CHLOROPHYLL-PROTEINS AND REACTION CENTER PREPARATIONS FROM PHOTOSYNTHETIC BACTERIA, ALGAE AND HIGHER PLANTS,‡

Kinetics and energetics of electron transfer in reaction centers of the photosynthetic bacterium Roseiflexus castenholzii

The kinetics and thermodynamics of the photochemical reactions of the purified reaction center (RC)-cytochrome (Cyt) complex from the chlorosome-lacking, filamentous anoxygenic phototroph, Roseiflexus castenholzii are presented. The RC consists of L- and M-polypeptides containing three bacteriochlorophyll (BChl), three bacteriopheophytin (BPh) and two quinones (Q(A) and Q(B)), and the Cyt is a tetraheme subunit. Two of the BChls form a dimer P that is the primary electron donor. At 285K, the lifetimes of the excited singlet state, P*, and the charge-separated state P(+)H(A)(-) (where H(A) is the photoactive BPh) were found to be 3.2±0.3 ps and 200±20 ps, respectively. Overall charge separation P*→→ P(+)Q(A)(-) occurred with ≥90% yield at 285K. At 77K, the P* lifetime was somewhat shorter and the P(+)H(A)(-) lifetime was essentially unchanged. Poteniometric titrations gave a P(865)/P(865)(+) midpoint potential of +390mV vs. SHE. For the tetraheme Cyt two distinct midpoint potentials of +85 and +265mV were measured, likely reflecting a pair of low-potential hemes and a pair of high-potential hemes, respectively. The time course of electron transfer from reduced Cyt to P(+) suggests an arrangement where the highest potential heme is not located immediately adjacent to P. Comparisons of these and other properties of isolated Roseiflexus castenholzii RCs to those from its close relative Chloroflexus aurantiacus and to RCs from the purple bacteria are made.

Escape probability and trapping mechanism in purple bacteria: revisited

Despite intensive research for decades, the trapping mechanism in the core complex of purple bacteria is still under discussion. In this article, it is attempted to derive a conceptionally simple model that is consistent with all basic experimental observations and that allows definite conclusions on the trapping mechanism. Some experimental data reported in the literature are conflicting or incomplete. Therefore we repeated two already published experiments like the time-resolved fluorescence decay in LH1-only purple bacteria Rhodospirillum rubrum and Rhodopseudomonas viridis chromatophores with open and closed (Q(A)(-)) reaction centers. Furthermore, we measured fluorescence excitation spectra for both species under the two redox-conditions. These data, all measured at room temperature, were analyzed by a target analysis based on a three-state model (antenna, primary donor, and radical pair). All states were allowed to react reversibly and their decay channels were taken into consideration. This leads to seven rate constants to be determined. It turns out that a unique set of numerical values of these rate constants can be found, when further experimental constraints are met simultaneously, i.e. the ratio of the fluorescence yields in the open and closed (Q(A)(-)) states F(m)/F(o) approximately 2 and the P(+)H(-)-recombination kinetics of 3-6 ns. The model allows to define and to quantify escape probabilities and the transfer equilibrium. We conclude that trapping in LH1-only purple bacteria is largely transfer-to-the-trap-limited. Furthermore, the model predicts properties of the reaction center (RC) in its native LH1-environment. Within the framework of our model, the predicted P(+)H(-)-recombination kinetics are nearly indistinguishable for a hypothetically isolated RC and an antenna-RC complex, which is in contrast to published experimental data for physically isolated RCs. Therefore RC preparations may display modified kinetic properties.

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.

Electrogenic steps in the redox reactions catalyzed by photosynthetic reaction-centre complex from Rhodopseudomonas viridis

Electrogenic and redox events in the reaction-centre complexes from Rhodopseudomonas viridis have been studied. In contrast to the previous points of view it is shown that all the four hemes of the tightly bound cytochrome c have different Em values (-60, +20, +310 and +380 mV). The first three hemes reveal alpha absorption maxima at 554 nm, 552 nm and 556 nm respectively. The 380-mV heme displays a split alpha band with a maximum at 559 nm and a shoulder at 552 nm. Such a splitting is due to non-degenerated Qx and Qy transitions in the iron-porphyrin ring as demonstrated by magnetic circular dichroism spectra. Fast kinetic measurements show that, at redox potentials when only high-potential hemes c-559 and c-556 are reduced, heme c-559 appears to be the electron donor to P-960+ (tau = 0.32 microsecond) whereas heme c-556 serves to rereduce c-559 (tau = 2.5 microsecond). Upon reduction of the third heme (c-552), the P-960+ reduction rate increases twofold (tau = 0.17 microsecond) and all photoinduced redox events within the cytochrome appear to be complete in less than 1 microsecond after the flash. The following sequence of the redox centers is tentatively suggested: c-554, c-556, c-552, c-559, P-960. To study electrogenesis, the reaction-centre complexes from Rps. viridis were incorporated into asolectin liposomes, and fast kinetics of laser flash-induced electric potential difference has been measured in proteoliposomes adsorbed on a phospholipid-impregnated film. The electrical difference induced by a single 15-ns flash was found to be as high as 100 mV. The photoelectric response has been found to involve four electrogenic stages associated with (I) QA reduction by P-960; (II) reduction of P-960+ by heme c-559; (III) reduction of c-559 by c-556 and (IV) protonation of Q2-B. The relative contributions of stages I, II, III and IV are found to be equal to 70%, 15%, 5% and 10%, respectively, of the overall electrogenic process. At the same time, the first three respective distances along the axis normal to the membrane plane covered by electrons, calculated from X-ray data of Deisenhofer et al. [J. Mol. Biol. 180, 385-398 (1984)], are 22%, 18.5% and 26%. This indicates that the efficiency of electrogenic phases depends first of all upon the value of the dielectric constant of the respective membrane regions rather than upon the distance between the redox groups involved.(ABSTRACT TRUNCATED AT 400 WORDS)

Structure of a bacterial photosynthetic membrane: integrity of reaction centers following proteolysis and detergent solubilization

The photosynthetic membranes of the purple bacterium Rhodopseudomonas viridis are composed of a semi-crystalline lattice of subunits. Proteolysis of isolated membranes with trypsin or pronase results in the degradation of polypeptides associated with the photosynthetic reaction center. However, two low molecular weight peptides which may form the light-harvesting complex survive the enzymatic treatment. The proteolysis does not affect the major absorbance peak (830 nm) associated with the reaction center. However, treatment of proteolyzed membranes with detergents such as LDAO abolishes the 830 nm absorbance peak. The 830 nm peak is stable following LDAO solubilization of non-proteolyzed membranes. These results suggest that a combination of covalent and non-covalent interactions are important in maintaining the configuration of the reaction center, and are consistent with a model of membrane organization in which the light-harvesting components are buried in a lipid phase of the membrane and reaction center components form the large structures which electron microscope studies have shown to extend from either membrane surface.

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

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

Absorption and circular dichroism spectra of chloroplast membrane fragments from spinach, barley and a barley mutant at room temperature and liquid nitrogen temperature

The absorption and CD spectra of chloroplast fragments from spinach, barley and a barley mutant (chlorophyll b-minus) were studied at temperatures of 23 degrees C and --196 degrees C. The CD spectrum of wild type barley and spinach at --196 degrees C showed troughs at 640, 653, 676 and 695 nm and a maximum at 667 nm. The CD spectrum of the barley mutant at --196 degrees C consisted of a large trough at 684 nm, a small trough at 695 nm and a positive peak at 670 nm. A new feature observed at --196 degrees C but not at 23 degrees C is the trough at 640 nm. This 640 nm CD signal is missing in the CD spectrum of the barley mutant. It is attributable to the light-harvesting chlorophyll a/b protein which appears to be missing in the mutant. Another new feature, the trough at 695 nm, was observed in the CD spectra of spinach, barley and the barley mutant at --196 degrees C. The 695 nm trough appears to be sensitive to detergents and it may be due to a labile chlorophyll a.protein complex. Possible interpretations of the data are discussed.

Excitation energy transfer in the light-harvesting chlorophyll a/b.protein

The "light-harvesting chlorophyll a/b.protein" described by Thornber has been prepared electrophoretically from spinach chloroplasts. The optical properties relevant to energy transfer have been measured in the red region (i.e. 600-700 nm). Measurements of the absorption spectrum, fluorescence excitation spectrum and excitation dependence of the fluorescence emission spectrum of this protein confirm that energy transfer from chlorophyll b to chlorophyll a is highly efficient, as is the case in concentrated chlorophyll solutions and in vivo. The excitiation dependence of the fluorescence polarization shows a minimum polarization of 1.9% at 650 nm which is the absorption maximum of chlorophyll b in the protein and rises steadily to a maximum value of 13.8% at 695 nm, the red edge of the chlorophyll a absorption band. Analysis of these measurements shows that at least two unresolved components must be responsible for the chlorophyll a absorption maximum. Comparison of polarization measurements with those observed in vivo shows that most of the depolarization observed in vivo can take place within a single protein. Circular dichroism measurements show a double structure in the chlorophyll b absorption band which suggest an exciton splitting not resolved in absorption. Analysis of these data yields information about the relative orientation of the So leads to S1 transition moments of the chlorophyll molecules within the protein.

Thermodynamic properties of the reaction center of Rhodopseudomonas viridis. In vivo measurement of the reaction center bacteriochlorophyll-primary acceptor intermediary electron carrier

The thermodynamic properties of redox components associated with the reaction center of Rhodopseudomonas viridis have been characterized with respect to their midpoint potentials and relationship with protons. In particular a midpoint potential for the intermediary electron carrier acting between the reaction center bacteriochlorophyll and the primary acceptor has been determined. The rationale for this measurement was that the light-induced triplet/biradical EPR signal would not be observed if this intermediate was chemically reduced before activation. The midpoint potential of the intermediary at pH 10.8 is about --400 mV (n=1).

Delayed fluorescence from Rhodopseudomonas viridis following single flashes

Delayed fluorescence from Rhodopseudomonas viridis membrane fragments has been studies using a phosphoroscope employing single, short actinic flashes, under conditions of controlled redox potential and temperature. The emission spectrum shows that delayed fluorescence is emitted by the bulk, antenna bacteriochlorophyll. The energy for delayed fluorescence, however, must be stored in a reaction-center complex including the photooxidized form (P+) of the primary electron-donor (P) and the photoreduced form (X MINUS) of the primary electron-acceptor. This is shown by the following observations: (1) Delayed luminescence is quenched (a) at low redox potentials which allow cytochromes to reduce P+ rapidly after the flash, (b) at higher redox potentials which, by oxidizing P chemically, prevent the photochemical formation of P+X minus, and (c) upon transfer of an electron from X minus to a secondary acceptor, Y. (2) Under conditions that prevent the reduction of P+ by cytochromes and the oxidation of X minus by Y, the decay kinetics of delayed fluorescence are identical with those of P+X minus, as measured from optical absorbance changes. The main decay route for P+X minus under these conditions has a rate-constant of approximately 10-3-s-minus 1. In contrase, a comparison of the intensities of delayed and prompt fluorescence indicates that the process in which P+X minus returns energy to the bulk bacteriochlorophyll has a rate-constant of 3.7 s-minus 1, at 295 degrees K and pH 7.8. The decay kinetics of P+X minus and delayed fluorescence change little with temperature, whereas the intensity of delayed fluorescence increases with increasing temperature, having an activation energy of 12.5 kcal mol-mol- minus 1. We conclude that the main decay route involves tunneling of an electron from X minus to P+, without the promotion of P to an excited state. Delayed fluorescence requires such a promotion, followed by transfer of energy to the bulk bacteriochlorophyll, and this combination of events is rare. The activation energy, taken with potentiometric data, indicates that the photochemical conversion of PX to P+X minus results in increases of both the energy and the entropy of the system, by 16.6 kcal-mol- minus 1 and 8.8 cal-mol- minus 1-deg- minus 1. The intensity of delayed fluorescence depends strongly on the pH; the origin of this effect remains unclear.