Photoreduction of dioxygen in photosystem I (PSI) of chloroplasts generates superoxide radicals as the primary product. In intact chloroplasts, the superoxide and the hydrogen peroxide produced via the disproportionation of superoxide are so rapidly scavenged at the site of their generation that the active oxygens do not inactivate the PSI complex, the stromal enzymes, or the scavenging system itself. The overall reaction for scavenging of active oxygens is the photoreduction of dioxygen to water via superoxide and hydrogen peroxide in PSI by the electrons derived from water in PSII, and the water-water cycle is proposed for these sequences. An overview is given of the molecular mechanism of the water-water cycle and microcompartmentalization of the enzymes participating in it. Whenever the water-water cycle operates properly for scavenging of active oxygens in chloroplasts, it also effectively dissipates excess excitation energy under environmental stress. The dual functions of the water-water cycle for protection from photoinihibition are discussed.

THE WATER-WATER CYCLE IN CHLOROPLASTS: Scavenging of Active Oxygens and Dissipation of Excess Photons

Infrared spectroscopy of proteins.

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.

https://ousar.lib.okayama-u.ac.jp/files/public/4/47455/20160528084139320094/Nature_473_55–60.pdf

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

We are just beginning to understand the metabolism of heavy metals and to use their metabolic functions in biotechnology, although heavy metals comprise the major part of the elements in the periodic table. Because they can form complex compounds, some heavy metal ions are essential trace elements, but, essential or not, most heavy metals are toxic at higher concentrations. This review describes the workings of known metal-resistance systems in microorganisms. After an account of the basic principles of homoeostasis for all heavy-metal ions, the transport of the 17 most important (heavy metal) elements is compared.

Microbial heavy-metal resistance

Photon yields of oxygen evolution at saturating CO2 were determined for 44 species of vascular plants, representing widely diverse taxa, habitats, life forms and growth conditions. The photonyield values on the basis of absorbed light (φa) were remarkably constant among plants possessing the same pathway of photosynthetic CO2 fixation, provided the plants had not been subjected to environmental stress. The mean φa value ±SE for 37 C3 species was 0.106±0.001 O2·photon-1. The five C4 species exhibited lower photon yields and greater variation than the C3 species (φa=0.0692±0.004). The φa values for the two Crassulaceanacid-metabolism species were similar to those of C3 species. Leaf chlorophyll content had little influence on φa over the range found in normal, healthy leaves. Chlorophyll fluorescence characteristics at 77 K were determined for the same leaves as used for the photon-yield measurements. Considerable variation in fluorescence emission both at 692 nm and at 734 nm, was found 1) among the different species; 2) between the upper and lower surfaces of the same leaves; and 3) between sun and shade leaves of the same species. By contrast, the ratio of variable to maximum fluorescence emission at 692 nm (Fv/FM, 692) remained remarkably constant (The mean value for the C3 species was 0.832±0.004). High-light treatments of shade leaves resulted in a reduction in both φa and the Fv/FM, 692 ratio. The extent of the reductions increased with time of exposure to bright light. A linear relationship was obtained when φa was plotted against Fv/FM, 692. The results show that determinations of the photon yield of O2 evolution and the Fv/FM, 692 ratio can serve as excellent quantitative measures of photoinhibition of overall photosynthetic energy-conversion system and of photochemistry of photosystem II, respectively. This is especially valuable in field work where it is often impossible to obtain appropriate controls.

Photon yield of O2 evolution and chlorophyll fluorescence characteristics at 77 K among vascular plants of diverse origins.

https://febs.onlinelibrary.wiley.com/doi/pdf/10.1016/0014-5793%2883%2980297-X

Mn-preserving extraction of 33-, 24- and 16-kDa proteins from O2-evolving PS II particles by divalent salt-washing

Cytochrome c-550, a low-potential c-type cytochrome, and a 12-kDa protein were recently shown to be associated extrinsically and stoichiometrically with purified photosystem II (PSII) complex of the thermophilic cyanobacterium Synechococcus vulcanus [Shen, J.-R., Ikeuchi, M., & Inoue, Y. (1992) FEBS Lett. 301, 145-149]. The binding and functional properties of these two extrinsic components in PSII were studied by means of release-reconstitution and thermoluminescence techniques. The following results were obtained: (i) cyt c-550 rebound appreciably to cyanobacterial PSII in the absence of the 33- and 12-kDa extrinsic proteins, but the presence of these two proteins facilitated the rebinding, affording a full level of binding equal to that in native PSII. (ii) The 12-kDa protein did not rebind to PSII at all unless the 33-kDa protein or cyt c-550 was present. It rebound only partially in the presence of either of these two proteins, but it rebound maximally when reconstituted together with both of them. (iii) Reconstitution with cyt c-550 or the 12-kDa protein alone in the absence of the 33-kDa protein did not restore the O2-evolving activity of CaCl2-washed PSII. Reconstitution with cyt c-550 in combination with the 33-kDa protein appreciably enhanced the activity, but the activity restoration was much more marked and reached a level close to that of the original activity when all three extrinsic proteins were included.(ABSTRACT TRUNCATED AT 250 WORDS)

Binding and functional properties of two new extrinsic components, cytochrome c-550 and a 12-kDa protein, in cyanobacterial photosystem II

A New 4.8-kDa Polypeptide Intrinsic to the PS II Reaction Center, as Revealed by Modified SDS-PAGE with Improved Resolution of Low-Molecular-Weight Proteins

Japan was one of the first countries toembrace structural genomics — Japanese-led projects focused on this effort were con-ceptualized as early as 1995. Officially,structural genomics efforts in Japan beganwith the Protein Folds Project (with S.Yokoyama as the project director), whichwas formed at the RIKEN Institute in April,1997 and was later transferred to the newly established RIKENGenomic Sciences Center (GSC;

Structural genomics projects in Japan.

https://febs.onlinelibrary.wiley.com/doi/pdf/10.1016/0014-5793%2888%2981039-1

Yorinao Inoue

Papers

Mn-preserving extraction of 33-, 24- and 16-kDa proteins from O2-evolving PS II particles by divalent salt-washing

Binding and functional properties of two new extrinsic components, cytochrome c-550 and a 12-kDa protein, in cyanobacterial photosystem II

A New 4.8-kDa Polypeptide Intrinsic to the PS II Reaction Center, as Revealed by Modified SDS-PAGE with Improved Resolution of Low-Molecular-Weight Proteins

Structural genomics projects in Japan.

A new photosystem II reaction center component (4.8 kDa protein) encoded by chloroplast genome