James Barber

X-ray crystallography identifies two chloride binding sites in the oxygen evolving centre of Photosystem II

Bromide anomalous X-ray diffraction analyses have been used to locate chloride binding sites in the vicinity of the water splitting/oxygen evolving centre (OEC) of Photosystem II. Three-dimensional crystals of PSII from Thermosynechococcus elongatus were grown from (i) isolated PSII crystals infiltrated with bromide or (ii) PSII obtained from cells cultured in a medium in which the chloride content was totally replaced by bromide. In either case, the anomalous diffraction yielded the same result, the existence of two bromide binding sites in the vicinity of the OEC. Neither are in the first coordination sphere of the Mn and Ca ions which form the catalytic centre of the OEC, being about 6 to 7 A from the metal-cluster. Site 1 is located close to the side chain nitrogen of D2-K317 and the backbone nitrogen of D1-Glu333 while Site 2 is adjacent to backbone nitrogens of CP43-Glu354 and D1-Asn338. Their positioning close to postulated hydrophilic channels may suggest a role in proton removal from, or substrate access to, the OEC.

Biological solar energy

Through the process of photosynthesis, the energy of sunlight has been harnessed, not only to create the biomass on our planet today, but also the fossil fuels. The overall efficiency of biomass formation, however, is low and despite being a valuable source of energy, it cannot replace fossil fuels on a global scale and provide the huge amount of power needed to sustain the technological aspirations of the world population now and in the future. However, at the heart of the photosynthetic process is the highly efficient chemical reaction of water splitting, leading to the production of hydrogen equivalents and molecular oxygen. This reaction takes place in an enzyme known as photosystem II, and the recent determination of its structure has given strong hints of how nature uses solar energy to generate hydrogen and oxygen from water. This new information provides a blue print for scientists to seriously consider constructing catalysts that mimic the natural system and thus stimulate new technologies to address the energy/CO2 problem that humankind must solve. After all, there is no shortage of water for this non-polluting reaction and the energy content of sunlight falling on our planet well exceeds our needs.

Photosynthetic acclimation: Structural reorganisation of light harvesting antenna - role of redox-dependent phosphorylation of major and minor chlorophyll a/b binding proteins

In order to carry out photosynthesis, plants and algae rely on the co‐operative interaction of two photosystems: photosystem I and photosystem II. For maximum efficiency, each photosystem should absorb the same amount of light. To achieve this, plants and green algae have a mobile pool of chlorophyll a/b‐binding proteins that can switch between being light‐harvesting antenna for photosystem I or photosystem II, in order to maintain an optimal excitation balance. This switch, termed state transitions, involves the reversible phosphorylation of the mobile chlorophyll a/b‐binding proteins, which is regulated by the redox state of the plastoquinone‐mediating electron transfer between photosystem I and photosystem II. In this review, we will present the data supporting the function of redox‐dependent phosphorylation of the major and minor chlorophyll a/b‐binding proteins by the specific thylakoid‐bound kinases (Stt7, STN7, TAKs) providing a molecular switch for the structural remodelling of the light‐harvesting complexes during state transitions. We will also overview the latest X‐ray crystallographic and electron microscopy‐derived models for structural re‐arrangement of the light‐harvesting antenna during State 1‐to‐State 2 transition, in which the minor chlorophyll a/b‐binding protein, CP29, and the mobile light‐harvesting complex II trimer detach from the light‐harvesting complex II–photosystem II supercomplex and associate with the photosystem I core in the vicinity of the PsaH/L/O/P domain.

Analysis of xenon binding to photosystem II by X-ray crystallography

In order to investigate oxygen binding and hydrophobic cavities in photosystem II (PSII), we have introduced xenon under pressure into crystals of PSII isolated from Thermosynechococcus elongatus and used X-ray anomalous diffraction analyses to identify the xenon sites in the complex. Under the conditions employed, 25 Xe-binding sites were identified in each monomer of the dimeric PSII complex. The majority of these were distributed within the membrane spanning portion of the complex with no obvious correlation with the previously proposed oxygen channels. One binding site was located close to the haem of cytochrome b559 in a position analogous to a Xe-binding site of myoglobin. The only Xe-binding site not associated with the intrinsic subunits of PSII was within the hydrophobic core of the PsbO protein.

Spectroscopic studies of the chlorophyll d containing photosystem I from the cyanobacterium, Acaryochloris marina.

Absorbance difference spectroscopy and redox titrations have been applied to investigate the properties of photosystem I from the chlorophyll d containing cyanobacterium Acaryochloris marina. At room temperature, the (P740(+)-P740) and (F(A/B)(-)-F(A/B)) absorbance difference spectra were recorded in the range between 300 and 1000 nm while at cryogenic temperatures, (P740(+)A(1)(-)-P740A(1)) and ((3)P740-P740) absorbance difference spectra have been measured. Spectroscopic and kinetic evidence is presented that the cofactors involved in the electron transfer from the reduced secondary electron acceptor, phylloquinone (A(1)(-)), to the terminal electron acceptor and their structural arrangement are virtually identical to those of chlorophyll a containing photosystem I. The oxidation potential of the primary electron donor P740 of photosystem I has been reinvestigated. We find a midpoint potential of 450+/-10 mV in photosystem I-enriched membrane fractions as well as in thylakoids which is very similar to that found for P700 in chlorophyll a dominated organisms. In addition, the extinction difference coefficient for the oxidation of the primary donor has been determined and a value of 45,000+/-4000 M(-1) cm(-1) at 740 nm was obtained. Based on this value the ratio of P740 to chlorophyll is calculated to be 1 : to approximately 200 chlorophyll d in thylakoid membranes. The consequences of our findings for the energetics in photosystem I of A. marina are discussed as well as the pigment stoichiometry and spectral characteristics of P740.

Recovery of photoinactivated photosystem II in leaves: retardation due to restricted mobility of photosystem II in the thylakoid membrane

The functionality of photosystem II (PS II) following high-light pre-treatment of leaf segments at a chilling temperature was monitored as Fv/Fm, the ratio of variable to maximum chlorophyll fluorescence in the dark-adapted state and a measure of the optimal photochemical efficiency in PS II. Recovery of PS II functionality in low light (LL) and at a favourable temperature was retarded by (1) water stress and (2) growth in LL, in both spinach and Alocasia macrorrhiza L. In spinach leaf segments, water stress per se affected neither Fv/Fm nor the ability of the adenosine triphosphate (ATP) synthase to be activated by far-red light for ATP synthesis, but it induced chloroplast shrinkage as observed in frozen and fractured samples by scanning electron microscopy. A common feature of water stress and growth of plants in LL is the enhanced anchoring of PS II complexes, either across the shrunken lumen in water-stress conditions or across the partition gap in larger grana due to growth in LL. We suggest that such enhanced anchoring restricts the mobility of PS II complexes in the thylakoid membrane system, and hence hinders the lateral migration of photoinactivated PS II reaction centres to the stroma-located ribosomes for repair.

The relationship between the yield factors for prompt and delayed fluorescence.

where J is the rate of chlorophyll singlet formation and @DF is the fluorescence yield of the chlorophyll molecules through which the delayed fluorescence exciton migrates before de-excitation. To understand the relationship between @DF and @, has important implications since it would reveal information about the extent of the migration of an exciton away from the trap from which it originated [2-51 . Unfortunately earlier studies designed to investigate the