Cryo-EM structural analyses of chlorophyll b-enriched PSI-LHC and PSII-LHC supercomplexes of the siphonous green alga Bryopsis corticulans.

A recently developed technique for dilution of the naturally high protein packing density in isolated grana membranes was applied to study the dependence of the light harvesting efficiency of photosystem (PS) II on macromolecular crowding. Slight dilution of the protein packing from 80% area fraction to the value found in intact grana thylakoids (70%) leads to an improved functionality of PSII (increased antenna size, enhanced connectivity between reaction centers). Further dilution induces a functional disconnection of light-harvesting complex (LHC) II from PSII. It is concluded that efficient light harvesting by PSII requires an optimal protein packing density in grana membranes that is close to 70%. We hypothesize that the decreased efficiency in overcrowded isolated grana thylakoids is caused by excited state quenching in LHCII, which has previously been correlated with neoxanthin distortion. Resonance Raman spectroscopy confirms this increase in neoxanthin distortion in overcrowded grana as compared with intact thylakoids. Furthermore, analysis of the changes in the antenna size in highly diluted membranes indicates a lipid-induced dissociation of up to two trimeric LHCII from PSII, leaving one trimer connected. This observation supports a hierarchy of LHCII-binding sites on PSII.

Iron deficiency induces a remodeling of the photosynthetic apparatus in Chlamydomonas reinhardtii. In this study we showed that a key mechanistic event in the remodeling process of photosystem I (PSI) and its associated light-harvesting proteins (LHCI) is the N-terminal processing of Lhca3. N-terminal processing of Lhca3 is documented independently by two-dimensional gel electrophoresis and tandem mass spectrometric (MS/MS) analysis as well as by quantitative comparative MS/MS peptide profiling using isotopic labeling of proteins. Dynamic remodeling of the LHCI complex under iron deficiency is further exemplified by depletion of Lhca5 and up-regulation of Lhca4 and Lhca9 polypeptides in respect to photosystem I. Most importantly, the induction of N-terminal processing of Lhca3 by progression of iron deficiency correlates with the functional drop in excitation energy transfer efficiency between LHCI and PSI as assessed by low temperature fluorescence emission spectroscopy. Using an RNA interference (RNAi) strategy, we showed that the truncated form of Lhca3 is essential for the structural stability of LHCI. Depletion of Lhca3 by RNAi strongly impacted the efficiency of excitation energy transfer between PSI and LHCI, as is the case for iron deficiency. However, in contrast to iron deficiency, comparative MS/MS peptide profiling using isotopic labeling of proteins demonstrated that RNAi depletion of Lhca3 caused strong reduction of almost all Lhca proteins in isolated PSI particles.

ApcD is necessary for efficient energy transfer from phycobilisomes to photosystem I and helps to prevent photoinhibition in the cyanobacterium Synechococcus sp. PCC 7002

A non-calcifying strain (Ply 92) of Emiliania huxleyi (Lohman) Hay et Mohler, was subjected to reciprocal step changes of photon flux density (PFD) between 50 and 800 µmol photons m−2 s−1 to establish the timescales of photoacclimation. Photoacclimation in E. huxleyi involved adjustment of cellular light harvesting pigment contents, but not cellular ribulose bisphosphate carboxylase-oxygenase (RuBisCO) or cellular RNA contents. There was considerable variability in both the magnitude and the rate of the response of physiological and biophysical variables to the step changes in PFD. The slowest response was observed in cellular chl a content, and the chl a/carbon ratio. Acclimation of these variables appeared to be due to dilution of pigment pools rather than to pigment turnover. Changes in the chl a-specific light absorption coefficient accompanied changes in cellular chl a content. Following the shifts in PFD, the efficiency of excitation energy transfer from the photosystem II antennae to reaction centres II (RCII), as assessed from , adjusted within 12 h. The efficiency of charge separation within RCII, as assessed from , adjusted on a similar timescale. The overall efficiency of photosystem II (PSII), as assessed from / , showed a step response to the change of PFD due to inverse changes in , and . Changes in non-photochemical excitation energy quenching were correlated with the relative and absolute abundance of the xanthophyll cycle pigments diadinoxanthin and diatoxanthin, which appeared to be reversibly interconverted with fucoxanthin. Thus, acclimation to PFD involved co-ordinated adjustment of the quantum efficiency of RCII, the efficiency of excitation energy transfer from the light-harvesting pigment bed to RCII, and cellular light absorption on overlapping time scales.

Excitation energy transfer kinetics and efficiency in phototrophic green sulfur bacteria

The present study was conducted to examine changes in photosynthetic pigment composition and functional state of the thylakoid membranes during the individual steps of preparation of samples that are intended for a separation of pigmentprotein complexes by nondenaturing polyacrylamide gel electrophoresis. The thylakoid membranes were isolated from barley leaves (Hordeum vulgare L.) grown under low irradiance (50 μmol m−2 s−1). Functional state of the thylakoid membrane preparations was evaluated by determination of the maximal photochemical efficiency of photosystem (PS) II (FV/FM) and by analysis of excitation and emission spectra of chlorophyll a (Chl a) fluorescence at 77 K. All measurements were done at three phases of preparation of the samples: (1) in the suspensions of osmotically-shocked broken chloroplasts, (2) thylakoid membranes in extraction buffer containing Tris, glycine, and glycerol and (3) thylakoid membranes solubilized with a detergent decyl-β-D-maltosid. FV/FM was reduced from 0.815 in the first step to 0.723 in the second step and to values close to zero in solubilized membranes. Pigment composition was not pronouncedly changed during preparation of the thylakoid membrane samples. Isolation of thylakoid membranes affected the efficiency of excitation energy transfer within PSII complexes only slightly. Emission and excitation fluorescence spectra of the solubilized membranes resemble spectra of trimers of PSII light-harvesting complexes (LHCII). Despite a disrupted excitation energy transfer from LHCII to PSII antenna core in solubilized membranes, energy transfer from Chl b and carotenoids to emission forms of Chl a within LHCII trimers remained effective.

Photosystem II, the oxygen-evolving complex of photosynthetic organisms, includes an intriguingly large number of low molecular weight polypeptides, including PsbM. Here we describe the first knock-out of psbM using a transplastomic, reverse genetics approach in a higher plant. Homoplastomic Delta psbM plants exhibit photoautotrophic growth. Biochemical, biophysical, and immunological analyses demonstrate that PsbM is not required for biogenesis of higher order photosystem II complexes. However, photosystem II is highly light-sensitive, and its activity is significantly decreased in Delta psbM, whereas kinetics of plastid protein synthesis, reassembly of photosystem II, and recovery of its activity are comparable with the wild type. Unlike wild type, phosphorylation of the reaction center proteins D1 and D2 is severely reduced, whereas the redox-controlled phosphorylation of photosystem II light-harvesting complex is reversely regulated in Delta psbM plants because of accumulation of reduced plastoquinone in the dark and a limited photosystem II-mediated electron transport in the light. Charge recombination in Delta psbM measured by thermoluminescence oscillations significantly differs from the 2/6 patterns in the wild type. A simulation program of thermoluminescence oscillations indicates a higher Q(B)/Q(-)(B) ratio in dark-adapted mutant thylakoids relative to the wild type. The interaction of the Q(A)/Q(B) sites estimated by shifts in the maximal thermoluminescence emission temperature of the Q band, induced by binding of different herbicides to the Q(B) site, is changed indicating alteration of the activation energy for back electron flow. We conclude that PsbM is primarily involved in the interaction of the redox components important for the electron flow within, outward, and backward to photosystem II.

Arrangement of photosystem II supercomplexes in crystalline macrodomains within the thylakoid membrane of green plant chloroplasts

Effect of 13-cis violaxanthin on organization of light harvesting complex II in monomolecular layers

The heart of oxygenic photosynthesis is photosystem II (PSII), a multisubunit protein complex that uses solar energy to drive the splitting of water and production of molecular oxygen. The effectiveness of the photochemical reaction center of PSII depends on the efficient transfer of excitation energy from the surrounding antenna chlorophylls. A kinetic model for PSII, based on the x-ray crystal structure coordinates of 37 antenna and reaction center pigment molecules, allows us to map the major energy transfer routes from the antenna chlorophylls to the reaction center chromophores. The model shows that energy transfer to the reaction center is slow compared with the rate of primary electron transport and depends on a few bridging chlorophyll molecules. This unexpected energetic isolation of the reaction center in PSII is similar to that found in the bacterial photosystem, conflicts with the established view of the photophysics of PSII, and may be a functional requirement for primary photochemistry in photosynthesis. In addition, the model predicts a value for the intrinsic photochemical rate constant that is 4 times that found in bacterial reaction centers.

Low-cost highlighter nanoparticles for highly efficient multistep excitation energy transfer.

In this paper, we model the excitation energy transfer (EET) of photosystem I (PSI) of the common pea plant Pisum sativum as a complex interacting network. The magnitude of the link energy transfer between nodes/chromophores is computed by Forster resonant energy transfer (FRET) using the pairwise physical distances between chromophores from the PDB 5L8R (Protein Data Bank). We measure the global PSI network EET efficiency adopting well-known network theory indicators: the network efficiency (Eff) and the largest connected component (LCC). We also account the number of connected nodes/chromophores to P700 (CN), a new ad hoc measure we introduce here to indicate how many nodes in the network can actually transfer energy to the P700 reaction centre. We find that when progressively removing the weak links of lower EET, the Eff decreases, while the EET paths integrity (LCC and CN) is still preserved. This finding would show that the PSI is a resilient system owning a large window of functioning feasibility and it is completely impaired only when removing most of the network links. From the study of different types of chromophore, we propose different primary functions within the PSI system: chlorophyll a (CLA) molecules are the central nodes in the EET process, while other chromophore types have different primary functions. Furthermore, we perform nodes removal simulations to understand how the nodes/chromophores malfunctioning may affect PSI functioning. We discover that the removal of the CLA triggers the fastest decrease in the Eff, confirming that CAL is the main contributors to the high EET efficiency. Our outcomes open new perspectives of research, such comparing the PSI energy transfer efficiency of different natural and agricultural plant species and investigating the light-harvesting mechanisms of artificial photosynthesis both in plant agriculture and in the field of solar energy applications.

Excitation energy transfer between acriflavine and rhodamine 6G as a pH sensor

The conversion of violaxanthin to zeaxanthin is essentially required for the pH-regulated dissipation of excess light energy in the antenna of photosystem II. Violaxanthin is bound to each of the antenna proteins of both photosystems. Former studies with recombinant Lhcb1 and different Lhca proteins implied that each antenna protein contributes specifically to violaxanthin conversion related to protein-specific affinities of the different violaxanthin binding sites. We investigated the violaxanthin de-epoxidation in the minor antenna proteins of photosystem II, Lhcb4-6. Recombinant proteins were reconstituted with different xanthophyll mixtures to study the conversion of violaxanthin at different xanthophyll binding sites in these proteins. The extent and kinetics of violaxanthin de-epoxidation were found to be dependent on the respective protein and, for each protein, also on the binding site of violaxanthin. In particular, violaxanthin bound to Lhcb4 was nearly inconvertible for de-epoxidation, whereas violaxanthin bound to Lhcb5 was fully convertible but with slow kinetics. Lhcb6 exhibited heterogeneous violaxanthin conversion characteristics, which could be assigned to different populations of reconstituted Lhcb6 complexes with respect to violaxanthin binding sites. The results support the proposed different binding affinities of violaxanthin to the three putative violaxanthin binding sites (V1, N1, and L2) in antenna proteins. Under consideration of former studies with Lhcb1 and Lhca proteins, the data imply that violaxanthin bound to the V1 and N1 binding site of antenna proteins is easily accessible for de-epoxidation in all antenna proteins, whereas violaxanthin bound to L2 is either only slowly or not convertible to zeaxanthin, depending on the respective protein.

Light-harvesting complex II (LHCII), the main light-harvesting antenna complex of plants, is a functionally flexible and mobile component of the photosynthetic membrane. Excitonic interactions between chlorophylls in the complex are the basis for fast and efficient excitation energy transfer (EET). In addition to its light-harvesting function, LHCII plays regulatory roles, e.g. balancing the energy flow between Photosystem II and Photosystem I (PSI) and activating photoprotective non-photochemical quenching (NPQ). These functions are controlled by the innate structural flexibility of the complex and its intermolecular interactions in the thylakoid membranes. The general aim of this thesis work is to clarify the changes in the molecular and excitonic structure of LHCII that are incurred by interactions with its environment, and how these changes affect EET in the complex and in the thylakoid membrane. Using CD and anisotropic CD (ACD) spectroscopy, we have identified specific changes in the excitonic states in LHCII related to protein-protein interactions in aggregates or induced by detergents and. Time-resolve fluorescence showed that the molecular environment strongly affects the excitation lifetime, hence the light-harvesting function, of LHCII. Due to self-segregation, LHCII formed protein-dense domains in reconstituted membranes wherein fluorescence quenching occurred with a mechanism similar to NPQ in vivo. The dynamics of EET in LHCII was followed by ultrafast two-dimensional electronic spectroscopy (2DES), resolving simultaneous uphill and downhill energy transfer pathways. Exciton equilibration in the Chl a domain was found to occur on timescales up to 5 ps at physiological temperature. EET in plant PSI–LHCI and isolated PSI core complexes were observed for the first time by 2DES. A refined kinetic model was proposed, according to which primary charge separation in PSI occurs after full equilibration of the excitations in the core antenna, with an effective time constant of 3–4 ps. Long-distance EET between LHCIIs and between LHCII and PSI was detected in reconstituted membranes. LHCII acted as efficient antenna of PSI increasing its functional antenna size by up to 50% with minor loss of photochemical efficiency.