Is there a direct chloride cofactor requirement in the oxygen-evolving reactions of photosystem II?

The dark incubation at room temperature of photosystem II (PS II) membrane fragments in a chloride-free medium at pH 6.3 slowly leads to large chloride-restorable and non-restorable O₂ evolution activity losses with time as compared with control samples incubated in the presence of 10 mM NaCl. The chloride requirement in O₂ evolution generated under these conditions reveals a complex interplay among various experimental parameters, including the source of the plant material, the times of incubation, the sample concentration, the chloride concentration, as well as those treatments which are believed to specifically displace chloride from PS II such as alkaline pH pretreatment and Na₂SO₄ addition. The results indicate that secondary, structural changes within the PS II complex are an important factor in determining the influence of chloride on the O₂ evolution activity and raise the question whether or not chloride ions actually play a direct cofactor role in the water-oxidizing reactions leading to O₂ evolution.Abbreviations Chl chlorophyll - EPR electron paramagnetic resonance - MES 2-(N-morpholino) ethanesulfonic acid - NMR nuclear magnetic resonance - PS II photosystem II     

Current status of the role of Cl− ion in the oxygen-evolving complex

This minireview summarizes the current state of knowledge concerning the role of Cl⁻ in the oxygen-evolving complex (OEC) of photosystem II (PSII). The model that proposes that Cl⁻ is a Mn ligand is discussed in light of more recent work. Studies of Cl⁻ specificity, stoichiometry, kinetics, and retention by extrinsic polypeptides are discussed, as are the results that fail to detect Cl⁻ ligation to Mn and results that show a lack of a requirement for Cl⁻ in PSII-catalyzed H₂O oxidation. Mutagenesis experiments in cyanobacteria and higher plants that produce evidence for a correlation between Cl⁻ retention and stable interactions among intrinsic and extrinsic polypeptides are summarized, and spectroscopic data on the interaction between PSII and Cl⁻ are discussed. Lastly, the question of the site of Cl⁻ action in PSII is discussed in connection with the current crystal structures of the enzyme.     

A M?ssbauer analysis of the low-potential iron-sulfur center in photosystem I: spectroscopic evidence that FX is a [4Fe-4S] cluster

We report the results of a M?ssbauer study of the low-potential iron-sulfur cluster FX in the Photosystem I core protein of Synechococcus 6301. The M?ssbauer spectrum of FX in the oxidized state shows an isomer shift of 0.42 mm/s, which is in good agreement with the 0.43 mm/s isomer shift found in [4Fe-4S] proteins but not with the isomer shift of 0.26 mm/s found in [2Fe-2S] proteins. In the reduced state the spectrum is asymmetrically broadened at 80 K, indicating the presence of two very closely spaced doublets with an average isomer shift of 0.55 mm/s, which is also in agreement with [4Fe-4S] proteins. At 4.2 K, the spectrum exhibits broadening and magnetic splitting similar to what is observed for [4Fe-4S] proteins and quite unlike [2Fe-2S] proteins. Given the assumption that the iron atoms of FX are tetrahedrally coordinated with sulfur ligands, the data strongly support the assignment of FX as a [4Fe-4S] cluster.     

Cutting across nature? The history of artificial insemination in pigs in the United Kingdom

Artificial insemination (AI) has a considerable cultural significance in addition to its economic and technical impact. This study is the first to examine the history of its application to pigs, and uses evidence provided directly by both the scientists involved in its development, and some of the farmers who were among the first to use it, in addition to archival and published sources, to show how the scientific studies of the 1950s evolved into a widely available commercial product by the 1980s. It describes the initial scientific work and quantifies the extent to which the technique was used at various points in time, showing that by 1990 nearly one half of UK pig herds were using AI for more than 25% of all services. It traces changes in the techniques employed and argues that these were the result of a multi-dimensional process of contemporaneous change. The various dimensions are identified firstly as authorities, meaning the people and organisations controlling the perception, administration, control, and so on, of the technique; secondly the discourses employed by the authorities; and thirdly the media by which the discourses were disseminated. Finally, it is suggested that this approach might be used more widely to examine the construction of other technologies.     

Effect of salt stress on photosystem Ⅱ heterogeneity in wheat leaves

In order to study the effects of salt stress on photosystem Ⅱ (PSⅡ) heterogeneity, signal from the fluorometer was digitized via a microcomputer interface to record PSⅡ fluorescence induction kinetics. Changes of parameters (Fm, Fp1 and F0) from the fast phase of fluorescence induction curve showed that the percentage of QB-nonreducing PSⅡ reaction centers dropped at first, and then rose with the increase of stress intensity and time. This indicated that the electron transfer from QA to QB is one of the sites where the photochemical and photophysical processes of PSⅡ are damaged by the salt stress.     

Effect of protein modification by malondialdehyde on the interaction between the oxygen-evolving complex 33 kDa protein and photosystem II core proteins

Previously we observed that the oxygen-evolving complex 33 kDa protein (OEC33) which stabilizes the Mn cluster in photosystem II (PSII), was modified with malondialdehyde (MDA), an end-product of peroxidized polyunsaturated fatty acids, and the modification increased in heat-stressed plants (Yamauchi et al. 2008). In this study, we examined whether the modification of OEC33 with MDA affects its binding to the PSII complex and causes inactivation of the oxygen-evolving complex. Purified OEC33 and PSII membranes that had been removed of extrinsic proteins of the oxygen-evolving complex (PSII∆OEE) of spinach (Spinacia oleracea) were separately treated with MDA. The binding was diminished when both OEC33 and PSII∆OEE were modified, but when only OEC33 or PSII∆OEE was treated, the binding was not impaired. In the experiment using thylakoid membranes, release of OEC33 from PSII and corresponding loss of oxygen-evolving activity were observed when thylakoid membranes were treated with MDA at 40°C but not at 25°C. In spinach leaves treated at 40°C under light, maximal efficiency of PSII photochemistry (F _v/F _m ratio of chlorophyll fluorescence) and oxygen-evolving activity decreased. Simultaneously, MDA contents in heat-stressed leaves increased, and OEC33 and PSII core proteins including 47 and 43 kDa chlorophyll-binding proteins were modified with MDA. In contrast, these changes were to a lesser extent at 40°C in the dark. These results suggest that MDA modification of PSII proteins causes release of OEC33 from PSII and it is promoted in heat and oxidative conditions.     

The stable tyrosyl radical in photosystem II: why D?

Two redox-active tyrosines are present in Photosysytem II, the water-oxidizing enzyme. While the tyrosine that is kinetically competent in electron transfer, TyrZ, may also have a role in the enzyme mechanism, the second tyrosine, TyrD, has a stable radical and is not directly involved in the redox chemistry associated with enzyme function. Nevertheless, reasonable mechanistic roles for TyrD have been postulated that satisfy desires to rationalise the presence of this cofactor, or, in English, we think we know what it does. First, the TyrD radical acts an oxidant of the Mn cluster in the lowest state of the redox accumulation cycle (i.e., S(0)), providing potential benefits in maintaining the cluster in the more stable higher valence states. This redox role may also be important during Mn assembly and indeed overreduced forms of the Mn cluster appear to be oxidised by TyrD(*). Second, the proton generated by the TyrD radical is thought to remain in its vicinity having an electrostatic influence on the location and potential of the chlorophyll cation, P(+). This effect may be important for the kinetics of TyrZ oxidation and may provide a significant thermodynamic boost to the enzyme. In addition, through its electrostatic influence, TyrD(*)(H(+)) may confine the highly oxidising cation P(+) to the chlorophyll nearest to TyrZ, thereby accelerating TyrZ oxidation and restricting the potentially damaging redox chemistry to one side of the reaction centre: the disposable D1 side. This second role, evidence for which is beginning to emerge, constitutes a new role for a redox-active tyrosine in biology: as a positive charge generator in a hydrophobic environment. In this short review, we focus on work relevant to these two roles.     

Interaction of chlorophyll a′ with the 65 kDa subunit protein of photosystem I reaction center

The 65 kDa polypeptide subunit depleted of P700 was prepared from a photosystem I reaction center preparation and mixed with chlorophyll a′ (C-10 epimer of chlorophyll a) to yield a complex exhibiting a tripleheaded spectrum with absorbance maxima at 673, 692 and 707 nm. The difference spectra (oxidized-minus-untreated and light-minus-dark) had a major trough at 707 nm and minor ones at 690 and 430 nm. The overall shape of the spectra resembled well that of P700 with a small red shift. A rapidly decaying flash-induced absorbance change was observed at 430 nm with a half decay time of less than 500 μs in a preparation supplemented with an electron donor system.     

Energy dissipation in photosynthesis: Does the quenching of chlorophyll fluorescence originate from antenna complexes of photosystem II or from the reaction center?

Dissipation of light energy was studied in the moss Rhytidiadelphus squarrosus (Hedw.) Warnst., and in leaves of Spinacia oleracea L. and Arabidopsis thaliana (L.) Heynh., using chlorophyll fluorescence as an indicator reaction. Maximum chlorophyll fluorescence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU)-treated spinach leaves, as produced by saturating light and studied between +5 and −20 °C, revealed an activation energy ΔE of 0.11 eV. As this suggested recombination fluorescence produced by charge recombination between the oxidized primary donor of photosystem II and reduced pheophytin, a mathematical model explaining fluorescence, and based in part on known characteristics of primary electron-transport reactions, was developed. The model permitted analysis of different modes of fluorescence quenching, two localized in the reaction center of photosystem II and one in the light-harvesting system of the antenna complexes. It predicted differences in the relationship between quenching of variable fluorescence F _v and quenching of basal, so-called F ₀ fluorescence depending on whether quenching originated from antenna complexes or from reaction centers. Such differences were found experimentally, suggesting antenna quenching as the predominant mechanism of dissipation of light energy in the moss Rhytidiadelphus, whereas reaction-center quenching appeared to be important in spinach and Arabidopsis. Both reaction-center and antenna quenching required activation by thylakoid protonation but only antenna quenching depended on or was strongly enhanced by zeaxanthin. De-protonation permitted relaxation of this quenching with half-times below 1 min. More slowly reversible quenching, tentatively identified as so-called q _I or photoinhibitory quenching, required protonation but persisted for prolonged times after de-protonation. It appeared to originate in reaction centers. Received: 8 April 2000 / Accepted: 31 August 2000     

Ecology in the university first year?

The inclusion of ecology in first-year university biology courses presents considerable difficulties. The new course being evolved at Bath centres round three main themes—spatial distribution, the flow of energy, and the gene as an element of continuity. Practical work presents particular problems on account of the large number of students and a lack of time. The difficulty can be overcome to some extent by the use of simulation experiments which aim to reproduce in miniature within the laboratory certain restricted aspects of the real environment. Bearing in mind that, at most, only one visit to a natural habitat will be possible, it is important that full use should be made of good visual aids such as films and slides. Facilities must also be provided for students to check their own progress through problems derived from second-hand sources such as research papers.     

PsbP-induced protein conformational changes around Cl<Superscript>−</Superscript> ions in the water oxidizing center of photosystem II

PsbP is an extrinsic protein of PSII having a function of Ca²⁺ and Cl^? retention in the water-oxidizing center (WOC). In order to understand the mechanism how PsbP regulates the Cl^? binding in WOC, we examined the effect of PsbP depletion on the protein structures around the Cl^? sites using Fourier transform infrared (FTIR) spectroscopy. Light-induced FTIR difference spectra upon the S¹→S₂ transition were obtained using Cl^?-bound and NO₃^?-substituted PSII membranes in the presence and absence of PsbP. A clear difference in the amide I band changes by PsbP depletion was observed between Cl^?-bound and NO₃^?-substituted PSII samples, indicating that PsbP binding perturbed the protein conformations around the Cl^?ion(s) in WOC. It is suggested that PsbP stabilizes the Cl^? binding by regulating the dissociation constant of Cl^? and/or an energy barrier of Cl^? dissociation through protein conformational changes around the Cl^? ion(s).     

Sequence variation at the oxygen-evolving centre of photosystem II: a new class of ‘rogue’ cyanobacterial D1 proteins

Photosystem II is the oxygen-evolving enzyme of photosynthesis. It is a membrane-bound protein-pigment complex. The oxygen is produced at the oxygen-evolving centre (OEC), a Mn₄CaO₅ metallocluster, which is largely ligated by amino acids of the D1 protein. The OEC-ligating residues are invariant between most cyanobacteria and higher plants. In this study, a new class of cyanobacterial D1 proteins has been identified in which the OEC metal-ligating residues are very different to the consensus. This new class of ‘rogue’ D1 proteins is associated with diazotrophic cyanobacteria. Their function, activity and origins are discussed.     

Does the first LH surge of the breeding season initiate the first full-length cycle in the ewe?

To determine whether the first LH surge of the breeding season initiates a transient rise in progesterone in most ewes, serum progesterone (daily) and LH (every 4 h) concentrations were measured in samples collected from 7 ewes between 19 July and first oestrus or 8 September, whichever came first. In 6 of the 7 ewes, the first LH surge of the breeding season was followed within 5 days by a transient, 2-day rise in progesterone. Within less than 5 (N = 4), or 9 (N = 1) or 10 (N = 1) days later, a second LH surge occurred, which was similar in maximum amplitude and duration to the first surge, and which initiated the first full-length luteal phase of the breeding season. In the remaining ewe, the first LH surge of the breeding season induced an abbreviated (9 days) and insufficient (maximum progesterone, 0.94 ng/ml) luteal phase. These results demonstrate that most ewes have more than one LH surge before the first full-length luteal phase, the first surge inducing a transient rise in progesterone. Therefore, although the seasonal decrease in response to oestradiol negative feedback is sufficient for initiation of the first LH surge of the breeding season, additional endocrine mechanisms may be necessary to induce the first full-length luteal phase.     

What is beta-carotene doing in the photosystem II reaction centre?

During photosynthesis carotenoids normally serve as antenna pigments, transferring singlet excitation energy to chlorophyll, and preventing singlet oxygen production from chlorophyll triplet states, by rapid spin exchange and decay of the carotenoid triplet to the ground state. The presence of two beta-carotene molecules in the photosystem II reaction centre (RC) now seems well established, but they do not quench the triplet state of the primary electron-donor chlorophylls, which are known as P(680). The beta-carotenes cannot be close enough to P(680) for triplet quenching because that would also allow extremely fast electron transfer from beta-carotene to P(+)(680), preventing the oxidation of water. Their transfer of excitation energy to chlorophyll, though not very efficient, indicates close proximity to the chlorophylls ligated by histidine 118 towards the periphery of the two main RC polypeptides. The primary function of the beta-carotenes is probably the quenching of singlet oxygen produced after charge recombination to the triplet state of P(680). Only when electron donation from water is disturbed does beta-carotene become oxidized. One beta-carotene can mediate cyclic electron transfer via cytochrome b559. The other is probably destroyed upon oxidation, which might trigger a breakdown of the polypeptide that binds the cofactors that carry out charge separation.     

The acceptor side of photosystem II is damaged more severely than the donor side of photosystem II in ‘Honeycrisp’ apple leaves with zonal chlorosis

To investigate the photoinhibition of photosynthesis in ‘Honeycrisp’ apple (Malus domestica Borkh. cv. Gala) leaves with zonal chlorosis, we compared pigments, CO₂ assimilation and chlorophyll (Chl) a fluorescence (OJIP) transient between chlorotic leaves and normal ones. Chl and carotenoids (Car) contents, Chl a/b ratio, and absorptance were lower in chlorotic leaves than in normal ones, whereas Car/Chl ratio was higher in the former. Although CO₂ assimilation and stomatal conductance were lower in chlorotic leaves, intercellular CO₂ concentration did not differ significantly between the two leaf types. Compared with normal leaves, chlorotic ones had increased deactivation of oxygen-evolving complexes (OEC), minimum fluorescence (F _o), dissipated energy, relative variable fluorescence at L-, W-, J- and I-steps, and decreased maximum fluorescence (F _m), maximum quantum yield for primary photochemistry (F _v /F _m or TR_o/ABS), quantum yield for electron transport (ET_o/ABS), quantum yield for the reduction of end acceptors of photosystem I (PSI) (φ_Ro and RE_o/ABS), maximum amplitude of IP phase, amount of active photosystem II (PSII) reaction centers (RCs) per cross section (CS) and total performance index (PI_tot,abs). In conclusion, photoinhibition occurs at both the donor (i.e., the OEC) and the acceptor sides of PSII in chlorotic leaves. The acceptor side is damaged more severely than the donor side, which possibly is the consequence of over-reduction of PSII due to the slowdown of Calvin cycle. In addition to decreasing light absorptance by lowering Chl level, energy dissipation is enhanced to protect chlorotic leaves from photo-oxidative damage.     

Digest: The role of linkage in mimicking “magic traits”

Can divergence in a mating trait increase local adaption by increasing ecological divergence? Servedio and Bürger propose that “pseudomagic traits,” tightly linked complexes consisting of an ecological locus under divergent selection and a locus acting as a mating cue, can effectively mimic pleiotropy. Such pseudomagic traits can form even when linkage between ecological and mating loci is limited.     

The PsbQ′ protein affects the redox potential of the Q<Subscript>A</Subscript> in photosystem II

Red alga contains four extrinsic proteins in photosystem II (PSII), which are PsbO, PsbV, PsbU, and PsbQ′. Except for the PsbQ′, the composition is the same in cyanobacterial PSII. Reconstitution analysis of cyanobacterial PSII has shown that oxygen-evolving activity does not depend on the presence of PsbQ′. Recently, the structure of red algal PSII was elucidated. However, the role of PsbQ′ remains unknown. In this study, the function of the acceptor side of PSII was analyzed in PsbQ′-reconstituted PSII by redox titration of Q_A and thermoluminescence. The redox potential of Q_A was positively shifted when PsbQ′ was attached to the PSII. The positive shift of Q_A is thought to cause a decrease in the amount of triplet chlorophyll in PSII. On the basis of these results, we propose that PsbQ′ has a photoprotective function when irradiated with strong light.     

Effect of phosphatidylglycerol on conformation and micro-environment of tyrosyl residue in photosystem Ⅱ

The structural aspects in the interaction of phosphatidylglycerol (PG) with photosystem II (PSII),mainly the effect of PG on conformation and microenvironment of tyrosine residues of PSII proteins were studied by Fourier transform infrared (FTIR) spectroscopy.It was found that the binding of PG to PSII particle induces changes in the conformation and micropolarity of phenol ring in the tyrosine residues.In other words,the PG effect on the PSII results in blue shift of the stretch vibrational band in the phenol ring from 1620 to 1500 cm-1 with the enhancement of the absorbance intensity.Additionally,a new spectrum of hydrogen bond was also observed.The results imply that the hydrogen-bond formation between the OH group of phenol and one of PG might cause changes in the structures of tyrosine residues in PSII proteins.     

Trolox, a water-soluble analogue of α-tocopherol, photoprotects the surface-exposed regions of the photosystem II reaction center in vitro. Is this physiologically relevant?

Can Trolox, a water-soluble analogue of α-tocopherol and a scavenger of singlet oxygen ((1)O(2)), provide photoprotection, under high irradiance, to the isolated photosystem II (PSII) reaction center (RC)? To answer the question, we studied the endogenous production of (1)O(2) in preparations of the five-chlorophyll PSII RC (RC5) containing only one β-carotene molecule. The temporal profile of (1)O(2) emission at 1270 nm photogenerated by RC5 in D(2)O followed the expected biexponential behavior, with a rise time, unaffected by Trolox, of 13 ± 1 μs and decay times of 54 ± 2 μs (without Trolox) and 38 ± 2 μs (in the presence of 25 μM Trolox). The ratio between the total (k(t)) and chemical (k(r)) bimolecular rate constants for the scavenging of (1)O(2) by Trolox in aqueous buffer was calculated to be ~1.3, with a k(t) of (2.4 ± 0.2) × 10(8) M(-1) s(-1) and a k(r) of (1.8 ± 0.2) × 10(8) M(-1) s(-1), indicating that most of the (1)O(2) photosensitized by methylene blue chemically reacts with Trolox in the assay buffer. The photoinduced oxygen consumption in the oxygen electrode, when RC5 and Trolox were mixed, revealed that Trolox was a better (1)O(2) scavenger than histidine and furfuryl alcohol at low concentrations (i.e., <1 mM). After its incorporation into detergent micelles in unbuffered solutions, Trolox was able to photoprotect the surface-exposed regions of the D1-D2 heterodimer, but not the RC5 pigments, which were oxidized, together with the membrane region of the protein matrix of the PSII RC, by (1)O(2). These results are discussed and compared with those of studies dealing with the physiological role of tocopherol molecules as a (1)O(2) scavenger in thylakoid membranes of photosynthetic organisms.