Efficiency of photochemical stages of photosynthesis in purple bacteria (A critical survey)

Based on currently available data, the energy transfer efficiency in the successive photophysical and photochemical stages has been analyzed for purple bacteria. This analysis covers the stages starting from migration of the light-induced electronic excitations from the bulk antenna pigments to the reaction centers up to irreversible stage of the electron transport along the transmembrane chain of cofactors-carriers. Some natural factors are revealed that significantly increase the rates of efficient processes in these stages. The influence on their efficiency by the “bottleneck” in the energy migration chain is established. The overall quantum yield of photosynthesis in these stages is determined.     

Leaf <Emphasis Type="Italic">vs</Emphasis>. inflorescence: differences in photosynthetic activity of grapevine

Using measures of gas exchange and photosynthetic chain activity, we found some differences between grapevine inflorescence and leaf in terms of photosynthetic activity and photosynthesis regulations. Generally, the leaf showed the higher net photosynthesis (P _N) and lower dark respiration than that of the inflorescence until the beginning of the flowering process. The lower (and negative) P _N indicated prevailing respiration over photosynthesis and could result from a higher metabolic activity rather than from a lower activity of the photosynthetic apparatus. Considerable differences were observed between both organs in the functioning and regulation of PSI and PSII. Indeed, in our conditions, the quantum yield efficiency and electron transport rate of PSI and PSII were higher in the inflorescence compared to that of the leaf; nevertheless, protective regulatory mechanisms of the photosynthetic chain were clearly more efficient in the leaf. This was in accordance with the major function of this organ in grapevine, but it highlighted also that inflorescence seems to be implied in the whole carbon balance of plant. During inflorescence development, the global PSII activity decreased and PSI regulation tended to be similar to the leaf, where photosynthetic activity and regulations remained more stable. Finally, during flowering, cyclic electron flow (CEF) around PSI was activated in parallel to the decline in the thylakoid linear electron flow. Inflorescence CEF was double compared to the leaf; it might contribute to photoprotection, could promote ATP synthesis and the recovery of PSII.     

缺铁对大豆叶片光合作用和光系统Ⅱ功能的影响

通过气体交换和叶绿素荧光测定研究了缺铁对大豆叶片碳同化和光系统Ⅱ的影响。缺铁条件下大豆光合速率(Pn)大幅下降;最大光化学效率(po)下降幅度较小;荧光诱导动力学曲线发生明显的变化,其中电子传递活性明显下降,K相(VK)相对荧光产量提高。缺铁大豆的天线转化效率(Fv'/Fm')、光化学猝灭系数(qP)和光系统Ⅱ实际光化学效率(ΦPSⅡ)降低,而非光化学猝灭(NPQ)则明显增加。此外,缺铁大豆的光后荧光上升增强。据此,认为铁缺乏伤害了光系统Ⅱ复合物供体侧和受体侧的电子传递;缺铁条件下光系统I环式电子传递的增强可能在维持激发能耗散和ATP供给方面起一定作用。     

外源钙对干旱胁迫下烤烟幼苗光系统Ⅱ功能的影响

以叶绿素快相荧光动力学曲线(OJIP)为探针,研究了外源钙对干旱胁迫下烤烟幼苗光系统Ⅱ(PSⅡ)功能的影响.结果表明:干旱胁迫降低了烤烟幼苗PSⅡ原初光能转换效率(Fv/Fm)和电子传递速率(ETR),抑制了光合作用的原初过程,烤烟幼苗叶片发生了明显的光抑制.叶面喷施10.0 mmol·L-1CaCl2溶液后烤烟叶片的光合电子传递能量比例(ФEo)在干旱胁迫下的降低幅度明显小于对照(喷施清水),电子转运效率(ET0/RC)在干旱胁迫下明显高于对照.叶面喷施CaC12溶液增加了PSⅡ捕获光能用于光合电子传递的比例、剩余有活性反应中心的效率和电子传递链中的能量传递,使烤烟叶片的光系统Ⅱ在干旱胁迫下保持相对较高的活性,从而提高了烤烟幼苗的抗旱能力.     

Regulation of the primary photosynthesis processes

The mechanisms of primary processes of photosynthesis and macromolecular conformational changes that control the efficiency of primary energy transformation in photosynthesis are discussed. Special attention is focused on the analysis of chlorophyll fluorescence as an integrated parameter indicative of the efficiency and dynamics of primary steps of photosynthesis. Sharp changes in environmental conditions and other unfavorable factors may lead to the distortions of the coupling between consecutive electron transfer steps. As a result, an excess of electrons and/or electronic excitation energy may form at some sites of the electron transport chain. This may lead to the generation of reactive oxygen species responsible for the subsequent oxidative stress. The results of the application of these data in the areas of biotechnology and ecology are demonstrated.     

Relationship between the Quantum Efficiencies of Photosystems I and II in Pea Leaves

The irradiance dependence of the efficiencies of photosystems I and II were measured for two pea (Pisum sativum [L.]) varieties grown under cold conditions and one pea variety grown under warm conditions. The efficiencies of both photosystems declined with increasing irradiance for all plants, and the quantum efficiency of photosystem I electron transport was closely correlated with the quantum efficiency of photosystem II electron transport. In contrast to the consistent pattern shown by efficiency of the photosystems, the redox state of photosystem II (as estimated from the photochemical quenching coefficient of chlorophyll fluorescence) exhibited relationships with both irradiance and the reduction of P-700 that varied with growth environment and genotype. This variability is considered in the context of the modulation of photosystem II quantum efficiency by both photochemical and nonphotochemical quenching of excitation energy.     

Theoretical study of the effect of several reverse reaction on the kinetics of the primary processes of photosynthesis

A theoretical model of energy migration and electron transport in photosynthesis of higher plants was considered. The set of different equations describing these processes takes into consideration the states of 4 components of electron transport chain and back reactions of electron transfer from the reduced acceptors to the oxidized reaction centres. The numerical integration of these equations was made for various kinetics parameters characterizing the electron transport chain.     

Phylogenetic Analysis of Proteins Associated in the Four Major Energy Metabolism Systems: Photosynthesis,Aerobic Respiration,Denitrification, and Sulfur Respiration

The four electron transfer energy metabolism systems, photosynthesis, aerobic respiration, denitrification, and sulfur respiration, are thought to be evolutionarily related because of the similarity of electron transfer patterns and the existence of some homologous proteins. How these systems have evolved is elusive. We therefore conducted a comprehensive homology search using PSI-BLAST, and phylogenetic analyses were conducted for the three homologous groups (groups 1–3) based on multiple alignments of domains defined in the Pfam database. There are five electron transfer types important for catalytic reaction in group 1, and many proteins bind molybdenum. Deletions of two domains led to loss of the function of binding molybdenum and ferredoxin, and these deletions seem to be critical for the electron transfer pattern changes in group 1. Two types of electron transfer were found in group 2, and all its member proteins bind siroheme and ferredoxin. Insertion of the pyridine nucleotide disulfide oxidoreductase domain seemed to be the critical point for the electron transfer pattern change in this group. The proteins belonging to group 3 are all flavin enzymes, and they bind flavin adenine dinucleotide (FAD) or flavin mononucleotide (FMN). Types of electron transfer in this group are divergent, but there are two common characteristics. NAD(P)H works as an electron donor or acceptor, and FAD or FMN transfers electrons from/to NAD(P)H. Electron transfer functions might be added to these common characteristics by the addition of functional domains through the evolution of group 3 proteins. Based on the phylogenetic analyses in this study and previous studies, we inferred the phylogeny of the energy metabolism systems as follows: photosynthesis (and possibly aerobic respiration) and the sulfur/nitrogen assimilation system first diverged, then the sulfur/nitrogen dissimilation system was produced from the latter system.     

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.     

Analysis of limitations to CO2 assimilation on exposure of leaves of two Brassica napus cultivars to UV-B

Apex and Bristol cultivars of oilseed rape (Brassica napus) were irradiated with 0.63 W m^?2 of UV-B over 5 d. Analyses of the response of net leaf carbon assimilation to intercellular CO₂ concentration were used to examine the potential limitations imposed by stomata, carboxylation velocity and capacity for regeneration of ribulose 1,5-bis-phosphate on leaf photosynthesis. Simultaneous measurements of chlorophyll fluorescence were used to estimate the maximum quantum efficiency of photosystem II (PSII) photochemistry, the quantum efficiency of linear electron transport at steady-state photosynthesis, and the light and CO₂-saturated rate of linear electron transport. Ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco) content and activities were assayed in vitro. In both cultivars the UV-B treatment resulted in decreases in the light-saturated rate of CO₂ assimilation, which were accompanied by decreases in carboxylation velocity and Rubisco content and activity. No major effects of UV-B were observed on end-product inhibition and stomatal limitation of photosynthesis or the rate of photorespiration relative to CO₂ assimilation. In the Bristol cultivar, photoinhibition of PSII and loss of linear electron transport activity were observed when CO₂ assimilation was severely inhibited. However, the Apex cultivar exhibited no major inhibition of PSII photochemistry or linear electron transport as the rate of CO₂ assimilation decreased. It is concluded that loss of Rubisco is a primary factor in UV-B inhibition of CO₂ assimilation.     

The enigmatic contribution of mitochondrial function in photosynthesis

Considerable cumulative evidence has accrued suggesting a vital role for mitochondrial function in optimizing photosynthesis. Both pharmacological approaches using respiratory inhibitors and reverse genetic approaches have recently underscored the high degree of interconnection between photosynthesis and respiration--the major pathways of energy production which are largely confined to the plastid and mitochondria, respectively. Here recent studies into the nature of these interactions are reviewed, with particular focus on (i) the recently described link between the mitochondrial electron transport chain activity, ascorbate biosynthesis, and photosynthesis; and (ii) the contribution of mitochondrial metabolism to the photorespiratory process. Whilst there is increasing evidence of a role for ascorbate in co-ordinating the rates of respiration and photosynthesis, some data are presented here for plants grown under extreme environmental conditions that suggest that this relationship is not absolute. It thus seems likely that interactions between these compartments are perhaps more numerous and complicated than previously thought. This observation suggests that although the elucidation of the genetic bases of both photorespiration and the Wheeler-Smirnoff pathway of ascorbate biosynthesis has recently been completed, much further research is probably necessary in order to understand fully how energy metabolism is co-ordinated in the illuminated leaf.     

线粒体呼吸链膜蛋白复合体的结构

线粒体作为真核细胞的重要“能量工厂”,是细胞进行呼吸作用的场所,呼吸作用包括柠檬酸循环和氧化磷酸化两个过程,其中氧化磷酸化过程的电子传递链（又称线粒体呼吸链）位于线粒体内膜上,由四个相对分子质量很大的跨膜蛋白复合体（Ⅰ、Ⅱ、Ⅲ、和Ⅳ）、介于Ⅰ／Ⅱ与Ⅲ之间的泛醌以及介于Ⅲ与Ⅳ之间的细胞色素c共同组成。线粒体呼吸链的功能是进行生物氧化,并与称之为复合物V的ATP合成酶（磷酸化过程）相偶联,共同完成氧化磷酸化过程,并生产能量分子ATP。线粒体呼吸链的结构生物学研究对于彻底了解电子传递和能量转化的机理是至关重要的,本文分别论述线粒体呼吸链复合体Ⅰ、Ⅱ、Ⅲ和Ⅳ的结构,并跟踪线粒体呼吸链超复合体的结构研究进展。     

光合作用氧释放机理研究进展

植物在光合作用过程中不仅为同化CO2提供能量和还原力,同时裂解水放出氧气。放氧反应主要由光系统Ⅱ(PSⅡ)氧化侧的4个锰原子组成的锰簇催化完成的。因此,锰簇在光合放氧过程中起看至关重要的作用。文章概述了对锰簇及其微环境的结构和功能的研究进展。     

The dipole moment of the electron carrier adrenodoxin is not critical for redox partner interaction and electron transfer

Dipole moments of proteins arise from helical dipoles, hydrogen bond networks and charged groups at the protein surface. High protein dipole moments were suggested to contribute to the electrostatic steering between redox partners in electron transport chains of respiration, photosynthesis and steroid biosynthesis, although so far experimental evidence for this hypothesis was missing. In order to probe this assumption, we changed the dipole moment of the electron transfer protein adrenodoxin and investigated the influence of this on protein-protein interactions and electron transfer. In bovine adrenodoxin, the [2Fe-2S] ferredoxin of the adrenal glands, a dipole moment of 803 Debye was calculated for a full-length adrenodoxin model based on the Adx(4-108) and the wild type adrenodoxin crystal structures. Large distances and asymmetric distribution of the charged residues in the molecule mainly determine the observed high value. In order to analyse the influence of the resulting inhomogeneous electric field on the biological function of this electron carrier the molecular dipole moment was systematically changed. Five recombinant adrenodoxin mutants with successively reduced dipole moment (from 600 to 200 Debye) were analysed for their redox properties, their binding affinities to the redox partner proteins and for their function during electron transfer-dependent steroid hydroxylation. None of the mutants, not even the quadruple mutant K6E/K22Q/K24Q/K98E with a dipole moment reduced by about 70% showed significant changes in the protein function as compared with the unmodified adrenodoxin demonstrating that neither the formation of the transient complex nor the biological activity of the electron transfer chain of the endocrine glands was affected. This is the first experimental evidence that the high dipole moment observed in electron transfer proteins is not involved in electrostatic steering among the proteins in the redox chain.     

强光下硫化氢通过促进光系统II的活性来缓解水稻的光抑制

以水稻品种‘II优084’为材料,测定了强光胁迫下,水稻光合速率、叶绿素荧光快速诱导曲线（OJIP）以及O2ˉ·和H2O2在水稻叶片中积累的影响。结果表明强光胁迫下,水稻的净光合速率及气孔导度下降;光系统II（PSII）反应中心关闭的比例以及电子传递链中光系统II受体侧原初醌受体（QA）的还原程度增加;PSII反应中心电子传递的量子产额、能量以及传递到下游电子链的比率下降;光抑制下PSII的过剩能量向PSI的状态装换减少;自由基的产生增加。而施加作为硫化氢（H2S）供体的外源硫氢化钠（NaHS）后,上述影响PSII活性的指标的负变化被缓解,捕光天线复合体LHC通过在两个光系统之间的移动,来调节两个光系统的能量分配。强光下H2S处理能促进LHC离开PSII,与PSI结合,从而减少PSII分配的激发能,增加PSI分配的激发能,缓解了PSII的过度还原。以上结果表明外源H2S通过促进PSII的光合活性来缓解水稻光抑制伤害。     

Effect of photosynthesis on dark mitochondrial respiration in green cells

Green plant cells can generate ATP in both chloroplasts and mitochondria. Hence the effect of photosynthesis on dark mitochondrial respiration can be considered at a variety of levels. Turnover of ceitric acid cycle dehydrogenases, which is essential for supply of carbon skeletons for amino acid synthesis, seems to be largely unaffected during photosynthesis. The source of carbon for the anaplerotic function of the citric acid cycle in light is however, not known with certainty. NADH generated in these reactions is probably not oxidised via the mitochondrial electron transfer chain coupled to ATP synthesis. However, it may be oxidised by the alternative cyanide-insensitive pathway, exported to the cytosol via the oxaloacetate-malate dicarboxylate shuttle or directly utilised for cytosolic nitrate reduction. Oxidation of succinate via cytochrome oxidase may also be similarly inhibited in light. Whether increase in the cytosolic ATP/ADP ratio in light is responsible for the inhibition of mitochondrial electron transfer to O₂ is not clearly established, because the ATP/ADP ratio is reported to be already quite high in the dark. Effective collaboration between photophosphorylation and oxidative phosphorylation in order to maintain the cytosolic energy charge at a present high level is discussed.     

The Absence of Alternative Oxidase AOX1A Results in Altered Response of Photosynthetic Carbon Assimilation to Increasing CO2 in Arabidopsis thaliana

In higher plants, the mitochondrial electron transport chain has non-phosphorylating alternative pathways that include the alternative terminal oxidase (AOX). This alternative pathway has been suggested to act as a sink for dissipating excess reducing power, minimizing oxidative stress and possibly optimizing photosynthesis in response to changing conditions. The expression patterns of the AOX genes have been well characterized under different growth conditions, particularly in response to light and temperature stress. Additionally, it has been suggested that mitochondrial electron transport is important for avoiding chloroplast over-reduction and balancing energy partitioning among photosynthesis, photorespiration and respiration. Nonetheless, the role AOX plays in optimizing photosynthetic carbon metabolism is unclear. Therefore, the response of photosynthesis to the disruption of AOX was investigated in the Arabidopsis thaliana T-DNA mutant aox1a (SALK_084897). Gas exchange analysis revealed a lower net CO(2) assimilation rate (A) at high CO(2) concentrations in the aox1a mutant compared to wild type. This decrease in A was accompanied by a lower maximum electron transport rate and quantum yield of PSII, and higher excitation pressure on PSII and non-photochemical quenching. The aox1a mutant also exhibited a lower estimated rate of ribulose 1,5-bisphosphate regeneration, and the ribulose 1,5-bisphosphate content was lower at high CO(2) concentrations, suggesting an ATP limitation of the Calvin-Benson cycle. Additionally, the activity of the malate-oxaloacetate shuttle was lower in the mutant compared to wild type. These results indicate that AOX is important for optimizing rates of photosynthetic CO(2) assimilation in response to rising CO(2) concentration by balancing the NAD(P)H/ATP ratio and rates of ribulose 1,5-bisphosphate regeneration within the chloroplast.     

Kok effect and the quantum yield of photosynthesis : light partially inhibits dark respiration

The linear response of photosynthesis to light at low photon flux densities is known to change abruptly in the vicinity of the light compensation point so that the quantum yield seems to decrease as radiation increases. We studied this `Kok effect' in attached sunflower (Helianthus annuus L. cv IS894) leaves using gas exchange techniques. The effect was present even though respiration was constant in the dark. It was observed at a similar photon flux density (7 to 11 micromole photons per square meter per second absorbed photosynthetically active radiation) despite a wide range of light compensation points as well as rates of photosynthesis. The effect was not apparent when photorespiration was inhibited at low pO₂ (1 kilopascal), but this result was complicated because dark respiration was quite O₂-sensitive and was partially suppressed under these conditions. The Kok effect was observed at saturating pCO₂ and, therefore, could not be explained by a change in photorespiration. Instead, the magnitude of the effect varied as dark respiration varied in a single leaf, and was minimized when dark respiration was minimized, indicating that a partial suppression of dark respiration by light is responsible. Quantum yields measured at photon flux densities between 0 and 7 to 11 micromole photons per square meter per second, therefore, represent the combined yields of photosynthesis and of the suppression of a component of dark respiration by light. This leads to an overestimate of the quantum yield of photosynthesis. In view of these results, quantum yields of photosynthesis must be measured (a) when respiration is constant in the dark, and (b) when dark respiration has been inhibited either at low pO₂ to eliminate most of the light-induced suppression of dark respiration or at photon flux densities above that required to saturate the light-induced suppression of dark respiration. Significant errors in quantum yields of photosynthesis can result in leaves exhibiting this respiratory behavior if these principles are not followed.     

Promotion of Energy Transfer and Oxygen Evolution in Spinach Photosystem II by Nano-anatase TiO<Subscript>2</Subscript>

Being a proven photocatalyst, nano-anatase is capable of undergoing electron transfer reactions under light. In previous studies we had proven that nano-anatase improved photosynthesis and greatly promoted spinach growth. The mechanisms by which nano-anatase promotes energy transfer and the conversion efficiency of the process are still not clearly understood. In the present paper, we report the results obtained with the photosystem II (PSII) isolated from spinach and treated by nano-anatase TiO₂ and studied the effect of nano-anatase TiO₂ on energy transfer in PSII by spectroscopy and on oxygen evolution. The results showed that nano-anatase TiO₂ treatment at a suitable concentration could significantly change PSII microenvironment and increase absorbance for visible light, improve energy transfer among amino acids within PSII protein complex, and accelerate energy transport from tyrosine residue to chlorophyll a. The photochemical activity of PSII (fluorescence quantum yield) and its oxygen-evolving rate were enhanced by nano-anatase TiO₂. This is viewed as evidence that nano-anatase TiO₂ can promote energy transfer and oxygen evolution in PSII of spinach.