Showing papers on "Photosynthetic reaction centre published in 2013"

Photosystem II: The Reaction Center of Oxygenic Photosynthesis*

Abstract: Photosystem II (PSII) uses light energy to split water into chemical products that power the planet. The stripped protons contribute to a membrane electrochemical potential before combining with the stripped electrons to make chemical bonds and releasing O2 for powering respiratory metabolisms. In this review, we provide an overview of the kinetics and thermodynamics of water oxidation that highlights the conserved performance of PSIIs across species. We discuss recent advances in our understanding of the site of water oxidation based upon the improved (1.9-A resolution) atomic structure of the Mn4CaO5 water-oxidizing complex (WOC) within cyanobacterial PSII. We combine these insights with recent knowledge gained from studies of the biogenesis and assembly of the WOC (called photoassembly) to arrive at a proposed chemical mechanism for water oxidation.

Quantum coherent energy transfer over varying pathways in single light-harvesting complexes.

Abstract: The initial steps of photosynthesis comprise the absorption of sunlight by pigment-protein antenna complexes followed by rapid and highly efficient funneling of excitation energy to a reaction center. In these transport processes, signatures of unexpectedly long-lived coherences have emerged in two-dimensional ensemble spectra of various light-harvesting complexes. Here, we demonstrate ultrafast quantum coherent energy transfer within individual antenna complexes of a purple bacterium under physiological conditions. We find that quantum coherences between electronically coupled energy eigenstates persist at least 400 femtoseconds and that distinct energy-transfer pathways that change with time can be identified in each complex. Our data suggest that long-lived quantum coherence renders energy transfer in photosynthetic systems robust in the presence of disorder, which is a prerequisite for efficient light harvesting.

Light-harvesting in photosystem I

Abstract: Water oxidation in photosynthesis takes place in photosystem II (PSII). This photosystem is built around a reaction center (RC) where sunlight-induced charge separation occurs. This RC consists of various polypeptides that bind only a few chromophores or pigments, next to several other cofactors. It can handle far more photons than the ones absorbed by its own pigments and therefore, additional excitations are provided by the surrounding light-harvesting complexes or antennae. The RC is located in the PSII core that also contains the inner light-harvesting complexes CP43 and CP47, harboring 13 and 16 chlorophyll pigments, respectively. The core is surrounded by outer light-harvesting complexes (Lhcs), together forming the so-called supercomplexes, at least in plants. These PSII supercomplexes are complemented by some “extra” Lhcs, but their exact location in the thylakoid membrane is unknown. The whole system consists of many subunits and appears to be modular, i.e., both its composition and organization depend on environmental conditions, especially on the quality and intensity of the light. In this review, we will provide a short overview of the relation between the structure and organization of pigment-protein complexes in PSII, ranging from individual complexes to entire membranes and experimental and theoretical results on excitation energy transfer and charge separation. It will become clear that time-resolved fluorescence data can provide invaluable information about the organization and functioning of thylakoid membranes. At the end, an overview will be given of unanswered questions that should be addressed in the near future.

The Photosynthetic Bacterial Reaction Center: Structure and Dynamics

Abstract: The Crystal Structure of the Photosynthetic Reaction Center from Rhodopseudomonas viridis.- Structure of the Reaction Center from Rhodobacter sphaeroides R-26 and 2.4.1.- Symmetry Breaking Structures Involved in the Docking of Cytochrome c and Primary Electron Transfer in Reaction Centers of Rhodobacter sphaeroides.- Crystallographic Studies of the Photosynthetic Reaction Center from Wild Type Rhodobacter sphaeroides (Y Strain).- Single Crystals of the Photochemical Reaction Center from Rhodobacter sphaeroides Wild Type Strain 2.4.1. Analyzed by Polarized Light.- Spectroscopic Studies of Crystallized Pigment-protein Complexes of R. palustris.- Protein-prosthetic Group Interactions in Bacterial Reaction Centers.- Circular Dichroism Spectroscopy of Photoreaction Centers.- Low Temperature Linear Dichroism Study of the Orientation of the Pigments in Reduced and Oxidized Reaction Centers of Rps. viridis and Rb. sphaeroides.- Anisotropic Magnetic Field Effects of the Photosynthetic Bacterial Reaction Center of Rhodobacter sphaeroides R-26, Studied by Linear Dichroic Magneto-optical Difference Spectroscopy (LD-MODS) in the Temperature Range 1.2-310K.- Bacterial Reaction Centers are Intrinsically Heterogeneous.- Reaction Centers of Purple Bacteria with Modified Chromophores.- Quantitative Analysis of Genetically Altered Reaction Centers Using an In Vitro Cytochrome Oxidation Assay.- Properties of Reaction Centers From the Green Photosynthetic Bacterium Chloroflexus aurantiacus.- Structural and Functional Properties of the Reaction Center of Green Bacteria and Heliobacteria.- Molecular Dynamics Simulation of the Primary Processes in the Photosynthetic Reaction Center of Rhodopseudomonas viridis.- The Stark Effect in Photosynthetic Reaction Centers from Rhodobacter sphaeroides R-26, Rhodopseudomonas viridis and the D1 D2 Complex of Photosystem II from Spinach.- The Nature of Excited States and Intermediates in Bacterial Photosynthesis.- On the Energetics of the States 1P*, 3P* and P+H? in Reaction Centers of Rb. sphaeroides.- The Possible Existence of a Charge Transfer State which Preceeds the Formation of (BChl)2+ BPh? in Rhodobacter sphaeroides Reaction Centers.- The Primary Electron Transfer in Photosynthetic Purple Bacteria: Long Range Electron Transfer in the Femtosecond Domain at Low Temperature.- The Problem of Primary Energy Conversion in Reaction Centers of Photosynthetic Bacteria.- Temperature Effects on the Ground State Absorption Spectra and Electron Transfer Kinetics of Bacterial Reaction Centers.- ENDOR of Exchangeable Protons of the Reduced Intermediate Acceptor in Reaction Centers from Rhodobacter sphaeroides R-26.- FTIR Spectroscopic Investigations of the Intermediary Electron Acceptor Photoreduction in Purple Photosynthetic Bacteria and Green Plants.- Charge Recombination at Low Temperature in Photosynthetic Bacteria Reaction Centers.- Temperature and - ?G Dependence of the Electron Transfer to and from QA in Reaction Center Protein from Rhodobacter sphaeroides.- The Effect of an Electric Field on the Charge Recombination Rate of D+QA? ? DQA in Reaction Centers from Rhodobacter sphaeroides R-26.- Pressure Effects on Electron Transfer in Bacterial Reaction Centers.- The Spectral Properties of Chlorophyll and Bacteriochlorophyll Dimers a Comparative Study.- Spectroscopic Properties and Electron Transfer Dynamics of Reaction Centers.- Analysis of A, LD, CD, ADMR and LD-ADMR Spectra for the Reaction Centers of Rps. viridis, Rb. sphaeroides, C. aurantiacus and Modified Rb. sphaeroides.- Modified-CI Model of Protein-induced Red Shifts of Reaction Center Pigment Spectra.- Temperature Dependence of the Long Wavelength Absorption Band of the Reaction Center of Rhodopseudomonas viridis.- Discussion of the Large Homogeneous Width of the P-Band in Bacterial- and Plant Reaction Centers.- Theoretical Models of Electrochromic and Environmental Effects on Bacterio-Chloropbylls and -Pheophytins in Reaction Centers.- Electrostatic Control of Electron Transfer in the Photosynthetic Reaction Center of Rhodopseudomonas viridis.- Molecular Orbital Studies on the Primary Donor P960 in Reaction Centers of Rps. viridis.- Early Steps in Bacterial Photosynthesis. Comparison of Three Mechanisms.- Mechanism of the Primary Charge Separation in Bacterial Photosynthetic Reaction Centers.- A Configuration Interaction (CI) Description of Vectorial Electron Transfer in Bacterial Reaction Centers.- Charge Transfer States and the Mechanism of Charge Separation in Bacterial Reaction Centers.- Light Reflections.

pH-dependent regulation of electron transport and ATP synthesis in chloroplasts.

Abstract: This review is focused on pH-dependent mechanisms of regulation of photosynthetic electron transport and ATP synthesis in chloroplasts. The light-induced acidification of the thylakoid lumen is known to decelerate the plastoquinol oxidation by the cytochrome b 6 f complex, thus impeding the electron flow between photosystem II and photosystem I. Acidification of the lumen also triggers the dissipation of excess energy in the light-harvesting antenna of photosystem II, thereby protecting the photosynthetic apparatus against a solar stress. After brief description of structural and functional organization of the chloroplast electron transport chain, our attention is focused on the nature of the rate-limiting step of electron transfer between photosystem II and photosystem I. In the context of pH-dependent mechanism of photosynthetic control in chloroplasts, the mechanisms of plastoquinol oxidation by the cytochrome b 6 f complex have been considered. The light-induced alkalization of stroma is another factor of pH-dependent regulation of electron transport in chloroplasts. Alkalization of stroma induces activation of the Bassham–Benson–Calvin cycle reactions, thereby promoting efflux of electrons from photosystem I to NADP+. The mechanisms of the light-induced activation of ATP synthase are briefly considered.

A Structure-Based Model of Energy Transfer Reveals the Principles of Light Harvesting in Photosystem II Supercomplexes

Abstract: Photosystem II (PSII) initiates photosynthesis in plants through the absorption of light and subsequent conversion of excitation energy to chemical energy via charge separation. The pigment binding proteins associated with PSII assemble in the grana membrane into PSII supercomplexes and surrounding light harvesting complex II trimers. To understand the high efficiency of light harvesting in PSII requires quantitative insight into energy transfer and charge separation in PSII supercomplexes. We have constructed the first structure-based model of energy transfer in PSII supercomplexes. This model shows that the kinetics of light harvesting cannot be simplified to a single rate limiting step. Instead, substantial contributions arise from both excitation diffusion through the antenna pigments and transfer from the antenna to the reaction center (RC), where charge separation occurs. Because of the lack of a rate-limiting step, fitting kinetic models to fluorescence lifetime data cannot be used to derive mechanistic insight on light harvesting in PSII. This model will clarify the interpretation of chlorophyll fluorescence data from PSII supercomplexes, grana membranes, and leaves.

Alleviation of heat damage to photosystem II by nitric oxide in tall fescue

Abstract: Nitric oxide (NO) has been found to mediate plant responses to heat stress. The objective of this study was to investigate the protective role of NO in the recovery process of photosystem II (PSII) in tall fescue (Festuca arundinacea) against heat stress. Treatment of tall fescue leaves with NO donor sodium nitroprusside significantly improved the overall behavior of PSII probed by the chlorophyll a fluorescence transients, while the inhibition of NO accumulation by 2-phenyl-4,4,5,5-tetramethyl-imidazoline-1-oxyl-3-oxide (PTIO, a NO scavenger) plus N G-nitro-l-arginine-methyl ester (L-NAME, NO synthase inhibitor) dramatically disrupted the operation of PSII. Specifically, under heat stress, the exogenous NO reduced the initial fluorescence (F 0), increased the maximal quantum yield (F V/F M), and disappeared the K-step of 0.3 ms. By the analysis of the JIP-test, the exogenous NO improved the quantum yield of the electron transport flux from Q A to Q B (ET0/ABS), and decreased the trapped excitation flux per reaction center (RC) (TR0/RC), electron transport flux per RC (ET0/RC), and electron flux reducing end electron acceptors per RC (RE0/RC). In addition, the exogenous NO reduced the content of H2O2, O 2 •− , and malondialdehyde and electrolyte leakage of tall fescue leaves. These data suggest that exogenous NO could protect plants, increase the amount of activated RC and improve the electron transport from oxygen evolving complex to D1 protein. Moreover, quantitative RT-PCR revealed that, in the presence of hydrogen peroxide, NO induced the gene expression of psbA, psbB, and psbC, which encode proteins belonging to subunits of PSII core reaction center (Psb) complex. These findings indicate that, as an important strategy to protect plants against heat stress, NO could improve the recovery process of PSII by the up regulation of the transcriptions of genes encoding PSII core proteins.

Photosynthetic Antenna‐Reaction Center Mimicry with a Covalently Linked Monostyryl Boron‐Dipyrromethene–Aza‐Boron‐Dipyrromethene–C60 Triad

Abstract: An efficient functional mimic of the photosynthetic antenna-reaction center has been designed and synthesized. The model contains a near-infrared-absorbing aza-boron-dipyrromethene (ADP) that is connected to a monostyryl boron-dipyrromethene (BDP) by a click reaction and to a fullerene (C60 ) using the Prato reaction. The intramolecular photoinduced energy and electron-transfer processes of this triad as well as the corresponding dyads BDP-ADP and ADP-C60 have been studied with steady-state and time-resolved absorption and fluorescence spectroscopic methods in benzonitrile. Upon excitation, the BDP moiety of the triad is significantly quenched due to energy transfer to the ADP core, which subsequently transfers an electron to the fullerene unit. Cyclic and differential pulse voltammetric studies have revealed the redox states of the components, which allow estimation of the energies of the charge-separated states. Such calculations show that electron transfer from the singlet excited ADP ((1) ADP*) to C60 yielding ADP(.+) -C60 (.-) is energetically favorable. By using femtosecond laser flash photolysis, concrete evidence has been obtained for the occurrence of energy transfer from (1) BDP* to ADP in the dyad BDP-ADP and electron transfer from (1) ADP* to C60 in the dyad ADP-C60 . Sequential energy and electron transfer have also been clearly observed in the triad BDP-ADP-C60 . By monitoring the rise of ADP emission, it has been found that the rate of energy transfer is fast (≈10(11) s(-1) ). The dynamics of electron transfer through (1) ADP* has also been studied by monitoring the formation of C60 radical anion at 1000 nm. A fast charge-separation process from (1) ADP* to C60 has been detected, which gives the relatively long-lived BDP-ADP(.+) C60 (.-) with a lifetime of 1.47 ns. As shown by nanosecond transient absorption measurements, the charge-separated state decays slowly to populate mainly the triplet state of ADP before returning to the ground state. These findings show that the dyads BDP-ADP and ADP-C60 , and the triad BDP-ADP-C60 are interesting artificial analogues that can mimic the antenna and reaction center of the natural photosynthetic systems.

Excitonic connectivity between photosystem II units: what is it, and how to measure it?

Abstract: In photosynthetic organisms, light energy is absorbed by a complex network of chromophores embedded in light-harvesting antenna complexes. In photosystem II (PSII), the excitation energy from the antenna is transferred very efficiently to an active reaction center (RC) (i.e., with oxidized primary quinone acceptor Q A), where the photochemistry begins, leading to O2 evolution, and reduction of plastoquinones. A very small part of the excitation energy is dissipated as fluorescence and heat. Measurements on chlorophyll (Chl) fluorescence and oxygen have shown that a nonlinear (hyperbolic) relationship exists between the fluorescence yield (Φ F ) (or the oxygen emission yield, $$ \Phi _{{{\text{O}}_{2} }} $$ ) and the fraction of closed PSII RCs (i.e., with reduced Q A). This nonlinearity is assumed to be related to the transfer of the excitation energy from a closed PSII RC to an open (active) PSII RC, a process called PSII excitonic connectivity by Joliot and Joliot (CR Acad Sci Paris 258: 4622–4625, 1964). Different theoretical approaches of the PSII excitonic connectivity, and experimental methods used to measure it, are discussed in this review. In addition, we present alternative explanations of the observed sigmoidicity of the fluorescence induction and oxygen evolution curves.

Exogenous melatonin affects photosynthesis in characeae Chara australis

Abstract: Melatonin was found in the fresh water characeae Chara australis. The concentrations (~4 μg/g of tissue) were similar in photosynthesizing cells, independent of their position on the plant and rhizoids (roots) without chloroplasts. Exogenous melatonin, added at 10 μM to the artificial pond water, increased quantum yield of photochemistry of photosystem II by 34%. The increased efficiency appears to be due to the amount of open reaction centers of photosystem II, rather than increased efficiency of each reaction center. More open reaction centers reflect better functionality of all photosynthetic transport chain constituents. We suggest that melatonin protection against reactive oxygen species covers not only chlorophyll, but also photosynthetic proteins in general.

Chlorophyll Biosynthesis Gene Evolution Indicates Photosystem Gene Duplication, Not Photosystem Merger, at the Origin of Oxygenic Photosynthesis

Abstract: An open question regarding the evolution of photosynthesis is how cyanobacteria came to possess the two reaction center (RC) types, Type I reaction center (RCI) and Type II reaction center (RCII). The two main competing theories in the foreground of current thinking on this issue are that either 1) RCI and RCII are related via lineage divergence among anoxygenic photosynthetic bacteria and became merged in cyanobacteria via an event of large-scale lateral gene transfer (also called "fusion" theories) or 2) the two RC types are related via gene duplication in an ancestral, anoxygenic but protocyanobacterial phototroph that possessed both RC types before making the transition to using water as an electron donor. To distinguish between these possibilities, we studied the evolution of the core (bacterio)chlorophyll biosynthetic pathway from protoporphyrin IX (Proto IX) up to (bacterio)chlorophyllide a. The results show no dichotomy of chlorophyll biosynthesis genes into RCI- and RCII-specific chlorophyll biosynthetic clades, thereby excluding models of fusion at the origin of cyanobacteria and supporting the selective-loss hypothesis. By considering the cofactor demands of the pathway and the source genes from which several steps in chlorophyll biosynthesis are derived, we infer that the cell that first synthesized chlorophyll was a cobalamin-dependent, heme-synthesizing, diazotrophic anaerobe.

Cytochrome c-coupled photosystem I and photosystem II (PSI/PSII) photo-bioelectrochemical cells

Abstract: Photo-bioelectrochemical cells are devices that use biomolecule-modified electrodes for the conversion of solar light to electrical power. We present the construction of a layered assembly of the native photosynthetic reaction centers photosystem I (PSI) and photosystem II (PSII), crosslinked by polyvinyl pyridine/methyl pyridinium and cytochrome c (Cyt. c) that act as an electron transfer mediating layer. Electrostatic interactions and glutaric dialdehyde crosslinking of the protein layers stabilize the biomolecules on the electrodes. The irradiation of the PSII/Cyt. c/PSI-modified electrodes facilitates an electron transfer cascade, where photoexcited PSII leads to O2 evolution and to the reduction of Cyt. c, with the concomitant ejection of electrons from PSI to the electrode, and the reduction of the P700+ sites by the reduced Cyt. c units.

Gymnosperms Have Increased Capacity for Electron Leakage to Oxygen (Mehler and PTOX reactions) in Photosynthesis Compared with Angiosperms

Abstract: Oxygen plays an important role in photosynthesis by participating in a number of O2-consuming reactions. O2 inhibits CO2 fixation by stimulating photorespiration, thus reducing plant production. O2 interacts with photosynthetic electron transport in the chloroplasts' thylakoids in two main ways: by accepting electrons from PSI (Mehler reaction); and by accepting electrons from reduced plastoquinone (PQ) mediated by the plastid terminal oxidase (PTOX). In this study, we show, using 101 plant species, that there is a difference in the potential for photosynthetic electron flow to O2 between angiosperms and gymnosperms. We found, from measurements of Chl fluorescence and leaf absorbance at 830 nm, (i) that electron outflow from PSII, as determined by decay kinetics of Chl fluorescence after application of a saturating light pulse, is more rapid in gymnosperms than in angiosperms; (ii) that the reaction center Chl of PSI (P700) is rapidly and highly oxidized in gymnosperms during induction of photosynthesis; and (iii) that these differences are dependent on oxygen. Finally, rates of O2 uptake measured by mass spectrometry in the absence of photorespiration were significantly promoted by illumination in dark-adapted leaves of gymnosperms, but not in those of angiosperms. The light-stimulated O2 uptake was around 10% of the maximum O2 evolution in gymnosperms and 1% in angiosperms. These results suggest that gymnosperms have increased capacity for electron leakage to oxygen in photosynthesis compared with angiosperms. The involvement of the Mehler reaction and PTOX in the electron flow to O2 is discussed.

Tight-binding model of the photosystem II reaction center: application to two-dimensional electronic spectroscopy

Abstract: We propose an optimized tight-binding electron-hole model of the photosystem II (PSII) reaction center (RC). Our model incorporates two charge separation pathways and spatial correlations of both static disorder and fast fluctuations of energy levels. It captures the main experimental features observed in time-resolved two-dimensional (2D) optical spectra at 77K: peak pattern, lineshapes and time traces. Analysis of 2D spectra kinetics reveals that specific regions of the 2D spectra of the PSII RC are sensitive to the charge transfer states. We find that the energy disorder of two peripheral chlorophylls is four times larger than the other RC pigments.

Artificial photosynthetic reaction center with a coumarin-based antenna system

Abstract: In photosynthesis, sunlight is absorbed mainly by antenna chromophores that transfer singlet excitation energy to reaction centers for conversion to useful electrochemical energy. Antennas may likewise be useful in artificial photosynthetic systems that use sunlight to make fuels or electricity. Here, we report the synthesis and spectroscopic properties of a molecular hexad comprising two porphyrin moieties and four coumarin antenna chromophores, all organized by a central hexaphenylbenzene core. Light absorbed by any of the coumarins is transferred to a porphyrin on the 1–10 ps time scale, depending on the site of initial excitation. The quantum yield of singlet energy transfer is 1.0. The energy transfer rate constants are consistent with transfer by the Forster dipole–dipole mechanism. A pyridyl-bearing fullerene moiety self-assembles to the form of the hexad containing zinc porphyrins to yield an antenna–reaction center complex. In the resulting heptad, energy transfer to the porphyrins is followed by...

The nonheme iron in photosystem II.

Abstract: Photosystem II (PSII), the light-driven water:plastoquinone (PQ) oxidoreductase of oxygenic photosynthesis, contains a nonheme iron (NHI) at its electron acceptor side. The NHI is situated between the two PQs QA and QB that serve as one-electron transmitter and substrate of the reductase part of PSII, respectively. Among the ligands of the NHI is a (bi)carbonate originating from CO2, the substrate of the dark reactions of oxygenic photosynthesis. Based on recent advances in the crystallography of PSII, we review the structure of the NHI in PSII and discuss ideas concerning its function and the role of bicarbonate along with a comparison to the reaction center of purple bacteria and other enzymes containing a mononuclear NHI site.

Fourier transform infrared difference spectroscopy for studying the molecular mechanism of photosynthetic water oxidation

Abstract: The photosystem II reaction center mediates the light-induced transfer of electrons from water to plastoquinone, with concomitant production of O2. Water oxidation chemistry occurs in the oxygen-evolving complex (OEC), which consists of an inorganic Mn4CaO5 cluster and its surrounding protein matrix. Light-induced Fourier transform infrared (FTIR) difference spectroscopy has been successfully used to study the molecular mechanism of photosynthetic water oxidation. This powerful technique has enabled the characterization of the dynamic structural changes in active water molecules, the Mn4CaO5 cluster, and its surrounding protein matrix during the catalytic cycle. This mini-review presents an overview of recent important progress in FTIR studies of the OEC and implications for revealing the molecular mechanism of photosynthetic water oxidation.

Membrane development in purple photosynthetic bacteria in response to alterations in light intensity and oxygen tension.

Abstract: Studies on membrane development in purple bacteria during adaptation to alterations in light intensity and oxygen tension are reviewed. Anoxygenic phototrophic such as the purple α-proteobacterium Rhodobacter sphaeroides have served as simple, dynamic, and experimentally accessible model organisms for studies of the photosynthetic apparatus. A major landmark in photosynthesis research, which dramatically illustrates this point, was provided by the determination of the X-ray structure of the reaction center (RC) in Blastochloris viridis (Deisenhofer and Michel, EMBO J 8:2149–2170, 1989), once it was realized that this represented the general structure for the photosystem II RC present in all oxygenic phototrophs. This seminal advance, together with a considerable body of subsequent research on the light-harvesting (LH) and electron transfer components of the photosynthetic apparatus has provided a firm basis for the current understanding of how phototrophs acclimate to alterations in light intensity and quality. Oxygenic phototrophs adapt to these changes by extensive thylakoid membrane remodeling, which results in a dramatic supramolecular reordering to assure that an appropriate flow of quinone redox species occurs within the membrane bilayer for efficient and rapid electron transfer. Despite the high level of photosynthetic unit organization in Rba. sphaeroides as observed by atomic force microscopy (AFM), fluorescence induction/relaxation measurements have demonstrated that the addition of the peripheral LH2 antenna complex in cells adapting to low-intensity illumination results in a slowing of the rate of electron transfer turnover by the RC of up to an order of magnitude. This is ascribed to constraints in quinone redox species diffusion between the RC and cytochrome bc 1 complexes arising from the increased packing density as the intracytoplasmic membrane (ICM) bilayer becomes crowded with LH2 rings. In addition to downshifts in light intensity as a paradigm for membrane development studies in Rba. sphaeroides, the lowering of oxygen tension in chemoheterotropically growing cells results in a gratuitous formation of the ICM by an extensive membrane biogenesis process. These membrane alterations in response to lowered illumination and oxygen levels in purple bacteria are under the control of a number of interrelated two-component regulatory circuits reviewed here, which act at the transcriptional level to regulate the formation of both the pigment and apoprotein components of the LH, RC, and respiratory complexes. We have performed a proteomic examination of the ICM development process in which membrane proteins have been identified that are temporally expressed both during adaptation to low light intensity and ICM formation at low aeration and are spatially localized in both growing and mature ICM regions. For these proteomic analyses, membrane growth initiation sites and mature ICM vesicles were isolated as respective upper-pigmented band (UPB) and chromatophore fractions and subjected to clear native electrophoresis for isolation of bands containing the LH2 and RC–LH1 core complexes. In chromatophores, increasing levels of LH2 polypeptides relative to those of the RC–LH1 complex were observed as ICM membrane development proceeded during light-intensity downshifts, along with a large array of other associated proteins including high spectral counts for the F1FO–ATP synthase subunits and the cytochrome bc 1 complex, as well as RSP6124, a protein of unknown function, that was correlated with increasing LH2 spectral counts. In contrast, the UPB was enriched in cytoplasmic membrane (CM) markers, including electron transfer and transport proteins, as well as general membrane protein assembly factors confirming the origin of the UPB from both peripheral respiratory membrane and sites of active CM invagination that give rise to the ICM. The changes in ICM vesicles were correlated to AFM mapping results (Adams and Hunter, Biochim Biophys Acta 1817:1616–1627, 2012), in which the increasing LH2 levels were shown to form densely packed LH2-only domains, representing the light-responsive antenna complement formed under low illumination. The advances described here could never have been envisioned when the author was first introduced in the mid-1960s to the intricacies of the photosynthetic apparatus during a lecture delivered in a graduate Biochemistry course at the University of Illinois by Govindjee, to whom this volume is dedicated on the occasion of his 80th birthday.

Age-dependent changes in the functions and compositions of photosynthetic complexes in the thylakoid membranes of Arabidopsis thaliana

Abstract: Photosynthetic complexes in the thylakoid membrane of plant leaves primarily function as energy-harvesting machinery during the growth period. However, leaves undergo developmental and functional transitions along aging and, at the senescence stage, these complexes become major sources for nutrients to be remobilized to other organs such as developing seeds. Here, we investigated age-dependent changes in the functions and compositions of photosynthetic complexes during natural leaf senescence in Arabidopsis thaliana. We found that Chl a/b ratios decreased during the natural leaf senescence along with decrease of the total chlorophyll content. The photosynthetic parameters measured by the chlorophyll fluorescence, photochemical efficiency (F v/F m) of photosystem II, non-photochemical quenching, and the electron transfer rate, showed a differential decline in the senescing part of the leaves. The CO2 assimilation rate and the activity of PSI activity measured from whole senescing leaves remained relatively intact until 28 days of leaf age but declined sharply thereafter. Examination of the behaviors of the individual components in the photosynthetic complex showed that the components on the whole are decreased, but again showed differential decline during leaf senescence. Notably, D1, a PSII reaction center protein, was almost not present but PsaA/B, a PSI reaction center protein is still remained at the senescence stage. Taken together, our results indicate that the compositions and structures of the photosynthetic complexes are differentially utilized at different stages of leaf, but the most dramatic change was observed at the senescence stage, possibly to comply with the physiological states of the senescence process.

Inhibition of chlorophyll biosynthesis at the protochlorophyllide reduction step results in the parallel depletion of Photosystem I and Photosystem II in the cyanobacterium Synechocystis PCC 6803

Abstract: In most oxygenic phototrophs, including cyanobacteria, two independent enzymes catalyze the reduction of protochlorophyllide to chlorophyllide, which is the penultimate step in chlorophyll (Chl) biosynthesis. One is light-dependent NADPH:protochlorophyllide oxidoreductase (LPOR) and the second type is dark-operative protochlorophyllide oxidoreductase (DPOR). To clarify the roles of both enzymes, we assessed synthesis and accumulation of Chl-binding proteins in mutants of cyanobacterium Synechocystis PCC 6803 that either completely lack LPOR or possess low levels of the active enzyme due to its ectopic regulatable expression. The LPOR-less mutant grew photoautotrophically in moderate light and contained a maximum of 20 % of the wild-type (WT) Chl level. Both Photosystem II (PSII) and Photosystem I (PSI) were reduced to the same degree. Accumulation of PSII was mostly limited by the synthesis of antennae CP43 and especially CP47 as indicated by the accumulation of reaction center assembly complexes. The phenotype of the LPOR-less mutant was comparable to the strain lacking DPOR that also contained <25 % of the wild-type level of PSII and PSI when cultivated under light-activated heterotrophic growth conditions. However, in the latter case, we detected no reaction center assembly complexes, indicating that synthesis was almost completely inhibited for all Chl-proteins, including the D1 and D2 proteins.

Photosynthetic electron transport in an anoxygenic photosynthetic bacterium Afifella (Rhodopseudomonas) marina measured using PAM fluorometry.

Abstract: Blue diode-based pulse amplitude modulation (PAM) technology can be used to measure the photosynthetic electron transport rate (ETR) in a purple nonsulfur anoxygenic photobacterium, Afifella (Rhodopseudomonas) marina. Rhodopseudomonads have a reaction center light harvesting antenna complex containing an RC-2 type bacteriochlorophyll a protein (BChl a RC-2-LH1) which has a blue absorption peak and variable fluorescence similar to PSII. Absorptance of cells filtered onto glass fiber disks was measured using a blue–diode-based absorptance meter (Blue-RAT) so that absolute ETR could be calculated from PAM experiments. Maximum quantum yield (Y) was ≈0.6, decreasing exponentially as irradiance increased. ETR vs irradiance (P vs E) curves fitted the waiting-in-line model (ETR = (ETRmax × E/Eopt) × exp(1 − E/Eopt)). Maximum ETR (ETRmax) was ≈1000–2000 μmol e− mg−1 BChl a h−1. Fe2+, bisulfite and thiosulfate act as photosynthetic electron donors. Optimum irradiance was ≈100 μmol m−2 s−1 PPFD even in Afifella grown in sunlight. Quantum efficiencies (α) were ≈0.3–0.4 mol e− mol hλ−1; or ≈11.8 ± 2.9 mol e− mol hλ−1 m2 μg−1 BChl a). An underlying layer of Afifella in a constructed algal/photosynthetic bacterial mat has little effect on the measured ETR of the overlying oxyphotoautotroph (Chlorella).

Beatings in electronic 2D spectroscopy suggest another role of vibrations in photosynthetic light harvesting

Abstract: Light harvesting by photosynthetic organisms is nature’s way to use solar energy for biomass growth The process starts with light absorption in so-called antenna pigments, and is followed by transfer of the excited-state energy to reaction center proteins, where the energy is converted to an electrochemical gradient across the photosynthetic membrane (1) This potential is used to drive all energy-consuming processes in the photosynthetic organisms Energy transfer in light harvesting occurs via various transport regimes The limiting cases are the Forster-type incoherent excitation hopping from pigment to pigment and the exciton relaxation between energy levels, which are coherently delocalized over several antenna molecules In both transfer regimes, vibrations play an important role in fulfilling the resonance condition of the rate equations However, this is not the only way vibrations are used in light harvesting The article in PNAS by Tiwari et al (2) discusses the role of anticorrelated nuclear motions in driving energy transfer via nonadiabatic coupling (Fig 1) The authors argue that the beatings observed in electronic 2D spectroscopy experiments of various antenna complexes are mainly of vibrational origin and provide evidence for this transport mechanism

Temporal and spectral characterization of the photosynthetic reaction center from Heliobacterium modesticaldum

Abstract: A time-resolved spectroscopic study of the isolated photosynthetic reaction center (RC) from Heliobacterium modesticaldum reveals that thermal equilibration of light excitation among the antenna pigments followed by trapping of excitation and the formation of the charge-separated state P800 +A0 – occurs within ~25 ps. This time scale is similar to that reported for plant and cyanobacterial photosystem I (PS I) complexes. Subsequent electron transfer from the primary electron acceptor A0 occurs with a lifetime of ~600 ps, suggesting that the RC of H. modesticaldum is functionally similar to that of Heliobacillus mobilis and Heliobacterium chlorum. The (A0 – − A0) and (P800 + − P800) absorption difference spectra imply that an 81-OH-Chl a F molecule serves as the primary electron acceptor and occupies the position analogous to ec3 (A0) in PS I, while a monomeric BChl g pigment occupies the position analogous to ec2 (accessory Chl). The presence of an intense photobleaching band at 790 nm in the (A0 – − A0) spectrum suggests that the excitonic coupling between the monomeric accessory BChl g and the 81-OH-Chl a F in the heliobacterial RC is significantly stronger than the excitonic coupling between the equivalent pigments in PS I.

Proton displacements coupled to primary electron transfer in the Rhodobacter sphaeroides reaction center.

Abstract: Using first-principles molecular dynamics (AIMD) and constrained density functional theory (CDFT) we identify the pathway of primary electron transfer in the R. Sphaeroides reaction center from the special pair excited state (P*) to the accessory bacteriochlorophyll (BA). Previous AIMD simulations on the special pair (PLPM) predicted a charge-transfer intermediate formation through the excited-state relaxation along a reaction coordinate characterized by the rotation of an axial histidine (HisM202). To account for the full electron transfer we extend the model to include the primary acceptor BA. In this extended model, the LUMO is primarily localized on the acceptor BA and extends over an interstitial water (water A) that is known to influence the rate of electron transfer (Potter et al. Biochemistry 2005 280, 27155-27164). A vibrational analysis of the dynamical trajectories gives a frequency of 30-35 cm(-1) for a molecular motion involving the hydrogen-bond network around water A, in good agreement with experimental findings (Yakovlev et al. Biochemistry, 2003, 68, 603-610). In its binding pocket water A can act as a switch by breaking and forming hydrogen bonds. With CDFT we calculate the energy required to the formation of the charge-separated state and find it to decrease along the predicted anisotropic reaction coordinate. Furthermore, we observe an increased coupling between the ground and charge-separated state. Water A adapts its hydrogen-bonding network along this reaction coordinate and weakens the hydrogen bond with HisM202. We also present AIMD simulations on the radical cation (P(•+)) showing a weakening of the hydrogen bond between HisL168 and the 3(1)-acetyl of PL. This work demonstrates how proton displacements are crucially coupled to the primary electron transfer and characterizes the reaction coordinate of the initial photoproduct formation.

Protein dynamics to optimize and control bacterial photosynthesis

Abstract: Proteins function by sampling conformational sub-states within a given fold. How this configurational flexibility and the associated protein dynamics affect the rates of chemical reactions are open questions. The difficulty in exploring this issue arises in part from the need to identify the relevant nuclear modes affecting the reaction rate for each characteristic time-scale of the reaction. Proteins as reaction media display a hierarchy of such nuclear modes, of increasingly collective character, that produce both a broad spectrum of static fluctuations and a broad spectrum of relaxation times. In order to understand the effect of protein dynamics on reaction rates, we have chosen to study a sub-nanosecond electron transfer reaction between the bacteriopheophytin and primary quinone cofactors of the photosynthetic bacterial reaction center. We show that dynamics affects the activation barrier of the reaction through a dynamical restriction of the configurational space sampled by the protein–water solvent on the reaction time-scale. The modes which become dynamically arrested on the reaction time-scale of hundreds of picoseconds are related to elastic motions of the protein that are strongly coupled to the hydration layer of water. Several mechanistic consequences for protein electron transfer emerge from this picture. Importantly, energy parameters used to define the activation barrier of electron transfer reactions lose their direct connection to equilibrium thermodynamics and become dependent in a very direct way on the relative magnitudes of the reaction and nuclear reorganization time-scales. As a result, the energetics of protein electron transfer need to be defined on each specific reaction time-scale. This perspective offers a mechanism to optimize protein electron transfer by tuning the reaction rate to the relaxation spectrum of the reaction coordinate.

Evolutionary Origins of the Photosynthetic Water Oxidation Cluster: Bicarbonate Permits Mn2+ Photo-oxidation by Anoxygenic Bacterial Reaction Centers

Abstract: The enzyme that catalyzes water oxidation in oxygenic photosynthesis contains an inorganic cluster (Mn4 CaO5 ) that is universally conserved in all photosystem II (PSII) protein complexes. Its hypothesized precursor is an anoxygenic photobacterium containing a type 2 reaction center as photo-oxidant (bRC2, iron-quinone type). Here we provide the first experimental evidence that a native bRC2 complex can catalyze the photo-oxidation of Mn(2+) to Mn(3+) , but only in the presence of bicarbonate concentrations that allows the formation of (bRC2)Mn(2+) (bicarbonate)1-2 complexes. Parallel-mode EPR spectroscopy was used to characterize the photoproduct, (bRC2)Mn(3+) (CO3 (2-) ), based on the g tensor and (55) Mn hyperfine splitting. (Bi)carbonate coordination extends the lifetime of the Mn(3+) photoproduct by slowing charge recombination. Prior electrochemical measurements show that carbonate complexation thermodynamically stabilizes the Mn(3+) product by 0.9-1 V relative to water ligands. A model for the origin of the water oxidation catalyst is presented that proposes chemically feasible steps in the evolution of oxygenic PSIIs, and is supported by literature results on the photoassembly of contemporary PSIIs.