Energy transfer between Photosystem II and Photosystem I in chloroplasts

A model for the photochemical apparatus of photosynthesis is presented which accounts for the fluorescence properties of Photosystem II and Photosystem I as well as energy transfer between the two photosystems. The model was tested by measuring at ?196 °C fluorescence induction curves at 690 and 730 nm in the absence and presence of 5 mM MgCl₂ which presumably changes the distribution of excitation energy between the two photosystems. The equations describing the fluorescence properties involve terms for the distribution of absorbed quanta, α, being the fraction distributed to Photosystem I, and β, the fraction to Photosystem II, and a term for the rate constant for energy transfer from Photosystem II to Photosystem I,k_T(II→I). The data, analyzed within the context of the model, permit a direct comparison of α andk_T(II→I) in the absence (?) and presence (+) of Mg²⁺:α/^?α⁺= 1.2andk/^?_T(II→I)k⁺_T(II→I)= 1.9. If the criterion thatα + β = 1 is applied absolute values can be calculated: in the presence of Mg²⁺,a⁺ = 0.27 and the yield of energy transfer,φ⁺_T(II→I) varied from 0.065 when the Photosystem II reaction centers were all open to 0.23 when they were closed. In the absence of Mg²⁺,α^? = 0.32 andφ_T(II→I) varied from 0.12 to 0.28.The data were also analyzed assuming that two types of energy transfer could be distinguished; a transfer from the light-harvseting chlorophyll of Photosystem II to Photosystem I,k_T(II→I), and a transfer from the reaction centers of Photosystem II to Photosystem I,k_t(II→I). In that caseα/^?α⁺= 1.3,k/^?_T(II→I)k⁺_T(II→I)= 1.3 andk/^?_t(II→I)k⁺_(tII→I)= 3.0. It was concluded, however, that both of these types of energy transfer are different manifestations of a single energy transfer process.     

Intersystem exciton transfer in isolated chloroplasts

The following arguments in favor of exciton transfer between the two photosystems are presented:

1. (1) MgCl₂ (1–10 mM range) decreases the intersystem transfer but does not modify the partition of absorbed photons between the photosystems. MgCl₂ addition causes a simultaneous increase of excitation life time (τ) and of fluorescence intensity (F). The same linear relationship is obtained with or without added Mg²⁺.

2. (2) The deactivation of Photosystem II by the Photosystem II to Photosystem I transfer increases with the level of reduced Photosystem II traps. When all Photosystem II traps are closed, half of Photosystem II excitons are deactivated by transfer to Photosystem I.

3. (3) From the relative values of the 685-nm fluorescence yield and System II electron transport rate in limiting light, measured with and without MgCl₂, the values of rate constants of Photosystem II deactivation were calculated.

4. (4) The intersystem transfer determines a 715-nm variable fluorescence, which is lowered by MgCl₂ addition. When this transfer is decreased by MgCl₂ the efficiency of the transfer between Photosystem II-connected units is enhanced, and a more sigmoidal fluorescence rise is obtained.

A double-layer model of the thylakoid membrane where each photosystem is restricted to one leaflet is proposed to explain the decrease of the intersystem transfer after adding cations. It is suggested that MgCl₂ decreases the thickness of the Photosystem I polar region, increasing the distance between the pigments of the two photosystems.     

Evolution and functional properties of photosystem II light harvesting complexes in eukaryotes

Photoautotrophic organisms, the major agent of inorganic carbon fixation into biomass, convert light energy into chemical energy. The first step of photosynthesis consists of the absorption of solar energy by pigments binding protein complexes named photosystems. Within photosystems, a family of proteins called Light Harvesting Complexes (LHC), responsible for light harvesting and energy transfer to reaction centers, has evolved along with eukaryotic organisms. Besides light absorption, these proteins catalyze photoprotective reactions which allowed functioning of oxygenic photosynthetic machinery in the increasingly oxidant environment. In this work we review current knowledge of LHC proteins serving Photosystem II. Balance between light harvesting and photoprotection is critical in Photosystem II, due to the lower quantum efficiency as compared to Photosystem I. In particular, we focus on the role of each antenna complex in light harvesting, energy transfer, scavenging of reactive oxygen species, chlorophyll triplet quenching and thermal dissipation of excess energy. This article is part of a Special Issue entitled: Photosystem II.     

Excitation spectra for photosystem I and photosystem II in chloroplasts and the spectral characteristics of the distributions of quanta between the two photosystems.

The parameters listed in the title were determined within the context of a model for the photochemical apparatus of photosynthesis. The fluorescence of variable yield at 750 nm at -196 degrees C is due to energy transfer from Photosystem II to Photosystem I. Fluorescence excitation spectra were measured at -196 degrees C at the minimum, FO, level and the maximum, FM, level of the emission at 750 nm. The difference spectrum, FM-FO, which represents the excitation spectrum for FV is presented as a pure Photosystem II excitation spectrum. This spectrum shows a maximum at 677 nm, attributable to the antenna chlorophyll a of Photosystem II units, with a shoulder at 670 nm and a smaller maximum at 650 nm, presumably due to chlorophyll a and chlorophyll b of the light-harvesting chlorophyll complex. Fluoresence at the FO level at 750 nm can be considered in two parts; one part due to the fraction of absorbed quanta, alpha, which excites Photosystem I more-or-less directly and another part due to energy transfer from Photosystem II to Photosystem I. The latter contribution can be estimated from the ratio of FO/FV measured at 692 nm and the extent of FV at 750 nm. According to this procedure the excitation spectrum of Photosystem I at -196 degrees C was determined by subtracting 1/3 of the excitation spectrum of FV at 750 nm from the excitation spectrum of FO at 750 nm. The spectrum shows a relatively sharp maximum at 681 nm due to the antenna chlorophyll a of Photosystem I units with probably some energy transfer from the light-harvesting chlorophyll complex. The wavelength dependence of alpha was determined from fluorescence measurements at 692 and 750 nm at -196 degrees C. Alpha is constant to within a few percent from 400 to 680 nm, the maximum deviation being at 515 nm where alpha shows a broad maximum increasing from 0.30 to 0.34. At wavelengths between 680 and 700 nm, alpha increases to unity as Photosystem I becomes the dominant absorber in the photochemical apparatus.     

Phosphorylation of the light-harvesting chlorophyll-protein regulates excitation energy distribution between photosystem II and photosystem I

The kinetics of thylakoid membrane protein phosphorylation in the presence of light and adenosine triphosphate is correlated to an incease in the 77 °K fluorescence emission at 735 nm (F735) relative to that at 685 nm (F685). Analysis of detergent-derived submembrane fractions indicate phosphorylation only of the polypeptides of Photosystem II, and the light-harvesting chlorophyll-protein complex serving Photosystem II (LHC-II). Although several polypeptides are phosphorylated, only the dephosphorylation kinetics of LHC-II follow the kinetics of the decrease of the $\text{F735}\text{F685}$ fluorescence emission ratios. The relative quantum yield of Photosystem II was significantly lower in phosphorylated membranes compared to dephosphorylated membranes. Reversible LHC-II phosphorylation thus provides the physiological mechanism for the control of the distribution of absorbed excitation energy between the two photosystems.     

Fluorescent kinetics of chlorophyll in Photosystems I and II enriched fractions of spinach

The fluorescent emission kinetics of spinach subchloroplast Photosystems I and II particles have been studied on a picosecond time scale. Using picosecond laser pulses and an optical Kerr gate, the fluorescent decay times are measured to be 60±10 ps, and 200±20 ps for Photosystems I and II, respectively. The quantum yields are calculated to be 0.004 for Photosystem I and 0.013 for Photosystem II. Theory of exciton energy transfer and trapping is applied for the determination of intermolecular potential energy in the photosystems.     

Energy distribution in the photochemical apparatus of Porphyridium cruentum in state I and state II

Fluorescence of Porphyridium cruentum in state I (cells equilibrated in light absorbed predominantly by Photosystem I) and in state II (cells equilibrated in light absorbed appreciably by Photosystem II) was examined to determine how the distribution of excitation energy was altered in the transitions between state I and state II. Low temperature emission spectra of cells frozen in state I and state II confirmed that a larger fraction of the excitation energy is delivered to Photosystem II in state I. Low temperature measurements showed that the yield of energy transfer from Photosystem II to Photosystem I was greater in state II and calculations indicated that the photochemical rate constant for such energy transfer was approximately twice as large in state II. Measurements at low temperature also showed that the cross sections and the spectral properties of the photosystems did not change in the transitions between state I and state II. In agreement with predictions made from the parameters measured at low temperature, the action spectra for oxygen evolution measured at room temperature were found to be the same in state I and state II.     

Fluorescence decay kinetics of mutants of corn deficient in photosystem I and photosystem II

The fast fluorescence decay kinetics of two photosynthetic mutants of corn (Zea mays) have been compared with those of normal corn. The fluorescence of normal corn can be resolved into three exponential decay components of lifetime 900–1500 ps (slow), 300–500 ps (middle) and 50–120 ps (fast), the yields of which are affected by light intensity and Mg²⁺ levels. The Photosystem II-(PS II)-defective mutant hcf-3 has similar decay lifetimes (approx. 1200, 450 and 100 ps) but is not affected by light intensity, reflecting the absence of PS II charge recombination. However, yields do respond to Mg²⁺ in a fashion typical of normal corn, which may be correlated with the presence of normal levels of light-harvesting chlorophyll a + b complex (LHCP). The PS I mutant hcf-50 also shows three-component decay kinetics. In conjunction with the results on the LHCP-deficient mutant of barley presented in a recent paper (Karukstis, K.K. and Sauer, K. (1984) Biochim. Biophys. Acta 766, 148–155), these data suggest that the slow component of normal chloroplasts is kinetically controlled by the decay processes of the LHCP and that the energy comes from one of two sources: (a) charge recombination in the reaction centre or (b) energy transferred within or between LHCP units only. The fast component appears to originate from both PS I and PS II. The complex response of the middle component to cations and light intensity, and its presence in all of the mutants, suggests that it also may have multiple origins.     

Energy transfer from photosystem II to photosystem I in Porphyridium cruentum

Rates of photooxidation of P-700 by green (560 nm) or blue (438 nm) light were measured in whole cells of porphyridium cruentum which had been frozen to -196 degrees C under conditions in which the Photosystem II reaction centers were either all open (dark adapted cells) or all closed (preilluminated cells). The rate of photooxidation of P-700 at -196 degrees C by green actinic light was approx. 80% faster in the preilluminated cells than in the dark-adapted cells. With blue actinic light, the rates of P-700 photooxidation in the dark-adapted and preilluminated cells were not significantly different. These results are in excellent agreement with predictions based on our previous estimates of energy distribution in the photosynthetic apparatus of Porphyridium cruentum including the yield of energy transfer from Photosystem II to Photosystem I determined from low temperature fluorescence measurements.     

A demonstration of energy transfer from photosystem II to photosystem I in chloroplasts

Photosystem I activity of Tris-washed chloroplasts was measured at room temperature as the rate of photoreduction of NADP and as the rate of oxygen uptake mediated by methyl viologen in both cases using dichlorophenolindophenol plus ascorbate as the source of electrons for Photosystem I. With both assay systems the rate of electron transport by Photosystem I was stimulated approx. 20 % by the addition of 3-(3,4-dichlorophenyl)-1, 1-dimethylurea which caused the Photosystem II reaction centers to close. Photosystem I activity of chloroplasts was measured at low temperature as the rate of photooxidation of P-700. Chloroplasts suspended in the presence of hydroxylamine and 3-(3,4-dichlorophenyl)-1, 1-dimethylurea were frozen to ?196 °C after adaptation to darkness or after a preillumination at room temperature. The Photosystem II reaction centers of the frozen dark-adapted sample were all open; those of the preilluminated sample were all closed. The rate of photooxidation of P-700 at ?196 °C with the preilluminated sample was approx. 25 % faster than with the dark-adapted sample. We conclude from both the room temperature and the low temperature experiments that there is greater energy transfer from Photosystem II to Photosystem I when the Photosystem II reaction centers are closed and that these results are a direct demonstration of spillover.     

Selective disruption of energy flow from phycobilisomes to Photosystem I

Efficient production of ATP and NADPH by the light reactions of oxygen-evolving photosynthesis demands continuous adjustment of transfer of absorbed light energy from antenna complexes to Photosystem I (PS I) and II (PS II) reaction center complexes in response to changes in light quality. Treatment of intact cyanobacterial cells with N-ethylmaleimide appears to disrupt energy transfer from phycobilisomes to Photosystem I (PS I). Energy transfer from phycobilisomes to Photosystem II (PS II) is unperturbed. Spectroscopic analysis indicates that the individual complexes (phycobilisomes, PS II, PS I) remain functionally intact under these conditions. The results are consistent with the presence of connections between phycobiliproteins and both PS II and PS I, but they do not support the existence of direct contacts between the two photosystems.Abbreviations Chl chlorophyll - EPR electron paramagnetic resonance - NEM N-ethylmaleimide - PBS phycobilisome - PS photosystem     

Energy transfer from photosystem II to photosystem I in Porphyridium cruentum

Rates of photooxidation of P-700 by green (560 nm) or blue (438 nm) light were measured in whole cells of Porphyridium cruentum which had been frozen to ?196 °C under conditions in which the Photosystem II reaction centers were either all open (dark adapted cells) or all closed (preilluminated cells). The rate of photooxidation of P-700 at ?196 °C by green actinic light was approx. 80% faster in the preilluminated cells than in the dark-adapted cells. With blue actinic light, the rates of P-700 photooxidation in the dark-adapted and preilluminated cells were not significantly different. These results are in excellent agreement with predictions based on our previous estimates of energy distribution in the photosynthetic apparatus of Porphyridium cruentum including the yield of energy transfer from Photosystem II to Photosystem I determined from low temperature fluorescence measurements.     

Release and aggregation of the light-harvesting complex in intact leaves subjected to strong CO2 deficit

Tobacco plants were subjected to long-term CO₂ deficit. The stress caused photoinhibition of Photosystem (PS) II photochemistry and the aggregation of the light-harvesting complex of PS II (LHC II). The aggregation was shown by the appearance of the characteristic band at 698–700 nm (F₆₉₉) in 77 K fluorescence emission spectra. LHC II aggregates are considered to quench fluorescence and, therefore, the fluorescence yield was determined to verify their quenching capability. PS II photochemistry, measured as F_V/F_M, was largely depressed during first 4 days of the stress. Unexpectedly, the total fluorescence yield increased in this period. Fitting of emission spectra by Gaussian components approximating emission bands of LHC II, PS II core, PS I and F₆₉₉ revealed that mainly the bands at 680 and 699 nm, representing emission of LHC II aggregates, were responsible for the increase of the fluorescence yield. This shows an interruption of the excitation energy transfer between LHC II and both photosystems and, thus, a physical disconnection of LHC II from photosystems. PS II and PS I emissions were not quenched in this period. Therefore, it was concluded that these LHC II aggregates were accumulated out of PS II antenna, and, thus they cannot be involved in dumping of excess excitation. The total fluorescence yield turned to decrease only after the large depression of PS II photochemistry, when LHC II aggregation was considerably speeded up and the fluorescence yields of PS I and II turned to decline.     

Cation-mediated regulation of excitation energy distribution in chloroplasts lacking organized Photosystem II complexes

Despite the total loss of Photosystem II activity, thylakoids isolated from the green nuclear maize mutant hcf1-3 contain normal amounts of the light-harvesting chlorophyll $\text{a}\text{b}$ pigment-protein complex (LHC). We interpret the spectroscopic and ultrastructural characteristics of these thylakoids to indicate that the LHC present in these membranes is not associated with Photosystem II reaction centers and thus exists in a ‘free’ state within the thylakoid membrane. In contrast, the LHC found in wild-type maize thylakoids shows the usual functional association with Photosystem II reaction centers. Several lines of evidence suggest that the free LHC found in thylakoids isolated from hcf1-3 is able to mediate cation-dependent changes in both thylakoid appression and energy distribution between the photosystems: (1) Thylakoids isolated from hcf1-3 and wild-type seedlings exhibit a similar Mg²⁺-dependent increase in the short/long wavelength fluorescence emission peak ratio at 77 K. This Mg²⁺ effect is lost following incubation of thylakoids isolated from either source with low concentrations of trypsin. Such treatment results in the partial proteolysis of the LHC in both membrane types. (2) Thylakoids isolated from both hcf1-3 and wild-type seedlings show a similar Mg²⁺ dependence for the enhancement of the maximal yield of room temperature fluorescence and light scattering; both Mg²⁺ effects are abolished by brief incubation of the thylakoids with low concentrations of trypsin (3) Mg²⁺ acts to reduce the relative quantum efficiency of Photosystem I-dependent electron transport at limiting 650 nm light in thylakoids isolated from hcf1-3. (4) The pattern of digitonin fractionation of thylakoid membranes, which is dependent upon structural membrane interactions and upon LHC in the thylakoids, is similar in thylakoids isolated from both hcf1-3 and wild-type seedlings. We conclude that the surface-exposed segment of the LHC, but not the LHC-Photosystem II core association, is necessary for the cation-dependent changes in both thylakoid appression and energy distribution between the two photosystems, and that the LHC itself is able to transfer excitation energy directly to Photosystem I in a Mg²⁺-dependent fashion in the absence of Photosystem II reaction centers. The latter phenomenon is equivalent to a cation-induced change in the absorptive cross-section of Photosystem I.     

Excitation energy transfer to Photosystem I in filaments and heterocysts of Nostoc punctiforme

Cyanobacteria adapt to varying light conditions by controlling the amount of excitation energy to the photosystems. On the minute time scale this leads to redirection of the excitation energy, usually referred to as state transitions, which involves movement of the phycobilisomes. We have studied short-term light adaptation in isolated heterocysts and intact filaments from the cyanobacterium Nostoc punctiforme ATCC 29133. In N.punctiforme vegetative cells differentiate into heterocysts where nitrogen fixation takes place. Photosystem II is inactivated in the heterocysts, and the abundancy of Photosystem I is increased relative to the vegetative cells. To study light-induced changes in energy transfer to Photosystem I, pre-illumination was made to dark adapted isolated heterocysts. Illumination wavelengths were chosen to excite Photosystem I (708 nm) or phycobilisomes (560 nm) specifically. In heterocysts that were pre-illuminated at 708 nm, fluorescence from the phycobilisome terminal emitter was observed in the 77 K emission spectrum. However, illumination with 560 nm light caused quenching of the emission from the terminal emitter, with a simultaneous increase in the emission at 750 nm, indicating that the 560 nm pre-illumination caused trimerization of Photosystem I. Excitation spectra showed that 560 nm pre-illumination led to an increase in excitation transfer from the phycobilisomes to trimeric Photosystem I. Illumination at 708 nm did not lead to increased energy transfer from the phycobilisome to Photosystem I compared to dark adapted samples. The measurements were repeated using intact filaments containing vegetative cells, and found to give very similar results as the heterocysts. This demonstrates that molecular events leading to increased excitation energy transfer to Photosystem I, including trimerization, are independent of Photosystem II activity.     

Structure of photosystem I.

In plants and cyanobacteria, the primary step in oxygenic photosynthesis, the light induced charge separation, is driven by two large membrane intrinsic protein complexes, the photosystems I and II. Photosystem I catalyses the light driven electron transfer from plastocyanin/cytochrome c(6) on the lumenal side of the membrane to ferredoxin/flavodoxin at the stromal side by a chain of electron carriers. Photosystem I of Synechococcus elongatus consists of 12 protein subunits, 96 chlorophyll a molecules, 22 carotenoids, three [4Fe4S] clusters and two phylloquinones. Furthermore, it has been discovered that four lipids are intrinsic components of photosystem I. Photosystem I exists as a trimer in the native membrane with a molecular mass of 1068 kDa for the whole complex. The X-ray structure of photosystem I at a resolution of 2.5 A shows the location of the individual subunits and cofactors and provides new information on the protein-cofactor interactions. [P. Jordan, P. Fromme, H.T. Witt, O. Klukas, W. Saenger, N. Krauss, Nature 411 (2001) 909-917]. In this review, biochemical data and results of biophysical investigations are discussed with respect to the X-ray crystallographic structure in order to give an overview of the structure and function of this large membrane protein.     

Probing the kinetics of photosystem I and photosystem II fluorescence in pea chloroplasts on a picosecond pulse fluorometer.

Picosecond fluorescence kinetics of pea chloroplasts have been investigated at room temperature using a pulse fluorometer with a resolution time of 10-11 s. Fluorescence has been excited by both a ruby and neodymium-glass mode-locked laser and has been reocrded within the 650 to 800 nm spectral region. We have found three-component kinetics of fluorescence from pea chloroplasts with lifetimes of 80, 300 and 4500 ps, respectively. The observed time dependency of the fluorescence of different components on the functional state of the photosynthetic mechanism as well as their spectra enabled us to conclude that Photosystem I fluoresces with a lifetime of 80 ps (tauI) and Photosystem II fluoresces with a lifetime of 300 ps (tauII). Fluorescence with a lifetime of 4500 ps (tauIII) may be interpreted as originating from chlorophill monomeric forms which are not involved in photosynthesis. It was determined that the rise time of Photosystem I and Photosystem II fluorescence after 530 nm photoexcitation is 200 ps, which corrsponds to the time of energy migration to them from carotenoids.     

Xanthophyll-induced aggregation of LHCII as a switch between light-harvesting and energy dissipation systems

The xanthophyll cycle pigments, violaxanthin and zeaxanthin, present outside the light-harvesting pigment-protein complexes of Photosystem II (LHCII) considerably enhance specific aggregation of proteins as revealed by analysis of the 77 K chlorophyll a fluorescence emission spectra. Analysis of the infrared absorption spectra in the Amide I region shows that the aggregation is associated with formation of intermolecular hydrogen bonding between the alpha helices of neighboring complexes. The aggregation gives rise to new electronic energy levels, in the Soret region (530 nm) and corresponding to the Q spectral region (691 nm), as revealed by analysis of the resonance light scattering spectra. New electronic energy levels are interpreted in terms of exciton coupling of protein-bound photosynthetic pigments. The energy of the Q excitonic level of chlorophyll is not high enough to drive the light reactions of Photosystem II but better suited to transfer excitation energy to Photosystem I, which creates favourable energetic conditions for the state I-state II transition. The lack of fluorescence emission from this energy level, at physiological temperatures, is indicative of either very high thermal energy conversion rate or efficient excitation quenching by carotenoids. Chlorophyll a fluorescence was quenched up to 61% and 34% in the zeaxanthin- and violaxanthin-containing samples, respectively, as compared to pure LHCII. Enhanced aggregation of LHCII, observed in the presence of the xanthophyll cycle pigments, is discussed in terms of the switch between light-harvesting and energy dissipation systems.     

Energy transfer in the photochemical apparatus of flashed bean leaves

Fluorescence and energy transfer properties of bean leaves greened by brief, repetitive xenon flashes were studied at −196 °C. The bleaching of P-700 has no influence on the yield of fluorescence at any wavelength of emission. The light-induced fluorescence yield changes which are observed in both the 690 and 730 nm emission bands in the low temperature fluorescence spectra are due to changes in the state of the Photosystem II reaction centers. The fluorescence yield changes in the 730 nm band are attributed to energy transfer from Photosystem II to Photosystem I. Such energy transfer was also confirmed by measurements of the rate of photooxidation of P-700 at −196 °C in leaves in which the Photosystem II reaction centers were either all open or all closed. It is concluded that energy transfer from Photosystem II to Photosystem I occurs in the flashed bean leaves which lack the light-harvesting chlorophyll a/b protein.     

Phycocyanin sensitizes both photosystem I and photosystem II in cryptophyte Chroomonas CCMP270 cells

This article presents an investigation of the energy migration dynamics in intact cells of the unicellular photosynthetic cryptophyte Chroomonas CCMP270 by steady-state and time-resolved fluorescence measurements. By kinetic modeling of the fluorescence data on chlorophyll and phycocyanin 645 excitation (at 400 and 582 nm respectively), it has been possible to show the excited state energy distribution in the photosynthetic antenna of this alga. Excitation energy from phycocyanin 645 is distributed nearly equally between photosystem I and photosystem II with very high efficiency on a 100-ps timescale. The excitation energy trapping times for both photosystem I (∼30 ps) and photosystem I (200 and ∼540 ps) correspond well to those obtained from experiments on isolated photosystems. The results are compared with previous results for another cryptophyte species, Rhodomonas CS24, and suggest a similar membrane organization for the cryptophytes with the phycobiliproteins tightly packed in the thylakoid lumen around the periphery of the photosystems.