Capture frequency of excitations and energy transfer between photosynthetic units in the photosystem II

The connections which exist, in the photosynthetic apparatus, between the spatial arrangement of chlorophyll and the movement of excitation energy are discussed.The capture frequency of excitations in the photosystem II is analysed. At the microscopic level of a photosynthetic unit two stages are studied: the propagation of the excitation to the reaction centre and the photochemical utilization of the excitation by the centre. It is shown that the transport process is not a limiting one. It implies that the capture frequency depends on the reaction centre state. Thus it is possible to distinguish eight states for a photosynthetic unit of system II.At the macroscopic level of a set of units, the analysis of the fluorescence yield-fluctuations shows that these units are not isolated. It also indicates that the fluorescence emitted by the photosynthetic apparatus originates almost entirely from the system II, and that the reaction centres are traps for excitations whatever their states.     

Photosystem II reaction centre quenching: mechanisms and physiological role

Dissipation of excess absorbed light energy in eukaryotic photoautotrophs through zeaxanthin- and DeltapH-dependent photosystem II antenna quenching is considered the major mechanism for non-photochemical quenching and photoprotection. However, there is mounting evidence of a zeaxanthin-independent pathway for dissipation of excess light energy based within the PSII reaction centre that may also play a significant role in photoprotection. We summarize recent reports which indicate that this enigma can be explained, in part, by the fact that PSII reaction centres can be reversibly interconverted from photochemical energy transducers that convert light into ATP and NADPH to efficient, non-photochemical energy quenchers that protect the photosynthetic apparatus from photodamage. In our opinion, reaction centre quenching complements photoprotection through antenna quenching, and dynamic regulation of photosystem II reaction centre represents a general response to any environmental condition that predisposes the accumulation of reduced Q(A) in the photosystem II reaction centres of prokaryotic and eukaryotic photoautotrophs. Since the evolution of reaction centres preceded the evolution of light harvesting systems, reaction centre quenching may represent the oldest photoprotective mechanism.     

In vitro models for photosynthetic energy conversion

A summary is presented of recent work on the photochemistry of chlorophyll in solution. It is shown that reactions occur which are close counterparts ofin vivo photoprocesses. These are (a) photoproduction of chlorophyll cation radical (analog of photosystem I reaction centre primary photoprocess), (b) one-electron phototransfer from bacterio-chlorophyll to quinone (analog of bacterial reaction centre primary photoprocess), (c) chlorophyll photosensitized one-electron transfer from hydroxylic compounds to quinone (analog of photosystem II reaction centre photoprocess). The mechanisms of these reactions and their implications for photosynthetic energy conversion are discussed.     

Photosynthetic antenna proteins: 100 ps before photochemistry starts

All photosynthetic organisms require a light harvesting system to funnel excitation energy towards the photosynthetic reaction centre, a process which can take 100 ps. Laser spectroscopy allows us to measure rates of energy transfer between pigments of the light harvesting system for the first time. These rates are correlated with models of the light harvesting apparatus.     

Re-emerging structures: continuing crystallography of the bacterial reaction centre

The reaction centre is nature's solar battery, and is found in a number of variations on a common theme in plants, algae and photosynthetic bacteria. During the last 20 years, a combination of X-ray crystallography, spectroscopy and mutagenesis has provided increasingly detailed insights into the mechanism of light energy transduction in the bacterial reaction centre. This mini-review looks at the application of X-ray crystallography to the bacterial reaction centre, focussing in particular on recent information on the structural consequences of site-directed mutagenesis, the roles played by water molecules in the reaction centre, the mechanism of ubiquinone reduction, and studies of the phospholipid environment of the protein.     

Reaction centre quenching of excess light energy and photoprotection of photosystem II

In addition to the energy dissipation of excess light occurring in PSII antenna via the xanthophyll cycle, there is mounting evidence of a zeaxanthin-independent pathway for non-photochemical quenching based within the PSII reaction centre (reaction centre quenching) that may also play a significant role in photoprotection. It has been demonstrated that acclimation of higher plants, green algae and cyanobacteria to low temperature or high light conditions which potentially induce an imbalance between energy supply and energy utilization is accompanied by the development of higher reduction state of Q_A and higher resistance to photoinhibition (Huner et al., 1998). Although this is a fundamental feature of all photoautotrophs, and the acquisition of increased tolerance to photoinhibition has been ascribed to growth and development under high PSII excitation pressure, the precise mechanism controlling the redox state of Q_A and its physiological significance in developing higher resistance to photoinhibition has not been fully elucidated. In this review we summarize recent data indicating that the increased resistance to high light in a broad spectrum of photosynthetic organisms acclimated to high excitation pressure conditions is associated with an increase probability for alternative non-radiative P680⁺Q_A ^- radical pair recombination pathway for energy dissipation within the reaction centre of PSII. The various molecular mechanisms that could account for non-photochemical quenching through PSII reaction centre are also discussed.     

Is the 9 kDa thylakoid membrane phosphoprotein functionally and structurally analogous to the 'H' subunit of bacterial reaction centres?

Although the amino acid sequence of the 9 kDa (phospho)protein of chloroplasts has been determined, the function of this thylakoid membrane protein in photosynthetic electron transport and the reason for its physiological control remains unclear. In this paper, I briefly review the evidence which indicates that the phosphorylation of the 9 kDa protein results in a partial inhibition of photosynthetic oxygen evolution by increasing the stability of the semiquinone bound to QA the primary, plastoquinone-binding site of photosystem II (PS II). I propose that in its dephosphorylated state, the 9 kDa thylakoid membrane protein may serve PS II to ensure efficient photochemical charge separation by aiding the transfer of reducing equivalents out of the reaction centre to the attendant plastoquinone pool. This function is analogous to that proposed for the H-subunit of the reaction centre of photosynthetic eubacteria. Whether these two proteins have evolved from a common ancestral reaction centre protein is discussed in the light of a comparison of their amino acid sequences and predicted secondary structures.     

A general model for regulation of photosynthetic unit function by protein phosphorylation

We propose that regulatory effects of membrane protein phosphorylation in photosynthetic systems result in all cases from simultaneous phosphorylation by a single kinase of the polypeptides of two intrinsic pigment-protein complexes, with phosphorylation leading to their mutual electrostatic repulsion in a direction parallel to the membrane plane and therefore to decreased excitation energy transfer between them. One complex is a peripheral light-harvesting complex and the other is bound to the reaction centre and functions as a link in excitation energy transfer. Immediate effects of phosphorylation are therefore decreased absorption cross-section together with decreased cooperativity of photosynthetic units. This general model applies equally to photosystem II of green plants, algae and cyanobacteria, as well as to the single photosystem of purple bacteria. Special cases of this general model permit increased excitation energy transfer to one type of reaction centre at the expense of another, and this may occur even in laterally homogeneous membranes that are uniformly unappressed.     

Variety in excitation energy transfer processes from phycobilisomes to photosystems I and II

Photosynthesis Research - The light-harvesting antennas of oxygenic photosynthetic organisms capture light energy and transfer it to the reaction centers of their photosystems. The light-harvesting...     

Photosystem II photochemistry and photosynthetic pigment composition in salt-adapted halophyte Artimisia anethifolia grown under outdoor conditions

The effects of high salinity (0-400 mmol/L NaCl) on photosystem II (PSII) photochemistry and photosynthetic pigment composition were investigated in the halophyte Artimisia anethifolia grown under outdoor conditions and exposed to full sunlight. High salinity resulted in an inhibition in plant growth and a significant accumulation of sodium and chloride in leaves. However, high salinity induced no effects on the actual PSII efficiency, the efficiency of excitation energy capture by open PSII reaction centres, photochemical quenching, and non-photochemical quenching at midday. High salinity also induced neither changes in the maximum efficiency of PSII photochemistry, the efficiency with which a trapped exciton can move an electron into the electron transport chain further than QA and the quantum yield of electron transport beyond QA, nor changes in absorption, trapping and electron transport fluxes per PSII reaction centre. No significant changes were observed in the levels of neoxanthin, lutein, beta-carotene, violaxanthin, antheraxanthin, and zeaxanthin expressed on a total chlorophyll basis in salt-adapted plants. Our results suggest that Artimisia anethifolia showed high resistance not only to high salinity, but also to photoinhibition even if it was treated with high salinity as high as 400 mmol/L NaCl and exposed to full sunlight. The results indicate that tolerance of PSII to high salinity and photoinhibition can be viewed as an important strategy for Artimisia anethifolia, a halophyte plant, to grow in very high saline soil.     

On the Role of Basic Residues in Adapting the Reaction Centre–LH1 Complex for Growth at Elevated Temperatures in Purple Bacteria

The purple photosynthetic bacterium Thermochromatium tepidum is a moderate thermophile, with a growth optimum of 48–50 °C. The X-ray crystal structure of the reaction centre from this organism has been determined, and compared with that from mesophilic bacteria such as Blastochloris viridis and Rhodobacter sphaeroides (Nogi T et al. (2000) Proc Natl Acad Sci USA 97: 13561–13566). Structural features that could contribute to the enhanced thermal stability of the Thermochromatium tepidum reaction centre were discussed, including three arginine residues exposed at the periplasmic side of the membrane that are not present in reaction centres from mesophilic organisms, and potentially could increase the affinity of the complex for the surrounding membrane. In the present report these arginine residues, plus a histidine identified from an extensive sequence alignment, were engineered into structurally homologous positions in the Rhodobacter sphaeroides reaction centre, and the effect on the thermal stability of the Rhodobacter sphaeroides complex was examined. We find that these residues do not enhance the thermal stability of the reaction centre, as assessed by absorbance spectroscopy of the bacteriochlorin cofactors in membrane-bound reaction centres. Possible roles of these residues in the Thermochromatium tepidum reaction centre are discussed, and it is proposed that they facilitate stronger binding of the reaction centre to the encircling LH1 antenna complex, through ionic interactions with acidic residues at the C-terminal end of the LH1 α-polypeptide. Such an interaction could enhance the stability of the so-called ‘RC–LH1 core’ complex that is formed between the reaction centre and the LH1 antenna, and which represents the minimal functional photosynthetic unit in all known purple photosynthetic bacteria. Stronger bonding interactions between the two complexes could also contribute to an increase in the rigidity of the photosynthetic membrane in Thermochromatium tepidum, in accord with the general finding that the cytoplasmic membrane from thermophilic eubacteria is less fluid than its counterpart in mesophilic bacteria.     

Mutations that modify or exclude binding of the QA ubiquinone and carotenoid in the reaction center from Rhodobacter sphaeroides

Three single-site mutations have been introduced at positions close to the Q_A ubiquinone in the reaction centre from Rhodobacter sphaeroides. Two of these mutations, Ala M260 to Trp and Ala M248 to Trp, result in a reaction centre that does not support photosynthetic growth of the bacterium, and in which electron transfer to the Q_A ubiquinone is abolished. In the reaction centre with an Ala to Trp mutation at the M260 residue, electron transfer from the primary donor to the acceptor bacteriopheophytin is not affected by the mutation, but electron transfer from the acceptor bacteriopheophytin to Q_A is not observed. The most likely basis for these effects is that the mutation produces a structural change that excludes binding of the Q_A ubiquinone. A third mutation, Leu M215 to Trp, produces a reaction centre that has an impaired capacity for supporting photosynthetic growth. The mutation changes the nature of ubiquinone binding at the Q_A site, and renders the site sensitive to quinone site inhibitors such as o- phenanthroline. Adopting a similar approach, in which a small residue located close to a cofactor is changed to a more bulky residue, we show that the reaction centre can be rendered carotenoid-less by the mutation Gly M71 to Leu.     

Effects of exogenous phenolic acids on photosystem functions and photosynthetic electron transport rate in strawberry leaves

Our study investigated the physiological and biochemical basis for the effects of exogenous phenolic acids on the function of the photosynthetic apparatus and photosynthetic electron transport rate in strawberry seedlings. Potted seedlings of the strawberry (Fragaria × ananassa Duch.) were used. Syringic acid inhibited net photosynthetic rate and water-use efficiency decreased. Additionally, primary quinone electron acceptor of the PSII reaction centre, the PSII reaction centre and the oxygen evolving complex were also impaired. Both the maximum quantum yield of the PSII primary photochemistry and the performance index on absorption basis were depressed, resulting in reduced function of the photosynthetic electron transport chain. Otherwise, low phthalic acid concentrations enhanced photosynthetic capacity, while high concentrations showed opposite effects. Syringic acid exhibited a higher toxic effect than that of phthalic acid which was more evident at higher concentrations.     

Lipids in photosynthetic reaction centres: structural roles and functional holes

Photosynthetic proteins power the biosphere. Reaction centres, light harvesting antenna proteins and cytochrome b(6)f (or bc(1)) complexes are expressed at high levels, have been subjected to an intensive spectroscopic, biochemical and mutagenic analysis, and several have been characterised to an informatively high resolution by X-ray crystallography. In addition to revealing the structural basis for the transduction of light energy, X-ray crystallography has brought molecular insights into the relationships between these multicomponent membrane proteins and their lipid environment. Lipids resolved in the X-ray crystal structures of photosynthetic proteins bind light harvesting cofactors, fill intra-protein cavities through which quinones can diffuse, form an important part of the monomer-monomer interface in multimeric structures and may facilitate structural flexibility in complexes that undergo partial disassembly and repair. It has been proposed that individual lipids influence the biophysical properties of reaction centre cofactors, and so affect the rate of electron transfer through the complex. Lipids have also been shown to be important for successful crystallisation of photosynthetic proteins. Comparison of the three types of reaction centre that have been structurally characterised reveals interesting similarities in the position of bound lipids that may point towards a generic requirement to reinforce the structure of the core electron transfer domain. The crystallographic data are also providing new opportunities to find molecular explanations for observed effects of different types of lipid on the structure, mechanism and organisation of reaction centres and other photosynthetic proteins.     

Kinetic analysis of the chlorophyll fluorescence inductions from chloroplasts blocked with 3-(3,4-dichlorophenyl)-1,1-dimethylurea.

1. The induction of Photosystem II chlorophyll fluorescence from chloroplasts blocked with 3-(3,4-dichlorophenyl)-1,1-dimethylurea and uncoupled with gramicidin has been measured. 2. In agreement with other authors it was found that the addition of cations to chloroplasts suspended in a low-cation medium not only stimulated the intensity of fluorescence but also changed the shape of the induction from being nearly exponential to being sigmoid. 3. A new theory of the photosynthetic unit of Photosystem II (Paillotin, G. (1976) J. Theor. Biol. 58, 237--252) was used to analyse the fluorescence inductions. 4. A comparison of the results of the Paillotin model with the experimental data suggests that excitation energy is not able to migrate between all the photosynthetic units of a photosynthetic domain. However, it is concluded that excitation energy may migrate from one photosynthetic unit to another, and that the energy migration is in competition with other processes leading to the decay of the excitation within Photosystem II. 5. It is suggested that the size of the "functional" photosynthetic unit, defined as the number of chlorophyll molecules that may communicate with a reaction centre, is variable.     

Regulation of the photosynthetic apparatus under fluctuating growth light

Safe and efficient conversion of solar energy to metabolic energy by plants is based on tightly inter-regulated transfer of excitation energy, electrons and protons in the photosynthetic machinery according to the availability of light energy, as well as the needs and restrictions of metabolism itself. Plants have mechanisms to enhance the capture of energy when light is limited for growth and development. Also, when energy is in excess, the photosynthetic machinery slows down the electron transfer reactions in order to prevent the production of reactive oxygen species and the consequent damage of the photosynthetic machinery. In this opinion paper, we present a partially hypothetical scheme describing how the photosynthetic machinery controls the flow of energy and electrons in order to enable the maintenance of photosynthetic activity in nature under continual fluctuations in white light intensity. We discuss the roles of light-harvesting II protein phosphorylation, thermal dissipation of excess energy and the control of electron transfer by cytochrome b₆f, and the role of dynamically regulated turnover of photosystem II in the maintenance of the photosynthetic machinery. We present a new hypothesis suggesting that most of the regulation in the thylakoid membrane occurs in order to prevent oxidative damage of photosystem I.     

Evolutionary relationships between "Q-type" photosynthetic reaction centres: hypothesis-testing using parsimony

Hypotheses concerning the evolutionary relationships between "Q-type" photosynthetic reaction centres are tested using amino acid parsimony analysis of subunit sequences and an alignment based on dot matrix comparisons. Strong evidence is found for independent gene duplications having produced the L and M subunits of the photosynthetic purple bacterial reaction centre and D1 and D2 of Photosystem-II. Much support is also found for the L and M subunits of the green filamentous bacterium Chloroflexus aurantiacus arising from the same gene duplication as the purple bacterial subunits, suggesting there was an ancestral bacterial heterodimeric reaction centre. These conclusions caution against over-extrapolation from the purple bacterial reaction centre to Photosystem-II, and suggest that the latter is more ancient than previously supposed.     

Localisation of reaction centre and light harvesting complexes in the photosynthetic unit of Rhodopseudomonas viridis

The photosynthetic unit of Rhodopseudomonas viridis contains a reaction centre (P960) and a light harvesting complex (B1015). Immune electron microscopy combined with image processing has allowed the central core of the photosynthetic unit to be identified as the reaction centre and the surrounding protein ring as the light harvesting complex. This light harvesting complex, subdivided into twelve subunits was shown to contain 24 bacteriochlorophyll b molecules. A model is presented which may account for the far red shift of the Qy absorption of the bacteriochlorophyll b molecules in vivo.     

Singlet oxygen production in photosynthesis

A photosynthetic organism is subjected to photo-oxidative stress when more light energy is absorbed than is used in photosynthesis. In the light, highly reactive singlet oxygen can be produced via triplet chlorophyll formation in the reaction centre of photosystem II and in the antenna system. In the antenna, triplet chlorophyll is produced directly by excited singlet chlorophyll, while in the reaction centre it is formed via charge recombination of the light-induced charge pair. Changes of the mid-point potential of the primary quinone acceptor in photosystem II modulate the pathway of charge recombination in photosystem II and influence the yield of singlet oxygen production. Singlet oxygen can be quenched by beta-carotene, alpha-tocopherol or can react with the D1 protein of photosystem II as target. If not completely quenched, it can specifically trigger the up-regulation of the expression of genes which are involved in the molecular defence response of plants against photo-oxidative stress.     

Lateral heterogeneity in the distribution of chlorophyll-protein complexes of the thylakoid membranes of spinach chloroplasts

The lateral distribution of the main chlorophyll-protein complexes between appressed and non-appressed thylakoid membranes has been studied. The reaction centre complexes of Photosystems I and II and the light-harvesting complex have been resolved by an SDS-polyacrylamide gel electrophoretic method which permits most of the chlorophyll to remain protein-bound.

The analyses were applied to subchloroplast fractions shown to be derived from different thylakoid regions. Stroma thylakoids were separated from grana stacks by centrifugation following chloroplast disruption by press treatment or digitonin. Vesicles derived from the grana partitions were isolated by aqueous polymer two-phase partition. A substantial depletion in the amount of Photosystem I chlorophyll-protein complex and an enrichment in the Photosystem II reaction centre complex and the light-harvesting complex occurred in the appressed grana partition region. The high enrichment in this fraction compared to grana stack fractions derived from press or digitonin treatments, suggests that the grana Photosystem I is restricted mainly to the non-appressed grana end membranes and margins, and that the grana partitions possess mainly Photosystem II reaction centre complex and the light-harvesting complex.

In contrast, stroma thylakoids are highly enriched in the Photosystem I reaction centre complex. They possess also some 10–20% of the total Photosystem II reaction centre complex and the light-harvesting complex.

The ratio of light-harvesting complex to Photosystem II reaction centre complex is rather constant in all subchloroplast fractions suggesting a close association between these complexes. This was not so for the ratio of light-harvesting complex and the Photosystem I reaction centre complex.

The lateral heterogeneity in the distribution of the photosystems between appressed and non-appressed membranes must have a profound impact on current understanding of both the distribution of excitation energy and photosynthetic electron transport between the photosystems.