Photochemical charge separation can be described, in general terms, as the reaction through which the lowest singlet excited state residing on the chromophores composing the reaction centre (RC*) is converted into a radical pair of the form [D+A], where D(+) is the (primary) electron donor and A() is the (primary) electron acceptor. Nuijs AM, van Gorkom HJ, Plijter JJ, Duysens LNM. Analysis of a range of PSII-LHCII supercomplexes harbouring antenna of progressively increasing sizes, purified by mild detergent treatment of purified grana membranes [62], showed that the average excited state migration time did indeed show an almost linear dependence to the dimension of chromophore array, thus respecting, to good approximation, the theoretical predictions. Kitajima M, Butler WL. Forti G, Elli G. Stimulation of photophosphorylation by ascorbate as a function of light intensity. Barber J, Morris E, Buchel C. Revealing the structure of the photosystem II chlorophyll binding proteins CP43 and CP47. Despite differences in the monomeric Lhca [54, 114], each dimer has remarkably similar properties, as both dimers have a Chl a/b ratio of ~3.7 and a Chls/Cars ratio of ~4.6-4.8 [115]. Remelli R, Varotto C, Sandona D, Croce R, Bassi R. Chlorophyll binding to monomeric light-harvesting complex.A mutation analysis of chromophore-binding residues. Sinclair J, Park YI, Chow WS, Anderson JM. Miloslavina Y, Wehner A, Lambrev PH, Wientjes E, Reus M, Garab G, Croce R, Holzwarth AR. chlorophyll Table of Contents Similar conclusions were obtained by the analysis of in vitro reconstituted antenna complexes by changing the carotenoid content during protein refolding [312]. Photosynthetic reaction centres are most commonly hetero-dimer pigment-protein complexes and this is the case for all known oxygenic photosystems. Nanosecond reduction kinetics of photooxodized chlorophyll aII (P-680) in single flashes as a probe for the electron pathway, H+- release and charge accumulation in the O-2-evolving complex. Holzwarth AR, Miloslavina Y, Nilkens M, Jahns P. Identification of two quenching sites active in the regulation of photosynthetic light-harvesting studied by time-resolved fluorescence. The modular architecture of PSII core subunits, compared with the more monolithic organisation of the PSI core, appears to serve at the level of the rapid turnover of the PSII reaction centre, due to its higher sensitivity to photoinhibition. Roach T, Krieger-Liszkay A. In State I instead, a thylakoid peripheral protein (TAP38/PPH1) [420, 421] dephosphorylates LHCII upon which it migrates back to PSII. Sarvikas P, Tyystjarvi T, Tyystjarvi E. Kinetics of prolonged photoinhibition revisited photoinhibited Photosystem II centres do not protect the active ones against loss of oxygen evolution. Carbonera D, Agostini G, Morosinotto T, Bassi R. Quenching of chlorophyll triplet states by carotenoids in reconstituted Lhca4 subunit of peripheral light-harvesting complex of photosystem I. Croce R, Mozzo M, Morosinotto T, Romeo A, Hienerwadel R, Bassi R. Singlet and triplet state transitions of carotenoids in the antenna complexes of higher-plant Photosystem I. Trebst A, Depka B. Styring S, Virgin I, Ehrenberg A, Andersson B. Differently from the case of Chl, the population of 3Car* by intersystem crossing occurs with a very low yield, due to the extremely short excited state lifetime of 1Car* (1-3 ps). [3], This photosystem is known as PSI because it was discovered before PhotosystemII, although future experiments showed that Photosystem II is actually the first enzyme of the photosynthetic electron transport chain. LHCII, the major antenna complex of PSII, is a heterotrimer composed by Lhcb1, Lhcb2 and Lhcb3 subunits [103, 117-119]. van Roon H, van Breemen JF, de Weerd FL, Dekker JP, Boekema EJ. Supramolecular organization of thylakoid membrane proteins in green plants. Jennings RC, Garlaschi FM, Forti G. Studies on the slow fluorescence decline in isolated chloroplasts. This is particularly evident for the Lhcb1 and Lhcb2 proteins which are encoded by several nuclear genes found in variable but always elevated number in different plant genomes [97, 103, 104]. sharing sensitive information, make sure youre on a federal Croce R, Chojnicka A, Morosinotto T, Ihalainen JA, van Mourik F, Dekker JP, Bassi R, van Grondelle R. The low-energy forms of photosystem I light-harvesting complexes Spectroscopic properties and pigment-pigment interaction characteristics. Groot ML, Frese RN, de Weerd FL, Bromek K, Pettersson A, Peterman EJ, van Stokkum IH, van Grondelle R, Dekker JP. It is possible to compare the properties of the two photosystems by an approximation in which the events occurring on two active branches of PSI are considered as an overall process that can be reduced to the monodirectional case. Hiyama T, Ke B. In both cases, most of the plant PSII complexes are present as dimers. In particular, it has been suggested to act as a superoxide anion scavenger [140], to be involved in the control of LHCII trimer-trimer interactions [141], as well as to undergo molecular rearrangements which can be used as spectroscopic markers accompanying the onset of NPQ [142-144]. 3P680*, like other Chl triplet states, reacts with molecular oxygen to produce singlet oxygen that can easily destroy proteins and pigments of PSII [243, 319, 327, 328]. Even though similar overall structural arrangements of the cofactors involved in primary charge separation reactions exist in PSI and PSII, it is now solidly established that there are profound differences in the molecular mechanisms of such reactions. Toward an understanding of the mechanism of nonphotochemical quenching in green plants. Federal government websites often end in .gov or .mil. Moreover, NPQ has also the effect of modulating (lowering) PSII quantum efficiency as a function of light intensity, a process that is also likely to contribute to the balance of electron flow through the thylakoid membrane, which is the principal source of ATP production in the leaves, avoiding, for instance, that the PQ remains in an almost fully reduced state under saturating light regimes. Analyses of different PSI mutants showed that the PsaH subunit is essential for the docking of LHCII [422], but also other subunits are important (for instance PsaL, PsaO and probably PsaP) for the formation of the interaction. [15], Ferredoxin (Fd) is a soluble protein that facilitates reduction of NADP+ to NADPH. Biochemistry. There are some clear differences between the special pairs of PSI and PSII: i) the absorption spectrum is red shifted in PSI [238-240], showing a bleaching upon oxidation at ~700 nm (P700) compared to that of P680 (~685 in intact systems) [241-243]; ii) when comparing the P+P (or 3PP) difference spectra, a very resolved spectral structure is observed in the case of P700 showing features at ~ 685 and 675 nm [244-247], whereas almost a single bleaching peaking at 682-685 nm, with a shoulder near 680 nm, is observed for the case of P680+P680 difference spectrum [241, 248-250]; these differences are likely to arise from the stronger interaction with the neighbouring pigments in the case of PSI reaction centre pigments compared to those comprising the PSII RC; iii) there is a the large difference in redox potential of the P700+/P700 (~500 mV) [251, 252] with respect to the P680+/P680 (~1.2 V) [41, 230, 253] redox couples, which is required for the different catalytic activity of the two reactions centres (Table 11); iv) finally, the cation species have markedly different lifetimes. First of all it is a constant feature of such organisms as red algae and cyanobacteria that perform photosynthesis at efficiencies similar to those of higher plants. On the other hand considerable evidence has also accumulated indicating that there is a partial but significant diffusion limited component in PSII, which accounts for 20-30% of the overall trapping time. Photosystem This constitutes a diffusion limitation to the overall trapping kinetics, which is of a different nature with respect to the pure antenna size affect (see below). Both of the two cofactor chains is coordinated preferentially by one of the two RC subunits, so that normally the ET chain is referred to accordingly, even though there are also cross over in the cofactor co-ordination pattern which is the result of the complex handshake arrangement of the TMH in the RC heterodimer. The long-wavelength chlorophyll states of plant LHCI at room temperature: a comparison with PSI-LHCI. Several other mechanisms that can lead to photooxidative damage, particularly at the level of PSII, have been discussed. However, each Lhc complex can have different isoforms. Sugiura M, Inoue Y. Beddard GS, Porter G. Concentration quenching in chlorophyll. The suggestion that entropic forces may be involved [83] does not seem to take into account the fact that photosystem separation, associated with grana formation, leads to a large decrease in the membrane configurational entropy, due to the lateral separation of the two photosystems, which must be considered if this idea is to be considered feasible. Mathis P, Butler WL, Satoh K. Carotenoid triplet state and chlorophyll fluorescence quenching in chloroplasts and sub chloroplasts particles. Ruban AV, Young A, Horton P. Modulation of chlorophyll fluorescence quenching in isolated light har-vesting complex of Photosystem II. Anderson JM, Chow WS, Goodchild DJ. oxidation of the accessory Chl eC2 and concomitant reduction of Chl A0. Johnson MP, Ruban AV. CD spectroscopy provides evidence for excitonic interactions involving red-shifted chlorophyll forms in photosystem I. Morosinotto T, Castelletti S, Breton J, Bassi R, Croce R. Mutation analysis of Lhca1 antenna complex - Low energy absorption forms originate from pigment-pigment interactions. This analysis showed that the properties of the single Lhca are different from each other [53, 54, 114]: in particular Lhca3 and Lhca4 have a far-red shifted absorption/fluorescence spectrum as compared with Lhca1 and Lhca2, due to the presence of a particular environment that gives rise to low energy Chls forms [53], with the most red-shifted form associated with Lhca4. The Cyt b559, tightly bound to the D1-D2 heterodimer, is involved in alternative electron transfer processes within the PSII RC [28]. Photosystem II (PSII) is a multi-component pigment-protein complex that is responsible for water splitting, oxygen evolution, and plastoquinone reduction. Surface charges the heterogeneous lateral distribution of the two photosystems, and thylakoid stacking. Crouchman S, Ruban A, Horton P. PsbS enhances nonphotochemical fluorescence quenching in the absence of zeaxanthin. In the following paragraph we will discuss aspects that can be considered common to both reaction centres and those in which they differ markedly. These electrons are moved in pairs in an oxidation/reduction process from P700* to electron acceptors, leaving behind P700+. In Light Harvesting Antenna in Photosynthesis. Therefore a structural model of plant PSII would be of inestimable value for the understanding of PSII functioning as well as for the interpretation of biochemical and spectroscopic data. [4] Louis Duysens first proposed the concepts of Photosystems I and II in 1960, and, in the same year, a proposal by Fay Bendall and Robert Hill assembled earlier discoveries into a coherent theory of serial photosynthetic reactions. WebThe photosystem in green bacteria is related to photosystem I of higher plants, whereas that in purple bacteria is related to photosystem II, which provides some indication of an evolutionary trail from bacteria to plants ( see photosynthesis: The process of photosynthesis: the light reactions). Protein-cofactor interactions in bioenergetic complexes the role of the A1A and A1B phylloquinones in Photosystem I. Srinivasan N, Santabarbara S, Rappaport F, Carbonera D, Redding K, van der Est A, Golbeck JH. At the same time, it is worth noting that both zeaxanthin and lutein are important for the establishment of Non Photochemical Quenching in plants, as reviewed in [134]. Moreover, different species harbour red forms absorbing and emitting at different wavelengths in the 710-750 nm range [50, 229]. It is however clear, also from the analysis of the whole Lhca pool [114], that the Chl b content is lower in Lhca complexes as compared with Lhcb complexes. Brettel K. Electron transfer and arrangement of the redox cofactors in photosystem I. Webber AN, Lubitz W. P700 the primary electron donor of photosystem I. Rappaport F, Guergova-Kuras M, Nixon PJ, Diner BA, Lavergne J. Kinetics and pathways of charge recombination in photosystem II. Yet, in a series of studies in which the effect of excited state quenching induced by artificial quinone quenchers was investigated systematically, both in intact cells [314] and in isolated thylakoids [269, 313], it was shown that the rate of net photo-inhibitory damage is not affected to a great extent by this quenching, with protection estimated to be 30%, which is much less than that expected on the basis of the linear-dose response relation between light intensity and photoinhibitory damage [269, 313-315, 337, 423]. Photoregulation of the composition, function, and structure of thylakoid membranes. is populated by two sequential events, the first of which is more likely the Actually, considering an average value for the overall rate associated with primary photochemistry for PSI, i.e., ~200 ns-1 and considering the presence of two equal branches (slight differences have been reported but this will not affect our reasoning), this would correspond to a rate of about ~100 ns-1 on each branch which falls about the mean values of the spread considered for PSII reactions. Nilkens M, Kress E, Lambrev P, Miloslavina Y, Muller M, Holzwarth AR, Jahns P. Identification of a slowly inducible zeaxanthin-dependent component of non-photochemical quenching of chlorophyll fluorescence generated under steady-state conditions in Arabidopsis. PSI: One-step charge separation: Acidification of the lumen is responsible for at least two factors determining Therefore, it is considered principally as a strategy to down-regulate the efficiency of PSII light harvesting under conditions in which the incident photon flux exceeds the maximal utilisation capacity by the thylakoid electron transfer chain. The acceptor side photoinibition occurs when the plastoquinone pool is highly reduced, i.e., under conditions that saturate the thylakoid transport chain and the QB binding pocket in PSII remains unoccupied due to the limited availability of oxidized plastoquinones molecules. What is the end product? Inspection of the PSII core high resolution structures obtained in cyanobacteria [20], which probably can be considered as an acceptable model also for the higher plant core, suggests that the porphyrin plane of all molecules bound to the PSII reaction centre subunits are more distorted with respect to those bound to PSI, which appear to overlay with a reference plane passing through the nitrogen atoms in the ring. [23] The number and degree of similarities between the two photosystems strongly indicates that PSI and the analogous photosystem of green sulfur bacteria evolved from a common ancestral photosystem. Thus, of the two photosystems, PSI would appear to have a greater contribution by the kinetics of energy diffusion within the antenna to the photochemical trapping kinetics. The large PsaA and PsaB subunits (MW ~80 kDa), which contain 11 TMH each, form a hetero-dimer that binds the vast majority of cofactors for light harvesting (~80 Chls a and ~20 -carotenes) as well as cofactors involved in the electron transfer reactions (6 Chls a, 2 phylloquinones and a 4Fe-4S cluster, known as FX), with the exception of terminal electron acceptors (Fe-S clusters FA and FB), which are bound by the PsaC subunit (Fig. Photosystem I receives electrons from plastocyanin or cytochrome c6 on the lumenal side of the thylakoid membrane and uses light energy to transfer them across the membrane to ferredoxin on the stromal side. Demmig-Adams B. Carotenoids and photoprotection in plants A role for the xanthophyll zeaxanthin. The model that emerges is that of some 300 chlorophyll molecules and 40 or so beta carotenes and other accessory pigments acting as a light harvesting antenna surrounding one chlorophyll a molecule that is a part of an action center. Garab G, Jennings RC, Croce R, Dorra D, Garlaschi FM, Holzwarth AR, Rivadossi A, Zucchelli G, Garab G. In Photosynthesis Mechanism and Effects / XI. government site. Note the presence of two functional ET and kpc is a macroscopic constant which describes excited state trapping, including contribution from excited state equilibration. Zucchelli G, Morosinotto T, Garlaschi FM, Bassi R, Jennings RC. Tjus SE, Moller BM, Scheller HV. WebEach photosystem consists of a light-harvesting complex and a core complex. [23] Last, redox centres in complexes of both photosystems are constructed upon a protein subunit dimer. Ganeteg U, Klimmek F, Jansson S. Lhca5 an LHC-type protein associated with photosystem I. Ballottari M, Dall'Osto L, Morosinotto T, Bassi R. Contrasting behavior of higher plant photosystem I and II antenna systems during acclimation. It is therefore surprising, at first view, that PSI performs primary photochemistry much faster than PSII, in which uphill energy transfer is almost absent (see energy transfer section). Hideg E, Vass I. Singlet oxygen is not produced in photosystem-I under photoinhibitory conditions. The Qo site of cytochrome b6f complexes controls the activation of the LHCII kinase. It should however be underlined that, while spillover is often considered to be an uncontrolled and hence an unfavorable phenomenon, there is evidence that this is not always the case. Wormit M, Harbach PHP, Mewes JM, Amarie S, Wachtveitl J, Dreuw A. Excitation energy transfer and carotenoid radical cation formation in light harvesting complexes A theoretical perspective. Tyystjarvi E. Photoinhibition of Photosystem II and photodamage of the oxygen evolving manganese cluster. Indeed the presence of Chl triplet states (populated by intersystem crossing) in relatively intact systems such as isolated thylakoid membranes has been detected only recently [288-290] and only at very low temperatures (~2 K), whereas the 3Car* are commonly observed, both in intact systems [277, 281-283, 291, 292] as well as in the isolated core [293-295] and antenna complexes [296-302]. Barber J, Chow WS. The action spectra were interpreted in terms of a small, heterogeneous, population of Chl-binding complexes in which Chl-Chl energy coupling is substantially eliminated and thus Chl-Car interactions are perturbed, so that 3Chl* quenching is inefficient [313]. Analysis of picosecond laserinduced fluorescence phenomena in photosynthetic membranes utilizing a master equation approach. At the same time, the more extended the random walk network, the longer the time the excited state resides in the antenna bed rather than in the reaction centre pigments. However in the case of PSII, where the energy difference between the core complexes and the whole of the Lhc antenna complement has been reported to be only a fraction of the thermal energy at physiological temperatures [99] due to presence of Chl b in the external antenna, the isoenergetic approximation may be of use. Basically two suggestions have been made concerning this aspect: i) they may have a function in photo-protection in PSI. Photosystems are the functional units for photosynthesis, defined by a particular pigment organization and association patterns, whose work is the absorption and transfer of light energy, which implies transfer of electrons. Three main features are similar between the different photosystems. This, as mentioned above, is a clear manifestation of a diffusion-limited situation imposed by a kinetic bottleneck rather a limitation due to the size of the antenna. Electron extracted from water in the Mn center of the OEC goes through a Tyrosine (Tyrz) to the P680. separation: the radical pair (P680+ PheoD1115%; 'Times New Roman',serif">115%; 'Times New Roman',serif">) Demmig-Adams B, Adams WWIII, Mattoo AK, Jansson S. In Photoprotection photoinhibition gene regulation and environment.Advances in photosynthesis and respiration. The fact that PSI does not contain neoxanthin could be either due to the fact that ROS scavenging around PSI can be efficiently performed by the stroma soluble enzyme superoxide dismutase, which, on the contrary, cannot enter in the highly appressed grana where PSII is localized (Fig. De Luca C, Varotto C, Svendsen I, De Laureto PP, Bassi R. Multiple light-harvesting II polypeptides from maize mesophyll chloroplasts are distinct gene products. photosynthesis, the process by which green plants and certain other organisms transform light energy into chemical energy.

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