Chlorophylls (Chl) play pivotal roles in energy capture, transfer and charge separation in photosynthesis. Among Chls functioning in oxygenic photosynthesis, Chl f is the most red-shifted type first ...found in a cyanobacterium Halomicronema hongdechloris. The location and function of Chl f in photosystems are not clear. Here we analyzed the high-resolution structures of photosystem I (PSI) core from H. hongdechloris grown under white or far-red light by cryo-electron microscopy. The structure showed that, far-red PSI binds 83 Chl a and 7 Chl f, and Chl f are associated at the periphery of PSI but not in the electron transfer chain. The appearance of Chl f is well correlated with the expression of PSI genes induced under far-red light. These results indicate that Chl f functions to harvest the far-red light and enhance uphill energy transfer, and changes in the gene sequences are essential for the binding of Chl f.
Photosynthetic light-harvesting complexes (LHCs) play a pivotal role in collecting solar energy for photochemical reactions in photosynthesis. One of the major LHCs are fucoxanthin chlorophyll ...a/c-binding proteins (FCPs) present in diatoms, a group of organisms having important contribution to the global carbon cycle. Here, we report a 2.40-Å resolution structure of the diatom photosystem I (PSI)-FCPI supercomplex by cryo-electron microscopy. The supercomplex is composed of 16 different FCPI subunits surrounding a monomeric PSI core. Each FCPI subunit showed different protein structures with different pigment contents and binding sites, and they form a complicated pigment-protein network together with the PSI core to harvest and transfer the light energy efficiently. In addition, two unique, previously unidentified subunits were found in the PSI core. The structure provides numerous insights into not only the light-harvesting strategy in diatom PSI-FCPI but also evolutionary dynamics of light harvesters among oxyphototrophs.
Photosynthetic water oxidation is catalyzed by the Mn
CaO
cluster of photosystem II (PSII) with linear progression through five S-state intermediates (S
to S
). To reveal the mechanism of water ...oxidation, we analyzed structures of PSII in the S
, S
, and S
states by x-ray free-electron laser serial crystallography. No insertion of water was found in S
, but flipping of D1 Glu
upon transition to S
leads to the opening of a water channel and provides a space for incorporation of an additional oxygen ligand, resulting in an open cubane Mn
CaO
cluster with an oxyl/oxo bridge. Structural changes of PSII between the different S states reveal cooperative action of substrate water access, proton release, and dioxygen formation in photosynthetic water oxidation.
Fucoxanthin chlorophyll (Chl) a/c-binding proteins (FCPs) function as light harvesters in diatoms. The structure of a diatom photosystem II-FCPII (PSII-FCPII) supercomplex have been solved by ...cryo-electron microscopy (cryo-EM) previously; however, the FCPII subunits that constitute the FCPII tetramers and monomers are not identified individually due to their low resolutions. Here, we report a 2.5 Å resolution structure of the PSII-FCPII supercomplex using cryo-EM. Two types of tetrameric FCPs, S-tetramer, and M-tetramer, are identified as different types of hetero-tetrameric complexes. In addition, three FCP monomers, m1, m2, and m3, are assigned to different gene products of FCP. The present structure also identifies the positions of most Chls c and diadinoxanthins, which form a complicated pigment network. Excitation-energy transfer from FCPII to PSII is revealed by time-resolved fluorescence spectroscopy. These structural and spectroscopic findings provide insights into an assembly model of FCPII and its excitation-energy transfer and quenching processes.
Photosystem I (PSI) is one of the two photosystems functioning in light-energy harvesting, transfer, and electron transfer in photosynthesis. However, the oligomerization state of PSI is variable ...among photosynthetic organisms. We present a 3.8-Å resolution cryo-electron microscopic structure of tetrameric PSI isolated from the glaucophyte alga Cyanophora paradoxa, which reveals differences with PSI from other organisms in subunit composition and organization. The PSI tetramer is organized in a dimer of dimers with a C2 symmetry. Unlike cyanobacterial PSI tetramers, two of the four monomers are rotated around 90°, resulting in a completely different pattern of monomer-monomer interactions. Excitation-energy transfer among chlorophylls differs significantly between Cyanophora and cyanobacterial PSI tetramers. These structural and spectroscopic features reveal characteristic interactions and excitation-energy transfer in the Cyanophora PSI tetramer, suggesting that the Cyanophora PSI could represent a turning point in the evolution of PSI from prokaryotes to eukaryotes.
•Experimental results on the natural water oxidation catalyst are summarized.•Theoretical studies on the natural water oxidation catalyst are summarized and compared with the experimental ...results.•Comparisons between experimental and theoretical studies point to a unified mechanism for natural water oxidation.•The catalyst serves as a blueprint for artificial catalyst of water oxidation.
The aim of this review is to elucidate geometric structures of the catalytic CaMn4Ox (x = 5, 6) cluster in the Kok cycle for water oxidation in the oxygen evolving complex (OEC) of photosystem II (PSII) based on the high-resolution (HR) X-ray diffraction (XRD) and serial femtosecond crystallography (SFX) experiments using the X-ray free-electron laser (XFEL). Quantum mechanics (QM) and QM/molecular mechanics (MM) computations are performed to elucidate the electronic and spin structures of the CaMn4Ox (x = 5, 6) cluster in five states Si (i = 0 ∼ 4) on the basis of the X-ray spectroscopy, electron paramagnetic resonance (EPR) and related experiments. Interplay between the experiments and theoretical computations has been effective to elucidate the coordination structures of the CaMn4Ox (x = 5, 6) cluster ligated by amino acid residues of the protein matrix of PSII, valence states of the four Mn ions and total spin states by their exchange-couplings, and proton-shifted isomers of the CaMn4Ox (x = 5, 6) cluster. The HR XRD and SFX XFEL experiments have also elucidated the biomolecular systems structure of OEC of PSII and the hydrogen bonding networks consisting of water molecules, chloride anions, etc., for water inlet and proton release pathways in PSII. Large-scale QM/MM computations have been performed for elucidation of the hydrogen bonding distances and angles by adding invisible hydrogen atoms to the HR XRD structure. Full geometry optimizations by the QM and QM/MM methods have been effective for elucidation of the molecular systems structure around the CaMn4Ox (x = 5, 6) cluster in OEC. DLPNO-CCSD(T0) method has been applied to elucidate relative energies of possible intermediates in each state of the Kok cycle for water oxidation. Implications of these results are discussed in relation to the blueprint for developments of artificial catalysts for water oxidation.
In plants and green algae, the core of photosystem I (PSI) is surrounded by a peripheral antenna system consisting of light-harvesting complex I (LHCI). Here we report the cryo-electron microscopic ...structure of the PSI-LHCI supercomplex from the green alga Chlamydomonas reinhardtii. The structure reveals that eight Lhca proteins form two tetrameric LHCI belts attached to the PsaF side while the other two Lhca proteins form an additional Lhca2/Lhca9 heterodimer attached to the opposite side. The spatial arrangement of light-harvesting pigments reveals that Chlorophylls b are more abundant in the outer LHCI belt than in the inner LHCI belt and are absent from the core, thereby providing the downhill energy transfer pathways to the PSI core. PSI-LHCI is complexed with a plastocyanin on the patch of lysine residues of PsaF at the luminal side. The assembly provides a structural basis for understanding the mechanism of light-harvesting, excitation energy transfer of the PSI-LHCI supercomplex and electron transfer with plastocyanin.
Photosystem II (PSII) functions mainly as a dimer to catalyze the light energy conversion and water oxidation reactions. However, monomeric PSII also exists and functions in vivo in some cases. The ...crystal structure of monomeric PSII has been solved at 3.6 Å resolution, but it is still not clear which factors contribute to the formation of the dimer. Here, we solved the structure of PSII monomer at a resolution of 2.78 Å using cryo-electron microscopy (cryo-EM). From our cryo-EM density map, we observed apparent differences in pigments and lipids in the monomer-monomer interface between the PSII monomer and dimer. One β-carotene and two sulfoquinovosyl diacylglycerol (SQDG) molecules are found in the monomer-monomer interface of the dimer structure but not in the present monomer structure, although some SQDG and other lipid molecules are found in the analogous region of the low-resolution crystal structure of the monomer, or cryo-EM structure of an apo-PSII monomer lacking the extrinsic proteins from Synechocystis sp. PCC 6803. In the current monomer structure, a large part of the PsbO subunit was also found to be disordered. These results indicate the importance of the β-carotene, SQDG and PsbO in formation of the PSII dimer.
•We solved the structure of PSII monomer at a resolution of 2.78 Å using cryo-electron microscopy.•A β-carotene and one or two SQDGs located in the monomer-monomer interface contribute to the formation of PSII dimer.•PsbO may also be important for the formation of the dimer.•Some additional differences existed between the present monomer structure and the previous dimer structure.
Photosystem II (PSII) plays a key role in water-splitting and oxygen evolution. X-ray crystallography has revealed its atomic structure and some intermediate structures. However, these structures are ...in the crystalline state and its final state structure has not been solved. Here we analyzed the structure of PSII in solution at 1.95 Å resolution by single-particle cryo-electron microscopy (cryo-EM). The structure obtained is similar to the crystal structure, but a PsbY subunit was visible in the cryo-EM structure, indicating that it represents its physiological state more closely. Electron beam damage was observed at a high-dose in the regions that were easily affected by redox states, and reducing the beam dosage by reducing frames from 50 to 2 yielded a similar resolution but reduced the damage remarkably. This study will serve as a good indicator for determining damage-free cryo-EM structures of not only PSII but also all biological samples, especially redox-active metalloproteins.
Photosystem I (PSI) functions to harvest light energy for conversion into chemical energy. The organisation of PSI is variable depending on the species of organism. Here we report the structure of a ...tetrameric PSI core isolated from a cyanobacterium, Anabaena sp. PCC 7120, analysed by single-particle cryo-electron microscopy (cryo-EM) at 3.3 Å resolution. The PSI tetramer has a C2 symmetry and is organised in a dimer of dimers form. The structure reveals interactions at the dimer-dimer interface and the existence of characteristic pigment orientations and inter-pigment distances within the dimer units that are important for unique excitation energy transfer. In particular, characteristic residues of PsaL are identified to be responsible for the formation of the tetramer. Time-resolved fluorescence analyses showed that the PSI tetramer has an enhanced excitation-energy quenching. These structural and spectroscopic findings provide insights into the physiological significance of the PSI tetramer and evolutionary changes of the PSI organisations.
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