the rate of ubiquinol formation in RC. The magnitude of the reverse phase was constant within the pH range 7.5-10.0. It is assumed that the reverse phase is due to the electrogenic deprotonation reaction which takes place after the binding of the RC-generated ubiquinol to center C.

E. Antonini Plenary lecture. A structural basis of light energy and electron transfer in biology

Aspects of intramolecular light energy and electron transfer will be discussed for three protein cofactor complexes, whose three-dimensional structures have been elucidated by X-ray crystallography: components of light-harvesting cyanobacterial phycobilisomes, the purple bacterial reaction centre and the blue multi-copper oxidases. A wealth of functional data is available for these systems which allow specific correlations between structure and function, and general conclusions about light energy and electron transfer in biological materials to be made.

Nobel lecture. A structural basis of light energy and electron transfer in biology

Aspects of intramolecular light energy and electron transfer will be discussed for three protein cofactor complexes, whose three-dimensional structures have been elucidated by x-ray crystallography: Components of light harvesting cyanobacterial phycobilisomes, the purple bacterial reaction centre, and the blue multi-copper oxidases. A wealth of functional data is available for these systems which allow specific correlations between structure and function and general conclusions about light energy and electron transfer in biological materials to be made.

Reaction centers from Rhodopseudomonas sphaeroides in reconstituted phospholipid vesicles. II. Light-induced proton translocation

Unidirectional light-dependent proton translocation was demonstrated in a suspension of reconstituted reaction center (RC) vesicles supplemented with cytochrome c and 2,3-dimethoxy-5-methyl-1,4-benzoquinone (UQ0), a lipid- and water-soluble quinone. Proton translocation was detected only at alkaline pH. The pH dependence can be accounted for by the slow redox reaction between the reduced quinone (UQ0H2) and oxidized cytochrome c. This conclusion is based on (i) the pH dependence of partial reactions of the reconstituted proton translocation cycle, measured either optically or electrometrically and (ii) titration studies with cytochrome c and UQ0. At 250 and 25 microM UQ0 and cytochrome c, respectively, maximal proton translocation was observed at pH 9.6. This pH optimum can be extended to a more acidic pH by increasing the concentration of the soluble redox mediators in the reconstituted cyclic electron transfer chain. At the alkaline side of the pH optimum, proton translocation appears to be limited by electron transfer from the endogenous primary to the secondary quinone within the RCs. The light intensity limits the reconstituted proton pump at the optimal pH. The results are discussed in the context of a reaction scheme for the cyclic redox reactions and the associated proton translocation events.