Reconstitution of the heliobacterial photochemical reaction center and cytochrome c553 into a proteoliposome system

Photosynthesis Research - The heliobacterial reaction center (HbRC) is the simplest known photochemical reaction center, in terms of its polypeptide composition. In the heliobacterial cells, its...     

The carotenoid band shift in reaction centers from from Rhodopseudomonas sphaeroides.

A specific carotenoid associated with reaction centers purified from Rhodopseudomonas sphaeroides shows an optical absorbance change in response to photochemical activity, at temperatures down to 35 K. The change corresponds to a bathochromic shift of 1 nm of each absorption band. The same change is induced by either chemical oxidation or photo-oxidation of reaction center bacteriochlorophyll (P-870). Reduction of the electron acceptor of the reaction center, either chemically or photochemically, does not cause a carotenoid absorbance change or modify a change already induced by oxidation of P-870. The change of the carotenoid spectrum can therefore be correlated with the appearance of positive charge in the reaction center. In these studies we observed that at 35 K the absorption band of reaction center bacteriochlorophyll near 600 nm exhibits a shoulder at 605 nm. The resolution into two components is more pronounced in the light-dark difference spectrum. This observation is consistent with our earlier finding, that the "special pair" of bacteriochlorophyll molecules that acts as photochemical electron donor has a dimer-like absorption spectrum in the near infrared.     

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.Abbreviations BChl bacteriochlorophyll - P a dimer of BChl molecules - BPh bacteriopheophytin - Q_A and Q_B quinone molecules - L, M and H light, medium and heavy polypeptides of the reaction center     

The carotenoid band shift in reaction centers from Rhodopseudomonas sphaeroides

A specific carotenoid associated with reaction centers purified from Rhodopseudomonas sphaeroides shows an optical absorbance change in response to photochemical activity, at temperatures down to 35 K. The change corresponds to a bathochromic shift of 1 nm of each absorption band. The same change is induced by either chemical oxidation or photo-oxidation of reaction center bacteriochlorophyll (P-870). Reduction of the electron acceptor of the reaction center, either chemically or photochemically, does not cause a carotenoid absorbance change or modify a change already induced by oxidation of P-870. The change of the carotenoid spectrum can therefore be correlated with the appearance of positive charge in the reaction center. In these studies we observed that at 35 K the absorption band of reaction center bacteriochlorophyll near 600 nm exhibits a shoulder at 605 nm. The resolution into two components is more pronounced in the light-dark difference spectrum. This observation is consistent with our earlier finding, that the “special pair” of bacteriochlorophyll molecules that acts as photochemical electron donor has a dimer-like absorption spectrum in the near infrared.     

外源葡萄糖对盐胁迫下山楂叶中光系统Ⅱ光化学活性的影响

研究盐胁迫下外源葡萄糖对山楂叶中光系统II（PSII）光化学活性影响的结果表明,盐胁迫下,浇灌外源葡萄糖可增加山楂幼苗叶中PSII最大光化学效率（Fv/Fm）、暗适应后PSII最大光化学效率（ΦPo）、捕获的激子将电子传递到电子传递链中QA-下游的其它电子受体的概率（Ψo）及反应中心吸收的光能用于电子传递的量子产额（ΦEo）,降低照光2ms时反应中心的关闭程度（Vj）和单位反应中心吸收的能量（ABS/RC）,提高电子转运效率（ETo/RC）,降低放氧复合体（OEC）受伤害的程度。     

Expression and purification of affinity-tagged variants of the photochemical reaction center from Heliobacterium modesticaldum

Photosynthesis Research - The heliobacterial photochemical reaction center (HbRC) from the chlorophototrophic Firmicutes bacterium Heliobacterium modesticaldum is the only homodimeric type I RC...     

Spectrophotometric and voltage clamp characterization of monolayers of bacterial photosynthetic reaction centers

Bacterial photosynthetic reaction centers from Rhodopseudomonas sphaeroides have been spread on an air/aqueous interface, compressed, and transferred quantitatively to either glass or transparent, tin oxide-coated slides. These assemblies permit the concomitant measurement of both optical and electrical activities to be made on protein films under voltage-clamp conditions. Optical spectra of the monolayer-coated slides reveal characteristic reaction center absorptions. Linear dichroism spectra of the monolayers indicate that the reaction center is aligned on the air/aqueous interface with an angle of inclination which is essentially the same as it is with respect to the membrane plane in vivo. The kinetics of the light-induced absorbance changes of the reaction center in the deposited films are essentially unaltered from those in solution; however, there is some loss in the extent of photochemical activity. Measurement of the light-induced electrical transients shows capacitative charging and discharging currents, which can be readily associated with the reaction center bacteriochlorophyll dimer to ubiquinone electron transfer. The extent of the photochemical activity detected by the voltage-clamp is at best only 10–12% of that measured by optical assay. This suggests that on the air/aqueous interface, the reaction centers must be predominately oriented with opposing directions of electron transfer, having only a slight, variable tendency to align with the ubiquinone directed toward the aqueous phase. In spite of the present shortcomings, these assemblies appear to be uniquely useful to study the effect of clamped potentials on the kinetics and mechanisms of electron transfer.     

Influence of pigment substitution on the electrochemical properties of Rhodobacter sphaeroides 601 reaction centers

With the help of pigment substitution, self-assembled monolayer film and square wave voltammetry, the influence of pigment substitution on the electrochemical properties of Rhodobac-ter sphaeroides 601 reaction centers was investigated. Results showed that the charge separation could also be driven by externally electric field, similar to the primary photochemical reaction in purple bacterial reaction center. On the surface of Au electrode, a self-assembled monolayer film (the RC-PDDA-DMSA film) was made up of 2,3-dimercaptosuccinic acid (DMSA), poly-dimeth-yldiallylammonium chloride (PDDA) and reaction center (RC). When square wave voltammetry was used to study the RC-PDDA-DMSA film, four redox pairs in the photochemical reaction of RC were observed by changing frequency. With nonlinear fitting, the standard potential of P/P+ and the corresponding electrode reaction rate constant were determined to be 0.522 V and 13.04 S-1, respectively. It was found that the redox peak at -0.02 V changed greatly when b     

EPR properties of the reaction center of Rhodopseudomas gelatinosa in situ and in a detergent-solubilized form.

The photochemical reaction centers from a variety of purple photosynthetic bacteria are composed of a trimer of protein subunits. However, the recently isolated reaction center from Rhodopseudomonas gelatinosa appears to have only two subunits. In this paper we examine the EPR characteristics of the primary photochemical reactants in this species, and compare them with those of other species. Despite of the differences in protein composition, no dramatic differences in EPR properties are seen in vivo, although some interesting effects are seen upon solubilization of the reaction center, which may be related to the unusual lability of the isolated preparation. Perhaps the most noteworthy phenomenon seen in Rps. gelatinosa is the apparent ability of electrons on the reduced intermediary electron carrier to tunnel at low temperatures to the oxidized c-type cytochrome, which has not been seen in other species studied to date.     

Dissipation in bioenergetic electron transfer chains

This paper examines the processes by which wasteful dissipation of free energy may occur in bioenergetic electron transfer chains. Frictionless transfer requires high rate constants in order to achieve a quasi-equilibrium steady-state. Previous results concerning the maximum power available from a photochemical source are recalled. The energetic performance of the bacterial reaction center is discussed, characterizing the processes that decrease either the quantum yield (recombination and obstruction) or the chemical potential (friction and non-equilibrated mechanisms). Considering the whole chain, diffusive carriers are potentially weaker links, due to kinetic limitation and short-circuiting reactions. It is suggested that the evolutionary trend has been to limit their number by lumping them into tightly bound protein complexes or, in a more flexible way, into labile supercomplexes.Abbreviations Cyt cytochrome - F Faraday - H primary acceptor in the bacterial reaction center (bacteriopheophytin) - k _B Boltzmann's constant - P primary photochemical donor (special bacteriochlorophyll pair) - RC reaction center - Q_A, Q_B primary, secondary quinone acceptor     

Influence of pigment substitution on the electrochemical properties ofRhodobacter sphaeroides 601 reaction centers

With the help of pigment substitution, self-assembled monolayer film and square wave voltammetry, the influence of pigment substitution on the electrochemical properties ofRhodobacter sphaeroides 601 reaction centers was investigated. Results showed that the charge separation could also be driven by externally electric field, similar to the primary photochemical reaction in purple bacterial reaction center. On the surface of Au electrode, a self-assembled monolayer film (the RC-PDDA-DMSA film) was made up of 2,3-dimercaptosuccinic acid (DMSA), poly-dimethyldiallylammonium chloride (PDDA) and reaction center (RC). When square wave voltammetry was used to study the RC-PDDA-DMSA film, four redox pairs in the photochemical reaction of RC were observed by changing frequency. With nonlinear fitting, the standard potential of P/P⁺ and the corresponding electrode reaction rate constant were determined to be 0.522 V and 13.04 S^-1, respectively. It was found that the redox peak at −0.02 V changed greatly when bacteriopheophytin was substituted by plant pheophytin in the reaction center. Further studies indicated that this change resulted from the decrease in electron transfer rate between Bphe^-/Bphe (Phe^-/Phe) and Q_A ^-/Q_A after pigment substitution. After investigations of spectra and electrochemical properties of different RCs and comparisons of different function groups of pigments, it was indicated that the phytyl tail, similar to other substituted groups of pheophytin, affected the efficiencies of pigment substitution.     

On the question of the primary acceptor in bacterial photosynthesis: Manganese substituting for iron in reaction centers of Rhodopseudomonas spheroides R-26

Iron was partially replaced by manganese in a reaction center preparation of Rhodopseudomonas spheroides R-26. The reaction centers containing manganese were distinguished spectroscopically (EPR) from those containing iron. The low-temperature photochemical activities were found to be identical for both species. This makes it unlikely that the transition metal by itself is the primary electron acceptor.     

Influence of pigment substitution on the electrochemical properties of Rhodobacter sphaeroides 601 reaction centers

With the help of pigment substitution, self-assembled monolayer film and square wave voltammetry, the influence of pigment substitution on the electrochemical properties ofRhodobacter sphaeroides 601 reaction centers was investigated. Results showed that the charge separation could also be driven by externally electric field, similar to the primary photochemical reaction in purple bacterial reaction center. On the surface of Au electrode, a self-assembled monolayer film (the RC-PDDA-DMSA film) was made up of 2,3-dimercaptosuccinic acid (DMSA), poly-dimethyldiallylammonium chloride (PDDA) and reaction center (RC). When square wave voltammetry was used to study the RC-PDDA-DMSA film, four redox pairs in the photochemical reaction of RC were observed by changing frequency. With nonlinear fitting, the standard potential of P/P⁺ and the corresponding electrode reaction rate constant were determined to be 0.522 V and 13.04 S^-1, respectively. It was found that the redox peak at −0.02 V changed greatly when bacteriopheophytin was substituted by plant pheophytin in the reaction center. Further studies indicated that this change resulted from the decrease in electron transfer rate between Bphe^-/Bphe (Phe^-/Phe) and Q_A ^-/Q_A after pigment substitution. After investigations of spectra and electrochemical properties of different RCs and comparisons of different function groups of pigments, it was indicated that the phytyl tail, similar to other substituted groups of pheophytin, affected the efficiencies of pigment substitution.     

Early Archean origin of Photosystem II

Photosystem II is a photochemical reaction center that catalyzes the light‐driven oxidation of water to molecular oxygen. Water oxidation is the distinctive photochemical reaction that permitted the evolution of oxygenic photosynthesis and the eventual rise of eukaryotes. At what point during the history of life an ancestral photosystem evolved the capacity to oxidize water still remains unknown. Here, we study the evolution of the core reaction center proteins of Photosystem II using sequence and structural comparisons in combination with Bayesian relaxed molecular clocks. Our results indicate that a homodimeric photosystem with sufficient oxidizing power to split water had already appeared in the early Archean about a billion years before the most recent common ancestor of all described Cyanobacteria capable of oxygenic photosynthesis, and well before the diversification of some of the known groups of anoxygenic photosynthetic bacteria. Based on a structural and functional rationale, we hypothesize that this early Archean photosystem was capable of water oxidation to oxygen and had already evolved protection mechanisms against the formation of reactive oxygen species. This would place primordial forms of oxygenic photosynthesis at a very early stage in the evolutionary history of life.     

Kinetics of pigment-acceptor interaction induced by steady-state illumination in Synechocystis spaeroides photosystem I preparations cooled to 160 K in the dark and on light

The kinetics of dark reduction of chlorophyll P700 oxidized by steady-state illumination in photosystem I reaction center preparations of cyanobacterium Synechocystis sp. coolled in the dark to 160 K is greatly nonexponential. The characteristic times for the components of the reaction are from fractions of a second to minutes and more. During cooling reaction center preparations on actinic light, a great part of chlorophyll P700 is fixed at 160 K in oxidized state. The kinetics of dark reduction of P700+ in the fraction of reaction centers that retain the photochemical activity in these conditions is faster than the kinetics in samples cooled in the dark. A theoretical analysis of the substantial deceleration of the P700+ dark recovery kinetics was done for photosystem I reaction center preparations oxidized by steady-state illumination to 160 K in contrast with situation that arises after the oxidation of reaction centers by single short light pulses. The deceleration of the kinetics in samples activated by steady-state illumination can be explained by processes of microconformational relaxation, connected with proton shifts in the reaction center structure.     

长期施氮对旱地小麦灌浆期叶绿素荧光参数的影响

探讨了长期施氮对黄土旱塬冬小麦灌浆期叶绿紊荧光的影响．结果表明,PSⅡ反应中心的实际光化学效率(φPSⅡ)随叶片水分胁迫的加剧而降低,施氮可明显提高φPsⅡ．施氮量为0kg·hm^-2、90kg·hm^-2和180kg·hm^-2三个处理中午时叶片的φPSⅡ分别为0．197、0．279和0．283,与上午相比分别降低了57．7％、56．4％和40．2％;下午各处理的φPSⅡ均得以恢复,0kg·hm^-2和90kg·hm^-2处理分别恢复到上午的87．3％和81．5％,180kg·hm^-2处理则完全恢复,并稍高于上午．施氮可提高PSⅡ的最大光化学量子效率(Fv／Fm)、光化学猝灭系数(qP)和非光化学猝灭系数(qNP)等荧光参数,表明施氮一方面提高了光能转换效率和PSⅡ的潜在活性,另一方面增强了过剩光能的非光化学耗散,有利于保护光合机构免受环境胁迫的破坏．但90kg·hm^-2和180kg·hm^-2施氮处理之间对光合的促进作用差异不显著,表明过量施氮无助于光合性能的提高,作物的光合能力并不随施氮量而同比例的改善。     

EPR properties of the reaction center of Rhodopseudomas gelatinosa in situ and in a detergent-solubilized form

The photochemical reaction centers from a variety of purple photosynthetic bacteria are composed of a trimer of protein subunits. However, the recently isolated reaction center from Rhodopseudomonas gelatinosa appears to have only two subunits. In this paper we examine the EPR characteristics of the primary photochemical reactants in this species, and compare them with those of other species. Despite of the differences in protein composition, no dramatic differences in EPR properties are seen in vivo, although some interesting effects are seen upon solubilization of the reaction center, which may be related to the unusual lability of the isolated preparation. Perhaps the most noteworthy phenomenon seen in Rps. gelatinosa is the apparent ability of electrons on the reduced intermediary electron carrier to tunnel at low temperatures to the oxidized c-type cytochrome, which has not been seen in other species studied to date.     

The recognition of a special ubiquinone functionally central in the ubiquinone-cytochrome b-c2 oxidoreductase.

Although the energy conserving membranes of the photosynthetic bacterium Rhodopseudomonas sphaeroides contain a 25 (+/- 3)-fold molar excess of ubiquinone over the photochemical reaction center, the activity of the ubiquinone-cytochrome b-c2 oxidoreductase is unaffected by quinone extraction until only 3, or at most 4, ubiquinones remain; only then does further extraction prevent the function of the oxidoreductase. Since 2 of these last ubiquinones are integral parts of the photochemical reaction center, we conclude that the ubiquinone-cytochrome b-c2 oxidoreductase requires only 1, or at most 2, molecules of ubiquinone-10 for its function. Earlier kinetic data identified a major electron donor to ferricytochrome c2 as a single molecule (known as Z) which requires 2 electrons and 2 protons for its equilibrium reduction. Hence, we identify a single molecule of quinone, probably ubiquinone-10 in a special environment, as a major electron donor to ferricytochrome c2 in the ubiquinone cytochrome b-c2 oxidoreductase.     

Excited states of photosynthetic reaction centers at low redox potentials

In preparations of photochemical reaction centers from Rhodopseudomonas spheroides R-26, lowering the redox potential so as to reduce the primary electron acceptor prevents the photochemical transfer of an electron from bacteriochlorophyll to the acceptor. Measuring absorbance changes under these conditions, we found that a 20-ns actinic flash converts the reaction center to a new state, P^F, which then decays with a half-time that is between 1 and 10 ns at 295 °K. At 25 °K, the decay half-time is approx. 20 ns. The quantum yield of state P^F appears to be near 1.0, both at 295 and at 15 °K. State P^F could be an intermediate in the photochemical electron-transfer reaction which occurs when the acceptor is in the oxidized form.Following the decay of state P^F, we detected another state, P^R, with a decay half-time of 6 μs at 295 °K and 120 μs at 15 °K. The quantum yield of state P^R is approx. 0.1 at 295 °K, but rises to a value nearer 1.0 at 15 °K. The kinetics and quantum yields are consistent with the view that state P^R forms from P^F. State P^R seems likely to be a side-product, rather than an intermediate in the electron-transfer process.The decay kinetics indicate that state P^F cannot be identical with the lowest excited singlet state of the reaction center. One of the two states, P^F or P^R, probably is the lowest excited triplet state of the reaction center, but it remains unclear which one.