Modeling of the photosynthetic electron transport regulation in cyanobacteria

In this work we have performed a computer analysis of electron and proton transport in cyanobacterial cells using a mathematical model of light-dependent stages of photosynthesis taking into account the key stages of pH-dependent regulation of electron transport on both acceptor and donor sides of photosystem 1 (PS1). Comparison of theoretical and experimental data shows that the model adequately describes the multiphase kinetics of photoinduced redox transformations of P₇₀₀ (the primary electron donor in PS1). Our computer simulation describes the effect of variations of atmospheric gases (CO₂ and O₂) on the induction events in cyanobacteria (P₇₀₀ photooxidation, generation of transmembrane ΔpH), which strongly depends on the preillumination conditions (aerobic or anaerobic atmosphere). It has been shown that the variations of CO₂ concentration in the cell interior may noticeably affect the kinetics of electron transport, acidification of lumen, and ATP synthesis. The contributions of alternative pathways of electron transport (cyclic electron transport around PS1 and electron outflow to O₂) to the function of cyanobacterial photosynthetic apparatus have been analyzed. At the initial stage of induction period, cyclic electron flows around PS1 (“short” and “long” pathways) substantially contribute to photosynthetic electron transport. These flows, however, attenuate with the light-induced activation of the Calvin-Benson cycle reactions. In the meantime, the outflow of electrons from PS1 to O₂ (or to other metabolic chains) increases with oxygen accumulation in the medium. The effects of ferredoxin oxidation by hydrogenase catalyzing the H₂ formation on the kinetics of P700 photooxidation and distribution of electron flows on the acceptor side of PS1 have been modeled.     

Construction of CO2-response model of electron transport rate in C4 crop and its application

准确估算光合电子流对CO₂响应的变化趋势对深入了解光合过程具有重要意义。该研究在植物光合作用对CO₂响应新模型(模型I)的基础上构建了电子传递速率(J)对CO₂的响应模型(模型II), 并对用LI-6400-40便携式光合仪测量的玉米(Zea mays)和千穗谷(Amaranthus hypochondriacus)的数据进行了拟合。结果表明, 模型II可以很好地拟合玉米和千穗谷叶片J对CO₂浓度的响应曲线(J-C_a曲线), 得到玉米和千穗谷的最大电子传递速率分别为262.41和393.07 mmol·m ^-2·s ^-1, 与估算值相符合。在此基础上, 对光合电子流分配到其他路径进行了探讨。结果显示, 380 mmol·mol ^-1 CO₂浓度下玉米和千穗谷碳同化所需的电子流为247.92和285.16 mmol·m ^-2·s ^-1, 分配到其他途径的光合电子流为14.49和107.91 mmol·m ^-2·s ^-1(考虑植物CO₂的回收利用)。比较两种植物的其他途径光合电子流分配值发现, 两者相差6倍之多。分析认为这与千穗谷和玉米的催化脱羧反应酶种类以及脱羧反应发生的部位不同密切相关。该发现为人们研究C₄植物中烟酰胺腺嘌呤二核苷磷酸苹果酸酶型和烟酰胺腺嘌呤二核苷酸苹果酸酶型两种亚型之间的差异提供了一个新的视角。此外, 构建的电子传递速率对CO₂的响应模型为人们研究C₄植物的光合电子流的变化规律提供了一个可供选择的数学工具。     

The Effects of Cold Stress on Photosynthesis in Hibiscus Plants

The present work studies the effects of cold on photosynthesis, as well as the involvement in the chilling stress of chlororespiratory enzymes and ferredoxin-mediated cyclic electron flow, in illuminated plants of Hibiscus rosa-sinensis. Plants were sensitive to cold stress, as indicated by a reduction in the photochemistry efficiency of PSII and in the capacity for electron transport. However, the susceptibility of leaves to cold may be modified by root temperature. When the stem, but not roots, was chilled, the quantum yield of PSII and the relative electron transport rates were much lower than when the whole plant, root and stem, was chilled at 10°C. Additionally, when the whole plant was cooled, both the activity of electron donation by NADPH and ferredoxin to plastoquinone and the amount of PGR5 polypeptide, an essential component of the cyclic electron flow around PSI, increased, suggesting that in these conditions cyclic electron flow helps protect photosystems. However, when the stem, but not the root, was cooled cyclic electron flow did not increase and PSII was damaged as a result of insufficient dissipation of the excess light energy. In contrast, the chlororespiratory enzymes (NDH complex and PTOX) remained similar to control when the whole plant was cooled, but increased when only the stem was cooled, suggesting the involvement of chlororespiration in the response to chilling stress when other pathways, such as cyclic electron flow around PSI, are insufficient to protect PSII.     

Effects of mutations and of ionophore on chlororespiration in Chlamydomonas reinhardtii

It is shown that electron-transport elements of the photosynthetic chain located between plastoquinone and photosystem I centers are not involved in oxidation of plastoquinone in the chlororespiratory pathway. The ionophore dicyclohexyl-18-crown-6 is shown to induce a reduction of the plastoquinone pool in the dark. This is interpreted as indicating the existence of a coupling site in the chlororespiratory pathway between NAD(P)H and plastoquinone.     

Electron transfer through the acceptor side of photosystem I: Interaction with exogenous acceptors and molecular oxygen

This review considers the state-of-the-art on mechanisms and alternative pathways of electron transfer in photosynthetic electron transport chains of chloroplasts and cyanobacteria. The mechanisms of electron transport control between photosystems (PS) I and II and the Calvin–Benson cycle are considered. The redistribution of electron fluxes between the noncyclic, cyclic, and pseudocyclic pathways plays an important role in the regulation of photosynthesis. Mathematical modeling of light-induced electron transport processes is considered. Particular attention is given to the electron transfer reactions on the acceptor side of PS I and to interactions of PS I with exogenous acceptors, including molecular oxygen. A kinetic model of PS I and its interaction with exogenous electron acceptors has been developed. This model is based on experimental kinetics of charge recombination in isolated PS I. Kinetic and thermodynamic parameters of the electron transfer reactions in PS I are scrutinized. The free energies of electron transfer between quinone acceptors A_1A/A_1B in the symmetric redox cofactor branches of PS I and iron–sulfur clusters F_X, F_A, and F_B have been estimated. The second-order rate constants of electron transfer from PS I to external acceptors have been determined. The data suggest that byproduct formation of superoxide radical in PS I due to the reduction of molecular oxygen in the A1 site (Mehler reaction) can exceed 0.3% of the total electron flux in PS I.     

Activation of Diadinoxanthin De-Epoxidase Due to a Chiororespiratory Proton Gradient in the Dark in the Diatom Phaeodactylum tricornutum

Abstract: An exponential dynamic light regime with prolonged dark periods (light/dark cycle 8/40 h) was used to simulate deep mixing of algae in natural waters and to investigate the adaptation of the diatom Phaeodactylum tricornutum to these extreme light conditions. After prolonged dark periods Phaeo dactylum cells showed surprisingly high contents of diatoxan-thin, low photosynthetic efficiency and high non-photochemical quenching (NPQ) of chlorophyll fluorescence. Diatoxanthin con centrations and NPQ were low at the beginning of the dark peri od and increased with the duration of the dark incubation. Addi tion of the diadinoxanthin de-epoxidase inhibitor, DTT, prevent ed the formation of diatoxanthin, thereby excluding de novo synthesis of diatoxanthin during the prolonged dark period. Evi dence of chlororespiratory electron flow and the establishment of a diadinoxanthin de-epoxidase activating proton gradient in the dark was derived from two observations. First, uncoupling of electron transport with NH₄CI at the beginning of the dark period prevented the development of non-photochemical quenching of chlorophyll fluorescence and the formation of diatoxanthin during the dark period. Second, inhibition of the electron and proton consuming terminal redox component of chlororespiratory electron transport, cytochrome oxidase, by addition of KCN induced stronger NPQ and a higher de-epoxidation state of the xanthophyll cycle. These results strongly indi cate that the activation of diadinoxanthin de-epoxidase in the dark is the consequence of a chlororespiratory proton gradient. We furthermore present evidence that diatoxanthin formed by the chlororespiratory proton gradient has the same efficiency in the mechanism of enhanced heat dissipation as diatoxanthin induced by a light-driven ApH.     

Effect of exogenous glucose on electron flow to photosystem I and respiration in cyanobacterial cells

The effects of exogenous glucose on the rates of alternative pathways of photosystem II (PSII)-independent electron flow to PSI and of dark respiration in Synechocystis sp. 6803 cells were studied. The presence of glucose was shown to accelerate the electron flow to P700⁺, the PSI primary electron donor oxidized with Far-red light (FRL), which excites specifically only PSI. An increase in the glucose concentration was accompanied by a further activation of electron flow to PSI, which was supported by the dark donation of reducing equivalents to the electron transport chain. An increase in the external glucose concentration resulted also in the disappearance of lag-phase in the kinetics of P700⁺ reduction, which was observed in the cells incubated without glucose after FRL switching off. A similarity of nonphotochemical processes of electron transfer to PSI in cyanobacteria and higher plants was supposed, basing on the earlier observed fact of the occurrence of such lagphase in higher plants and its dependence on the exhausting of stromal reductants in the light. Acceleration of dark electron flow to PSI in the presence of glucose, a major respiratory substrate, may indicate the coupling between nonphotochemical processes in the photosynthetic and respiratory chains of electron transport in cyanobacterial cells. A close correlation between photosynthesis and respiration in cyanobacterial cells is also confirmed by a sharp acceleration of respiration with an increase in the glucose concentration in medium.     

Photosynthetic Properties of ac-31, a Mutant Strain of Chlamydomonas reinhardi Devoid of Chloroplast Membrane Stacking

A pale-green mutant strain of Chlamydomonas reinhardi, ac-31, is characterized by the absence of any stacking of its chloroplast membranes. The capacity for photosynthetic electron transport, phosphorylation, and CO₂ fixation in ac-31 is substantial, and it is concluded that these photosynthetic activities occur within the single membrane. The photosynthetic capacities of wild type and ac-31 as a function of increasing light intensity are compared. Saturation is attained at higher light intensities in ac-31, and the kinetics of the 2 sets of curves are distinctly different. The possibility that energy transfer is enhanced by membrane stacking is suggested by these results. The repeatedly-observed correlation between reduced stacking and disfunctional Photosystem II activities is discussed in view of the observation that ac-31 has no stacking but retains a functional Photosystem II.     

Redox and ATP control of photosynthetic cyclic electron flow in Chlamydomonas reinhardtii (I) aerobic conditions

Assimilation of atmospheric CO₂ by photosynthetic organisms such as plants, cyanobacteria and green algae, requires the production of ATP and NADPH in a ratio of 3:2. The oxygenic photosynthetic chain can function following two different modes: the linear electron flow which produces reducing power and ATP, and the cyclic electron flow which only produces ATP. Some regulation between the linear and cyclic flows is required for adjusting the stoichiometric production of high-energy bonds and reducing power. Here we explore, in the green alga Chlamydomonas reinhardtii, the onset of the cyclic electron flow during a continuous illumination under aerobic conditions. In mutants devoid of Rubisco or ATPase, where the reducing power cannot be used for carbon fixation, we observed a stimulation of the cyclic electron flow. The present data show that the cyclic electron flow can operate under aerobic conditions and support a simple competition model where the excess reducing power is recycled to match the demand for ATP.     

Stoichiometry of protein complexes in plant photosynthetic membranes

Hetero-oligomeric membrane protein complexes form the electron transport chain (ETC) of oxygenic photosynthesis. The ETC complexes undertake the light-driven vectorial electron and proton transport reactions, which generate energy-rich ATP and electron-rich NADPH molecules for carbon fixation. The rate of photosynthetic electron transport depends on the availability of photons and the relative abundance of electron transport complexes. The relative abundance of the two photosystems, critical for the quantum efficiency of photosynthesis in changing light quality conditions, has been determined successfully by optical methods. Due to the lack of spectroscopic signatures, however, relatively little is known about the stoichiometry of other non-photosystem complexes in plant photosynthetic membrane. Here we determine the ratios of all major thylakoid-bound ETC complexes in Arabidopsis by a label-free quantitative mass spectrometry technique. The calculated stoichiometries are consistent with known subunit composition of complexes and current estimates of photosystem and cytochrome b₆f concentrations. The implications of these stoichiometries for photosynthetic light harvesting and the partitioning of electrons between the linear and cyclic electron transport pathways of photosynthesis are discussed.     

Simulation of chlorophyll fluorescence rise and decay kinetics,and P<Subscript>700</Subscript>-related absorbance changes by using a rule-based kinetic Monte-Carlo method

A model of primary photosynthetic reactions in the thylakoid membrane was developed and its validity was tested by simulating three types of experimental kinetic curves: (1) the light-induced chlorophyll a fluorescence rise (OJIP transients) reflecting the stepwise transition of the photosynthetic electron transport chain from the oxidized to the fully reduced state; (2) the dark relaxation of the flash-induced fluorescence yield attributed to the Q_A^? oxidation kinetics in PSII; and (3) the light-induced absorbance changes near 820 or 705 nm assigned to the redox transitions of P₇₀₀ in PSI. A model was implemented by using a rule-based kinetic Monte-Carlo method and verified by simulating experimental curves under different treatments including photosynthetic inhibitors, heat stress, anaerobic conditions, and very high light intensity.     

The NADPH-dependent electron flow in chloroplasts of the higher plants

The NADPH-dependent reduction of some photosynthetic electron carriers in the dark, and the reduction of NADP+ associated with the glycolytic sequence and the oxidative pentose phosphate pathway in chloroplasts are reviewed. The postulated pathways of electron transports sensitive and insensitive to antimycin A are also evaluated. It is proposed that the electron flow, predominantly through cytochrome bf complex, may be also involved in the pathway of NADPH-dependent and antimycin A-insensitive back electron transport. An information on the chlororespiration in higher plants is also included.     

Photoacoustic measurements in vivo of energy storage by cyclic electron flow in algae and higher plants

Energy storage by cyclic electron flow through photosystem I (PSI) was measured in vivo using the photoacoustic technique. A wide variety of photosynthetic organisms were considered and all showed measurable energy storage by PSI-cyclic electron flow except for higher plants using the C-3 carbon fixation pathway. The capacity for energy storage by PSI-cyclic electron flow alone was found to be small in comparison to that of linear and cyclic electron flows combined but may be significant, nonetheless, under conditions when photosystem II is damaged, particularly in cyanobacteria. Light-induced dynamics of energy storage by PSI-cyclic electron flow were evident, demonstrating regulation under changing environmental conditions.     

Photochemical activities of two photosystems in Bratonia orchid protocorms cryopreserved by vitrification method

Functional activities of two photosystems in orchid-specific embryos (protocorms) of a tropical hybrid orchid Bratonia were investigated before and after their cryopreservation by vitrification method. The kinetics of light-induced absorbance changes at 830 nm was analyzed as indicator of P700 redox conversions; changes in the variable chlorophyll fluorescence served to indicate the oxidation-reduction changes of the primary acceptor Q_A. Untreated protocorms exhibited low photochemical activity of photosystem II (PSII). In freeze-treated Bratonia protocorms, examined immediately after thawing, photosynthetic electron transport was strongly inhibited. Nevertheless, the cells retained activities of noncyclic electron flow and of alternative electron transport pathways related solely to PSI. However, Bratonia protocorms subjected to deep-freezing lost the capability of P700 photooxidation during the first day of reculturing. Deep freezing of protocorms had virtually no effect on the kinetics of dark relaxation of chlorophyll variable fluorescence, when measurements were made immediately after thawing. Unlike chlorophyll fluorescence, the kinetics of dark reduction of P700⁺ in protocorms exposed to freezing-thawing was substantially modified compared to untreated protocorms. Two exponential components with half-decay times of 27 and 310 ms were distinguished in the kinetics of P700⁺ reduction in treated samples, whereas the absorbance relaxation attributed to P700⁺ reduction in untreated samples followed an exponential decay with a half-decay time of 24 ms. Despite the appearance of additional slow component in the kinetics of P700⁺ reduction, the dark relaxation of variable fluorescence remained unaltered after deep freezing of protocorms. This observation indicates that the freezing-thawing procedure caused partial disorders in linear electron transport between PSII and PSI. Apparently, the functional interactions among carriers in the electron-transport chain were disturbed between the plastoquinone pool and the PSI reaction center. It is concluded that the vitrification method applied to protocorm cryopreservation did not cause their immediate death, but the protocorms died later, on the first day after reculturing.     

Inhibition of chlororespiration by myxothiazol and antimycin A in Chlamydomonas reinhardtii

Myxothiazol and antimycin A are shown to suppress the oxygen transient previously attributed to the flash-induced inhibition of chlororespiration in Chlamydomonas reinhardtii (Peltier et al. 1987, Biochim Biophys Acta 893: 83–90). However, these two compounds do not affect the photosynthetic electron transport chain as inferred by the insensitivity of the CO₂-dependent photosynthetic O₂ evolution and of the flash-induced electrochromic effect. Chlorophyll fluorescence induction measurements carried out in dark-adapted cells of a mutant of Chlamydomonas lacking photosystem 1, show that myxothiazol and antimycin A significantly increase the redox state of the photosystem 2 acceptors. We conclude from these results that chlororespiration is inhibited by myxothiazol and antimycin A and that the site of inhibition is located on the dark oxidation pathway of the plastoquinone pool. This inhibition is interpreted through the involvement of a myxothiazol and antimycin A sensitive cytochrome in the chlororespiratory chain.Abbreviations cyt cytochrome - PQ plastoquinone - PS photosystem     

Thermodynamical and structural information on photosynthetic systems obtained from electroluminescence kinetics

The relationship between the thermodynamical and structural properties of photosynthetic reaction centers and kinetics and polarization of electric field-induced luminescence was studied. A general model is presented to describe the influence of an electric field on the individual electron transfer rate constants. Comparison of simulations with this model and experimental curves of Photosystem I electroluminescence showed that (a) at least three electrogenic electron transfer steps occur: P-700 to A₀(~30%), A₀ to A₁ (~50%), and A₁ to F_A(~20%), (b) the midpoint potential of A₁/A-₁ is ~ - 0.81 V, and (c) the emission moments of the pigments make on average an angle of 67° with the membrane normal. It is concluded that the analysis of electro-luminescence kinetics may be a powerful technique to obtain information on primary processes using relatively intact systems.     

Localization of cytochrome b6f complexes implies an incomplete respiratory chain in cytoplasmic membranes of the cyanobacterium Synechocystis sp. PCC 6803

The cytochrome b₆f complex is an integral part of the photosynthetic and respiratory electron transfer chain of oxygenic photosynthetic bacteria. The core of this complex is composed of four subunits, cytochrome b, cytochrome f, subunit IV and the Rieske protein (PetC). In this study deletion mutants of all three petC genes of Synechocystis sp. PCC 6803 were constructed to investigate their localization, involvement in electron transfer, respiration and photohydrogen evolution. Immunoblots revealed that PetC1, PetC2, and all other core subunits were exclusively localized in the thylakoids, while the third Rieske protein (PetC3) was the only subunit found in the cytoplasmic membrane. Deletion of petC3 and both of the quinol oxidases failed to elicit a change in respiration rate, when compared to the respective oxidase mutant. This supports a different function of PetC3 other than respiratory electron transfer. We conclude that the cytoplasmic membrane of Synechocystis lacks both a cytochrome c oxidase and the cytochrome b₆f complex and present a model for the major electron transfer pathways in the two membranes of Synechocystis. In this model there is no proton pumping electron transfer complex in the cytoplasmic membrane.Cyclic electron transfer was impaired in all petC1 mutants. Nonetheless, hydrogenase activity and photohydrogen evolution of all mutants were similar to wild type cells. A reduced linear electron transfer and an increased quinol oxidase activity seem to counteract an increased hydrogen evolution in this case. This adds further support to the close interplay between the cytochrome bd oxidase and the bidirectional hydrogenase.     

Excitation Dynamics in Phycoerythrin 545: Modeling of Steady-State Spectra and Transient Absorption with Modified Redfield Theory

We model the spectra and excitation dynamics in the phycobiliprotein antenna complex PE545 isolated from the unicellular photosynthetic cryptophyte algae Rhodomonas CS24. The excitonic couplings between the eight bilins are calculated using the CIS/6-31G method. The site energies are extracted from a simultaneous fit of the absorption, circular dichroism, fluorescence, and excitation anisotropy spectra together with the transient absorption kinetics using the modified Redfield approach. Quantitative fit of the data enables us to assign the eight exciton components of the spectra and build up the energy transfer picture including pathways and timescales of energy relaxation, thus allowing a visualization of excitation dynamics within the complex.     

A nucleus-encoded factor, CRR2, is essential for the expression of chloroplast ndhB in Arabidopsis

The chloroplast NDH complex, NAD(P)H dehydrogenase, reduces the plastoquinone pool non-photochemically and is involved in cyclic electron flow around photosystem I (PSI). A transient increase in chlorophyll fluorescence after turning off actinic light is a result of NDH activity. We focused on this subtle change in chlorophyll fluorescence to isolate nuclear mutants affected in chloroplast NDH activity in Arabidopsis by using chlorophyll fluorescence imaging. crr2-1 and crr2-2 (chlororespiratory reduction) are recessive mutant alleles in which accumulation of the NDH complex is impaired. Except for the defect in NDH activity, photosynthetic electron transport was unaffected. CRR2 encodes a member of the plant combinatorial and modular protein (PCMP) family consisting of more than 200 genes in Arabidopsis. CRR2 functions in the intergenic processing of chloroplast RNA between rps7 and ndhB, which is possibly essential for ndhB translation. We have determined the function of a PCMP family member, indicating that the family is closely related to pentatrico-peptide PPR proteins involved in the maturation steps of organellar RNA.     

Regulation of Cell Shape in Euglena gracilis: I. Involvement of the Biological Clock, Respiration, Photosynthesis, and Cytoskeleton

The alga Euglena gracilis Z. changes its shape two times per day when grown under the synchronizing effect of a daily light-dark cycle. At the beginning of the light period when photosynthetic capacity is low, the population of cells is largely spherical in shape. The mean cell length of the population increases to a maximum in the middle of the light period when photosynthetic capacity is greatest, and then decreases for the remainder of the 24-hour period. The population becomes spherical by the end of the 24-hour period when the cycle reinitiates. These changes are also observed under constant dim light conditions (up to 72 hours) and are therefore controlled by the biological clock and represent a circadian rhythm in cell shape. In constant dim light, the cell division rhythm is either arrested or slowed considerably, while the cell shape rhythm continues.

The involvement of respiratory and photosynthetic pathways in the cell shape changes was investigated with energy pathway inhibitors. Antimycin A and NaN₃ both inhibited the round to long and long to round shape changes, indicating that the respiratory pathways are involved. DCMU and atrazine inhibited the round to long shape change but did not affect the long to round transition, indicating that light-induced electron flow is necessary only for the round to long shape change.

The influence of the cell shape changes on the photosynthetic reactions was investigated by altering cell shape with the cytoskeletal inhibitors cytochalasin and colchicine. Both inhibitors blocked the round to long and long to round shape changes. Cytochalasin B was found to have minimal cytotoxic effects on the photosynthetic reactions, but colchicine significantly inhibited light-induced electron flow and the in vivo expression of the photosynthetic rhythm.