Keisuke Kawakami

Bio: Keisuke Kawakami is an academic researcher from Osaka City University. The author has contributed to research in topics: Photosystem II & Photosystem I. The author has an hindex of 24, co-authored 63 publications receiving 5068 citations. Previous affiliations of Keisuke Kawakami include Okayama University.

Crystal structure of oxygen-evolving photosystem II at a resolution of 1.9 Å.

Abstract: Photosystem II is the site of photosynthetic water oxidation and contains 20 subunits with a total molecular mass of 350 kDa. The structure of photosystem II has been reported at resolutions from 3.8 to 2.9 angstrom. These resolutions have provided much information on the arrangement of protein subunits and cofactors but are insufficient to reveal the detailed structure of the catalytic centre of water splitting. Here we report the crystal structure of photosystem II at a resolution of 1.9 angstrom. From our electron density map, we located all of the metal atoms of the Mn(4)CaO(5) cluster, together with all of their ligands. We found that five oxygen atoms served as oxo bridges linking the five metal atoms, and that four water molecules were bound to the Mn(4)CaO(5) cluster; some of them may therefore serve as substrates for dioxygen formation. We identified more than 1,300 water molecules in each photosystem II monomer. Some of them formed extensive hydrogen-bonding networks that may serve as channels for protons, water or oxygen molecules. The determination of the high-resolution structure of photosystem II will allow us to analyse and understand its functions in great detail.

Location of chloride and its possible functions in oxygen-evolving photosystem II revealed by X-ray crystallography

Abstract: The chloride ion, Cl−, is an essential cofactor for oxygen evolution of photosystem II (PSII) and is closely associated with the Mn4Ca cluster Its detailed location and function have not been identified, however We substituted Cl− with a bromide ion (Br−) or an iodide ion (I−) in PSII and analyzed the crystal structures of PSII with Br− and I− substitutions Substitution of Cl− with Br− did not inhibit oxygen evolution, whereas substitution of Cl− with I− completely inhibited oxygen evolution, indicating the efficient replacement of Cl− by I− PSII with Br− and I− substitutions were crystallized, and their structures were analyzed The results showed that there are 2 anion-binding sites in each PSII monomer; they are located on 2 sides of the Mn4Ca cluster at equal distances from the metal cluster Anion-binding site 1 is close to the main chain of D1-Glu-333, and site 2 is close to the main chain of CP43-Glu-354; these 2 residues are coordinated directly with the Mn4Ca cluster In addition, site 1 is located in the entrance of a proton exit channel These results indicate that these 2 Cl− anions are required to maintain the coordination structure of the Mn4Ca cluster as well as the proposed proton channel, thereby keeping the oxygen-evolving complex fully active

S1-State Model of the O2-Evolving Complex of Photosystem II

Abstract: We introduce a quantum mechanics/molecular mechanics model of the oxygen-evolving complex of photosystem II in the S1 Mn4(IV,III,IV,III) state, where Ca2+ is bridged to manganese centers by the carboxylate moieties of D170 and A344 on the basis of the new X-ray diffraction (XRD) model recently reported at 19 A resolution The model is also consistent with high-resolution spectroscopic data, including polarized extended X-ray absorption fine structure data of oriented single crystals Our results provide refined intermetallic distances within the Mn cluster and suggest that the XRD model most likely corresponds to a mixture of oxidation states, including species more reduced than those observed in the catalytic cycle of water splitting

Structure of the catalytic, inorganic core of oxygen-evolving photosystem II at 1.9 Å resolution

Abstract: The catalytic center for photosynthetic water-splitting consists of 4 Mn atoms and 1 Ca atom and is located near the lumenal surface of photosystem II. So far the structure of the Mn(4)Ca-cluster has been studied by a variety of techniques including X-ray spectroscopy and diffraction, and various structural models have been proposed. However, its exact structure is still unknown due to the limited resolution of crystal structures of PSII achieved so far, as well as possible radiation damages that might have occurred. Very recently, we have succeeded in solving the structure of photosystem II at 1.9 angstrom. which yielded a detailed picture of the Mn(4)CaO(5)-cluster for the first time. In the high resolution structure, the Mn(4)CaO(5)-cluster is arranged in a distorted chair form, with a cubane-like structure formed by 3 Mn and 1 Ca, 4 oxygen atoms as the distorted base of the chair, and 1 Mn and 1 oxygen atom outside of the cubane as the back of the chair. In addition, four water molecules were associated with the cluster, among which, two are associated with the terminal Mn atom and two are associated with the Ca atom. Some of these water molecules may therefore serve as the substrates for water-splitting. The high resolution structure of the catalytic center provided a solid basis for elucidation of the mechanism of photosynthetic water splitting. We review here the structural features of the Mn(4)CaO(5)-cluster analyzed at 1.9 angstrom resolution, and compare them with the structures reported previously.

Theoretical illumination of water-inserted structures of the CaMn4O5 cluster in the S2 and S3 states of oxygen-evolving complex of photosystem II: full geometry optimizations by B3LYP hybrid density functional.

Abstract: Full geometry optimizations of several inorganic model clusters, CaMn4O4XYZ(H2O)2 (X, Y, Z = H2O, OH− or O2−), by the use of the B3LYP hybrid density functional theory (DFT) have been performed to illuminate plausible molecular structures of the catalytic site for water oxidation in the S0, S1, S2 and S3 states of the Kok cycle for the oxygen-evolving complex (OEC) of photosystem II (PSII). Optimized geometries obtained by the energy gradient method have revealed the degree of symmetry breaking of the unstable three-center Mna–X–Mnd bond in CaMn4O4XYZ(H2O)2. The right-elongated (R) Mna–X⋯Mnd and left-elongated (L) Mna⋯X–Mnd structures appear to occupy local minima on a double-well potential for several key intermediates in these states. The effects of insertion of one extra water molecule to the vacant coordination site, Mnd (Mna), for R (L) structures have also been examined in detail. The greater stability of the L-type structure over the R-type has been concluded for key intermediates in the S2 and S3 states. Implications of the present DFT structures are discussed in relation to previous DFT and related results, together with recent X-ray diffraction results for model compounds of cubane-like OEC cluster of PSII.

Crystal structure of oxygen-evolving photosystem II at a resolution of 1.9 Å.

Abstract: Photosystem II is the site of photosynthetic water oxidation and contains 20 subunits with a total molecular mass of 350 kDa. The structure of photosystem II has been reported at resolutions from 3.8 to 2.9 angstrom. These resolutions have provided much information on the arrangement of protein subunits and cofactors but are insufficient to reveal the detailed structure of the catalytic centre of water splitting. Here we report the crystal structure of photosystem II at a resolution of 1.9 angstrom. From our electron density map, we located all of the metal atoms of the Mn(4)CaO(5) cluster, together with all of their ligands. We found that five oxygen atoms served as oxo bridges linking the five metal atoms, and that four water molecules were bound to the Mn(4)CaO(5) cluster; some of them may therefore serve as substrates for dioxygen formation. We identified more than 1,300 water molecules in each photosystem II monomer. Some of them formed extensive hydrogen-bonding networks that may serve as channels for protons, water or oxygen molecules. The determination of the high-resolution structure of photosystem II will allow us to analyse and understand its functions in great detail.

Metal–air batteries: from oxygen reduction electrochemistry to cathode catalysts

Abstract: Because of the remarkably high theoretical energy output, metal–air batteries represent one class of promising power sources for applications in next-generation electronics, electrified transportation and energy storage of smart grids. The most prominent feature of a metal–air battery is the combination of a metal anode with high energy density and an air electrode with open structure to draw cathode active materials (i.e., oxygen) from air. In this critical review, we present the fundamentals and recent advances related to the fields of metal–air batteries, with a focus on the electrochemistry and materials chemistry of air electrodes. The battery electrochemistry and catalytic mechanism of oxygen reduction reactions are discussed on the basis of aqueous and organic electrolytes. Four groups of extensively studied catalysts for the cathode oxygen reduction/evolution are selectively surveyed from materials chemistry to electrode properties and battery application: Pt and Pt-based alloys (e.g., PtAu nanoparticles), carbonaceous materials (e.g., graphene nanosheets), transition-metal oxides (e.g., Mn-based spinels and perovskites), and inorganic–organic composites (e.g., metal macrocycle derivatives). The design and optimization of air-electrode structure are also outlined. Furthermore, remarks on the challenges and perspectives of research directions are proposed for further development of metal–air batteries (219 references).

Artificial photosynthesis for solar water-splitting

Abstract: Hydrogen from solar-driven water splitting has the potential to provide clean energy. Current progress towards artificial photosynthetic devices is reviewed, with particular focus on visible light active nanostructures. A vision for a future sustainable hydrogen fuel community based on artificial photosynthesis is outlined.

Wireless Solar Water Splitting Using Silicon-Based Semiconductors and Earth-Abundant Catalysts

Abstract: We describe the development of solar water-splitting cells comprising earth-abundant elements that operate in near-neutral pH conditions, both with and without connecting wires. The cells consist of a triple junction, amorphous silicon photovoltaic interfaced to hydrogen- and oxygen-evolving catalysts made from an alloy of earth-abundant metals and a cobalt|borate catalyst, respectively. The devices described here carry out the solar-driven water-splitting reaction at efficiencies of 4.7% for a wired configuration and 2.5% for a wireless configuration when illuminated with 1 sun (100 milliwatts per square centimeter) of air mass 1.5 simulated sunlight. Fuel-forming catalysts interfaced with light-harvesting semiconductors afford a pathway to direct solar-to-fuels conversion that captures many of the basic functional elements of a leaf.

The artificial leaf

Abstract: To convert the energy of sunlight into chemical energy, the leaf splits water via the photosynthetic process to produce molecular oxygen and hydrogen, which is in a form of separated protons and electrons. The primary steps of natural photosynthesis involve the absorption of sunlight and its conversion into spatially separated electron–hole pairs. The holes of this wireless current are captured by the oxygen evolving complex (OEC) of photosystem II (PSII) to oxidize water to oxygen. The electrons and protons produced as a byproduct of the OEC reaction are captured by ferrodoxin of photosystem I. With the aid of ferrodoxin–NADP+ reductase, they are used to produce hydrogen in the form of NADPH. For a synthetic material to realize the solar energy conversion function of the leaf, the light-absorbing material must capture a solar photon to generate a wireless current that is harnessed by catalysts, which drive the four electron/hole fuel-forming water-splitting reaction under benign conditions and under 1 su...