Photosystem I functions as a sunlight energy converter, catalyzing one of the initial steps in driving oxygenic photosynthesis in cyanobacteria, algae, and higher plants. Functionally, Photosystem I captures sunlight and transfers the excitation energy through an intricate and precisely organized antenna system, consisting of a pigment network, to the center of the molecule, where it is used in the transmembrane electron transfer reaction. Our current understanding of the sophisticated mechanisms underlying these processes has profited greatly from elucidation of the crystal structures of the Photosystem I complex. In this report, we describe the developments that ultimately led to enhanced structural information of plant Photosystem I. In addition, we report an improved crystallographic model at 3.3-Å resolution, which allows analysis of the structure in more detail. An improved electron density map yielded identification and tracing of subunit PsaK. The location of an additional ten ␤-carotenes as well as five chlorophylls and several loop regions, which were previously uninterpretable, are now modeled. This represents the most complete plant Photosystem I structure obtained thus far, revealing the locations of and interactions among 17 protein subunits and 193 non-covalently bound photochemical cofactors. Using the new crystal structure, we examine the network of contacts among the protein subunits from the structural perspective, which provide the basis for elucidating the functional organization of the complex.During oxygenic photosynthesis, solar energy is converted into chemical energy for all higher forms of life on Earth. This process is driven by a photosynthetic apparatus within the thylakoid membranes of cyanobacteria, algae, and plants. The photochemical functions are performed by two photosystems: Photosystems I (PSI) and II (PSII). 4 These photosystems are multisubunit complexes that consist of protein and non-protein components, and drive light-dependent electron transfer reactions, resulting in the formation of high energy products: ATP and NADPH (1, 2). PSII catalyzes light-driven oxidation of water, providing electrons to PSI via the plastoquinone pool, the cytochrome b 6 f complex, and the water-soluble electron carrier plastocyanin. This electron transfer is coupled to the increase of a transmembrane electrochemical potential gradient (proton motive force), which powers ATP-synthase for phosphorylation of ADP to ATP. PSI catalyzes light-driven electron transport from plastocyanin at the inner face of the membrane (lumen) to ferredoxin on the outside of the membrane (stroma). The reduced ferredoxin is subsequently used for NADPH production, which provides the reducing power for the conversion of carbon dioxide to organic molecules. Although PSII is unique in its ability to extract electrons from water, PSI is arguably the most efficient photoelectric apparatus in nature, exhibiting a quantum efficiency of almost 100% in its utilization of light for electron transport (3). The ability of PS to...

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Photosynthetic water splitting, coupled to hydrogenase-catalyzed hydrogen production, is considered a promising clean, renewable source of energy. It is widely accepted that the oxygen sensitivity of hydrogen production, combined with competition between hydrogenases and NADPH-dependent carbon dioxide fixation are the main limitations for its commercialization. Here we provide evidence that, under the anaerobic conditions that support hydrogen production, there is a significant loss of photosynthetic electrons toward NADPH production in vitro. To elucidate the basis for competition, we bioengineered a ferredoxin-hydrogenase fusion and characterized hydrogen production kinetics in the presence of Fd, ferredoxin:NADP þ -oxidoreductase (FNR), and NADP þ . Replacing the hydrogenase with a ferredoxin-hydrogenase fusion switched the bias of electron transfer from FNR to hydrogenase and resulted in an increased rate of hydrogen photoproduction. These results suggest a new direction for improvement of biohydrogen production and a means to further resolve the mechanisms that control partitioning of photosynthetic electron transport.

Oxygenic photosynthesis supports virtually all life forms on earth. Light energy is converted by two photosystems—photosystem I (PSI) and photosystem II (PSII). Globally, nearly 50% of photosynthesis takes place in the Ocean, where single cell cyanobacteria and algae reside together with their viruses. An operon encoding PSI was identified in cyanobacterial marine viruses. We generated a PSI that mimics the salient features of the viral complex, named PSIPsaJF. PSIPsaJF is promiscuous for its electron donors and can accept electrons from respiratory cytochromes. We solved the structure of PSIPsaJF and a monomeric PSI, with subunit composition similar to the viral PSI, providing for the first time a detailed description of the reaction center and antenna system from mesophilic cyanobacteria, including red chlorophylls and cofactors of the electron transport chain. Our finding extends the understanding of PSI structure, function and evolution and suggests a unique function for the viral PSI.DOI: http://dx.doi.org/10.7554/eLife.01496.001

To identify the molecular composition of the low-energy states in cyanobacterial Photosystem I (PSI) of Synechocystis PCC6803, we focus on highresolution (low-temperature) absorption, emission, resonant, and nonresonant holeburned spectra obtained for wild-type (WT) PSI and three PSI mutants. In the Red_a mutant, the B33 chlorophyll (Chl) is added to the B31-B32 dimer; in Red_b, histidine 95 (His95) on PsaB (which coordinates Mg in the B7 Chl within the His95-B7-A31-A32cluster) is replaced with glutamine (Gln), while in the Red_ab mutant, both mutations are made. We show that the C706 state (B31-B32) changes to the C710 state (B31-B32-B33) in both Red_a and Red_ab mutants, while the C707 state in WT Synechocystis (localized on the His95-B7-A31-A32 cluster) is modified to C716 in both Red_b and Red_ab. Excitation energy transfer from C706 to the C714 trap in the WT PSI and Red_b mutant is hampered as reflected by a weak emission at 712 nm. Large electron−phonon coupling strength (exposed via resonant hole-burned spectra) is consistent with a strong mixing of excited states with intermolecular charge transfer states leading to significantly red-shifted emission spectra. We conclude that excitation energy transfer in PSI is controlled by fine-tuning the electronic states of a small number of highly conserved red states. Finally, we show that mutations modify the protein potential energy landscape as revealed by different shapes and shifts of the blue-and red-shifted antiholes.

The PSI 3 −IsiA 18 supercomplex is one of the largest and most complicated assemblies in photosynthesis. The IsiA ring, composed of 18 IsiA monomers (IsiA 18 ) surrounding the PSI trimer (PSI 3 ), forms under iron-deficient conditions in cyanobacteria and acts as a peripheral antenna. Based on the supercomplex structure recently determined via cryo-EM imaging, we model various optical spectra of the IsiA monomers and IsiA 18 ring. Comparison of the absorption and emission spectra of the isolated IsiA monomers and the full ring reveals that about 2.7 chlorophylls (Chls) are lost in the isolated IsiA monomers. The best fits for isolated monomers spectra are obtained assuming the absence of Chl 508 and Chl 517 and 70% loss of Chl 511. The best model describing all three hexamers and the entire ring suggests that the lowest energy pigments are Chls 511, 514, and 517. Based on the modeling results presented in this work, we conclude that there are most likely three entry points for EET from the IsiA 6 hexamer to the PSI core monomer, with two of these entry points likely being located next to each other (i.e., nine entry points from IsiA 18 to the PSI 3 trimer). Finally, we show that excitation energy transfer inside individual monomers is fast (<2 ps at T = 5 K) and at least 20 times faster than intermonomer energy transfer.

Light energy conversion to electrical energy in photosynthesis and in photovoltaic devices is initiated by separation of charges that generates a photovoltage across an energy barrier in the system. [1][2][3] In conventional devices, mostly based on semiconductors, [ 4 ] the photovoltage is limited by the bandgap energy. Anomalous photovoltaic effects, in which the generated photovoltage is larger than the bandgap, have been reported in polar materials such as ferroelectric crystals, [ 5 , 6 ] but also in various amorphous and polycrystalline semiconductors. [7][8][9][10] Can a photoactive biological protein also generate an anomalous photovoltage? Natural candidates are photosynthetic reaction centers such as plant photosystem I (PSI), which are highly effi cient light energy converters found in thylakoid membranes inside the chloroplasts of green plants. [ 11 , 12 ] Here we show that these dried micrometer-thick crystalline biological protein complexes made of plant PSI [ 13 ] can generate unprecedented high photovoltages (well above 10 V) when placed on a conducting solid surface and measured by Kelvin probe force microscopy (KPFM). The measured photovoltages give rise to internal electric fi elds as large as 100 kV cm − 1 , which are among the highest values ever reported in an inorganic material system. We suggest that these high values result from depletion of the P700 electron donor centers and trapping centers in the PSI complex. We also present an equation depicting the dependence of the large photovoltage phenomenon in plant PSI crystals on light intensity. The unprecedented photovoltage, the microscale dimensions, and the high quantum yield make the PSI crystals intriguing units for potential use in optoelectronic devices.Photosynthetic reaction centers such as PSI evolved over a billion years to become highly effi cient light energy converters.The nanosized membrane complex containing chlorophyll molecules can generate a photopotential of approximately 1 V with a quantum effi ciency of ≈ 100%. [ 11 ] In the crystals of plant PSI, the complexes are serially oriented in stacks of over several hundred layers ( Figure 1 ) that absorb most of the visible light. [ 12 -14 ] We show that such dried PSI crystals generate very large photovoltages compared to any semiconductor, ferroelectric, and/or organic crystals.Each PSI complex consists of 18 polypeptides including four light-harvesting protein chlorophyll complexes binding a total of over 170 light-harvesting chlorophyll and over 30 carotenoid molecules. [ 14 ] The electron transport chain contains a chlorophyll a special pair (P700), four monomeric chlorophyll a molecules, two phylloquinones, and three [4Fe-4S] iron-sulphur clusters (Figure 1 a). P700, which undergoes light-induced charge separation in 1 ps, transfers an electron across the protein through the sequential carriers within 250 ns to the fi nal acceptor. [ 11 ] The redox potential of the primary donor P700 is + 0.43 V and that of the fi nal acceptor (4Fe-4S) F B is -0.53 V, producing a red...

Photosystem I coordinates more than 90 chlorophylls in its core antenna while achieving near perfect quantum efficiency. Low energy chlorophylls (also known as red chlorophylls) residing in the antenna are important for energy transfer dynamics and yield, however, their precise location remained elusive. Here, we construct a chimeric Photosystem I complex in Synechocystis PCC 6803 that shows enhanced absorption in the red spectral region. We combine Cryo-EM and spectroscopy to determine the structure−function relationship in this red-shifted Photosystem I complex. Determining the structure of this complex reveals the precise architecture of the low energy site as well as large scale structural heterogeneity which is probably universal to all trimeric Photosystem I complexes. Identifying the structural elements that constitute red sites can expand the absorption spectrum of oxygenic photosynthetic and potentially modulate light harvesting efficiency.