Tuning of the ChlD1 and ChlD2 properties in photosystem II by site-directed mutagenesis of neighbouring amino acids

Photosystem II is the water/plastoquinone photo-oxidoreductase of photosynthesis. The photochemistry and catalysis occur in a quasi-symmetrical heterodimer, D1D2, that evolved from a homodimeric ancestor. Here, we studied site-directed mutants in PSII from the thermophilic cyanobacterium Thermosynechoccocus elongatus, focusing on the primary electron donor chlorophyll a in D1, ChlD1, and on its symmetrical counterpart in D2, ChlD2, which does not play a direct photochemical role. The main conserved amino acid specific to ChlD1 is D1/T179, which H-bonds the water ligand to its Mg2+, while its counterpart near ChlD2 is the non-H-bonding D2/I178. The symmetrical-swapped mutants, D1/T179I and D2/I178T, and a second ChlD2 mutant, D2/I178H, were studied. The D1 mutations affected the 686 nm absorption attributed to ChlD1, while the D2 mutations affected a 663 nm feature, tentatively attributed to ChlD2. The mutations had little effect on enzyme activity and forward electron transfer, reflecting the robustness of the overall enzyme function. In contrast, the mutations significantly affected photodamage and protective mechanisms, reflecting the importance of redox tuning in these processes. In D1/T179I, the radical pair recombination triplet on ChlD1 was shared onto a pheophytin, presumably PheD1 and the detection of 3PheD1 supports the proposed mechanism for the anomalously short lifetime of 3ChlD1; e.g. electron transfer quenching by QA- of 3PheD1 after triplet transfer from 3ChlD1. In D2/I178T, a charge separation could occur between ChlD2 and PheD2, a reaction that is thought to occur in ancestral precursors of PSII. These mutants help understand the evolution of asymmetry in PSII.

Absorption changes in Photosystem II in the Soret band region upon the formation of the chlorophyll cation radical [PD1PD2]. PHOTOSYNTHESIS RESEARCH 2023:10.1007/s11120-023-01049-3. [PMID: 37751034 DOI: 10.1007/s11120-023-01049-3] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

Flash-induced absorption changes in the Soret region arising from the [PD1PD2]+ state, the chlorophyll cation radical formed upon light excitation of Photosystem II (PSII), were measured in Mn-depleted PSII cores at pH 8.6. Under these conditions, TyrD is i) reduced before the first flash, and ii) oxidized before subsequent flashes. In wild-type PSII, when TyrD● is present, an additional signal in the [PD1PD2]+-minus-[PD1PD2] difference spectrum was observed when compared to the first flash when TyrD is not oxidized. The additional feature was "W-shaped" with troughs at 434 nm and 446 nm. This feature was absent when TyrD was reduced, but was present (i) when TyrD was physically absent (and replaced by phenylalanine) or (ii) when its H-bonding histidine (D2-His189) was physically absent (replaced by a Leucine). Thus, the simple difference spectrum without the double trough feature at 434 nm and 446 nm, seemed to require the native structural environment around the reduced TyrD and its H bonding partners to be present. We found no evidence of involvement of PD1, ChlD1, PheD1, PheD2, TyrZ, and the Cytb559 heme in the W-shaped difference spectrum. However, the use of a mutant of the PD2 axial His ligand, the D2-His197Ala, shows that the PD2 environment seems involved in the formation of "W-shaped" signal.

Energetics and proton release in photosystem II from Thermosynechococcus elongatus with a D1 protein encoded by either the psbA2 or psbA3 gene

In the cyanobacterium Thermosynechococcus elongatus, there are three psbA genes coding for the Photosystem II (PSII) D1 subunit that interacts with most of the main cofactors involved in the electron transfers. Recently, the 3D crystal structures of both PsbA2-PSII and PsbA3-PSII have been solved [Nakajima et al., J. Biol. Chem. 298 (2022) 102668.]. It was proposed that the loss of one hydrogen bond of Phe_D1 due to the D1-Y147F exchange in PsbA2-PSII resulted in a more negative E_m of Phe_D1 in PsbA2-PSII when compared to PsbA3-PSII. In addition, the loss of two water molecules in the Cl-1 channel was attributed to the D1-P173M substitution in PsbA2-PSII. This exchange, by narrowing the Cl-1 proton channel, could be at the origin of a slowing down of the proton release. Here, we have continued the characterization of PsbA2-PSII by measuring the thermoluminescence from the S₂Q_A^-/DCMU charge recombination and by measuring proton release kinetics using time-resolved absorption changes of the dye bromocresol purple. It was found that i) the E_m of Phe_D1^-•/Phe_D1 was decreased by ~30 mV in PsbA2-PSII when compared to PsbA3-PSII and ii) the kinetics of the proton release into the bulk was significantly slowed down in PsbA2-PSII in the S₂Tyr_Z^• to S₃Tyr_Z and S₃Tyr_Z^• → (S₃Tyr_Z^•)' transitions. This slowing down was partially reversed by the PsbA2/M173P mutation and induced by the PsbA3/P173M mutation thus confirming a role of the D1-173 residue in the egress of protons trough the Cl-1 channel.

Crystal structures of photosystem II from a cyanobacterium expressing psbA2 in comparison to psbA3 reveal differences in the D1 subunit

Three psbA genes (psbA₁, psbA₂, and psbA₃) encoding the D1 subunit of photosystem II (PSII) are present in the thermophilic cyanobacterium Thermosynechococcus elongatus and are expressed differently in response to changes in the growth environment. To clarify the functional differences of the D1 protein expressed from these psbA genes, PSII dimers from two strains, each expressing only one psbA gene (psbA₂ or psbA₃), were crystallized, and we analyzed their structures at resolutions comparable to previously studied PsbA1-PSII. Our results showed that the hydrogen bond between pheophytin/D1 (Pheo_D1) and D1-130 became stronger in PsbA2- and PsbA3-PSII due to change of Gln to Glu, which partially explains the increase in the redox potential of Pheo_D1 observed in PsbA3. In PsbA2, one hydrogen bond was lost in Pheo_D1 due to the change of D1-Y147F, which may explain the decrease in stability of Pheo_D1 in PsbA2. Two water molecules in the Cl-1 channel were lost in PsbA2 due to the change of D1-P173M, leading to the narrowing of the channel, which may explain the lower efficiency of the S-state transition beyond S₂ in PsbA2-PSII. In PsbA3-PSII, a hydrogen bond between D1-Ser270 and a sulfoquinovosyl-diacylglycerol molecule near Q_B disappeared due to the change of D1-Ser270 in PsbA1 and PsbA2 to D1-Ala270. This may result in an easier exchange of bound Q_B with free plastoquinone, hence an enhancement of oxygen evolution in PsbA3-PSII due to its high Q_B exchange efficiency. These results provide a structural basis for further functional examination of the three PsbA variants.

Probing the proton release by Photosystem II in the S1 to S2 high-spin transition

The stoichiometry and kinetics of the proton release were investigated during each transition of the S-state cycle in Photosystem II (PSII) from Thermosynechococcus elongatus containing either a Mn₄CaO₅ (PSII/Ca) or a Mn₄SrO₅ (PSII/Sr) cluster. The measurements were done at pH 6.0 and pH 7.0 knowing that, in PSII/Ca at pH 6.0 and pH 7.0 and in PSII/Sr at pH 6.0, the flash-induced S₂-state is in a low-spin configuration (S₂^LS) whereas in PSII/Sr at pH 7.0, the S₂-state is in a high-spin configuration (S₂^HS) in half of the centers. Two measurements were done; the time-resolved flash dependent i) absorption of either bromocresol purple at pH 6.0 or neutral red at pH 7.0 and ii) electrochromism in the Soret band of P_D1 at 440 nm. The fittings of the oscillations with a period of four indicate that one proton is released in the S₁ to S₂^HS transition in PSII/Sr at pH 7.0. It has previously been suggested that the proton released in the S₂^LS to S₃ transition would be released in a S₂^LSTyr_Z^• → S₂^HSTyr_Z^• transition before the electron transfer from the cluster to Tyr_Z^• occurs. The release of a proton in the S₁Tyr_Z^• → S₂^HSTyr_Z transition would logically imply that this proton release is missing in the S₂^HSTyr_Z^• to S₃Tyr_Z transition. Instead, the proton release in the S₁ to S₂^HS transition in PSII/Sr at pH 7.0 was mainly done at the expense of the proton release in the S₃ to S₀ and S₀ to S₁ transitions. However, at pH 7.0, the electrochromism of P_D1 seems larger in PSII/Sr when compared to PSII/Ca in the S₃ state. This points to the complex link between proton movements in and immediately around the Mn₄ cluster and the mechanism leading to the release of protons into the bulk.

Properties of Photosystem II lacking the PsbJ subunit

Photosystem II (PSII), the oxygen-evolving enzyme, consists of 17 trans-membrane and 3 extrinsic membrane proteins. Other subunits bind to PSII during assembly, like Psb27, Psb28, and Tsl0063. The presence of Psb27 has been proposed (Zabret et al. in Nat Plants 7:524-538, 2021; Huang et al. Proc Natl Acad Sci USA 118:e2018053118, 2021; Xiao et al. in Nat Plants 7:1132-1142, 2021) to prevent the binding of PsbJ, a single transmembrane α-helix close to the quinone Q_B binding site. Consequently, a PSII rid of Psb27, Psb28, and Tsl0034 prior to the binding of PsbJ would logically correspond to an assembly intermediate. The present work describes experiments aiming at further characterizing such a ∆PsbJ-PSII, purified from the thermophilic Thermosynechococcus elongatus, by means of MALDI-TOF spectroscopy, thermoluminescence, EPR spectroscopy, and UV-visible time-resolved spectroscopy. In the purified ∆PsbJ-PSII, an active Mn₄CaO₅ cluster is present in 60-70% of the centers. In these centers, although the forward electron transfer seems not affected, the Em of the Q_B/Q_B^- couple increases by ≥ 120 mV , thus disfavoring the electron coming back on Q_A. The increase of the energy gap between Q_A/Q_A^- and Q_B/Q_B^- could contribute in a protection against the charge recombination between the donor side and Q_B^-, identified at the origin of photoinhibition under low light (Keren et al. in Proc Natl Acad Sci USA 94:1579-1584, 1997), and possibly during the slow photoactivation process.

Probing the role of arginine 323 of the D1 protein in photosystem II function

The Mn₄ CaO₅ cluster of photosystem II (PSII) advances sequentially through five oxidation states (S₀ to S₄ ). Under the enzyme cycle, two water molecules are oxidized, O₂ is generated and four protons are released into the lumen. Umena et al. (2011) have proposed that, with other charged amino acids, the R323 residue of the D1 protein could contribute to regulate a proton egress pathway from the Mn₄ CaO₅ cluster and Tyr_Z via a proton channel identified from the 3D structure. To test this suggestion, a PsbA3/R323E site-directed mutant has been constructed and the properties of its PSII have been compared to those of the PsbA3-PSII by using EPR spectroscopy, polarography, thermoluminescence and time-resolved UV-visible absorption spectroscopy. Neither the oscillations with a period four nor the kinetics and S-state-dependent stoichiometry of the proton release were affected. However, several differences have been found: (1) the P₆₈₀ ⁺ decay in the hundreds of ns time domain was much slower in the mutant, (2) the S₂ Q_A ^- /DCMU and S₃ Q_A ^- /DCMU radiative charge recombination occurred at higher temperatures and (3) the S₀ Tyr_Z ^• , S₁ Tyr_Z ^• , S₂ Tyr_Z ^• split EPR signals induced at 4.2 K by visible light from the S₀ Tyr_Z , S₁ Tyr_Z , S₂ Tyr_Z , respectively, and the (S₂ Tyr_Z ^• )' induced by NIR illumination at 4.2 K of the S₃ Tyr_Z state differed. It is proposed that the R323 residue of the D1 protein interacts with Tyr_Z likely via the H-bond network previously proposed to be a proton channel. Therefore, rather than participating in the egress of protons to the lumen, this channel could be involved in the relaxations of the H-bonds around Tyr_Z by interacting with the bulk, thus tuning the driving force required for Tyr_Z oxidation.

Psoriasis associated with ACE inhibitors: an analysis of the FAERS database

Many case reports have been published concerning the development or exacerbation of psoriasis after administration of angiotensin-converting enzyme (ACE) inhibitors. The aim of the present study was to investigate the association between psoriasis and ACE inhibitors using the US Food and Drug Administration Adverse Event Reporting System (FAERS) data. After excluding patients with psoriasis-related primary diseases, the association of psoriasis with 14 ACE inhibitors was examined using disproportional analyses reporting odds ratio (ROR) and information component (IC). Signals were detected for all 14 ACE inhibitors combined (ROR: 1.25, 95% confidence interval [CI]: 1.14-1.37; IC: 0.31, 95% CI: 0.17-0.44) and individually for lisinopril (ROR: 1.20, 95% CI: 1.05-1.37; IC: 0.25, 95% CI: 0.06-0.45), perindopril (ROR: 1.86, 95% CI: 1.38-2.52; IC: 0.86, 95% CI: 0.43-1.30), and ramipril (ROR: 1.63, 95% CI: 1.36-1.96; IC: 0.69, 95% CI: 0.42-0.96). ACE inhibitors are widely used in patients with hypertension, heart failure, and diabetes mellitus, which are considered comorbidities of psoriasis. Our results suggest that the involvement of ACE inhibitors should be considered in patients on ACE inhibitor therapy who have developed (or show exacerbated) psoriasis.

What can we still learn from the electrochromic band-shifts in Photosystem II?

Electrochromic band-shifts have been investigated in Photosystem II (PSII) from Thermosynechoccocus elongatus. Firstly, by using Mn-depleted PsbA1-PSII and PsbA3-PSII in which the Q_X absorption of Phe_D1 differs, a band-shift in the Q_X region of Phe_D2 centered at ~ 544 nm has been identified upon the oxidation, at pH 8.6, of Tyr_D. In contrast, a band-shift due to the formation of either Q_A^•- or Tyr_Z^• is observed in PsbA3-PSII at ~ 546 nm, as expected with E130 H-bonded to Phe_D1 and at ~ 544 nm as expected with Q130 H-bonded to Phe_D1. Secondly, electrochromic band-shifts in the Chla Soret region have been measured in O₂-evolving PSII in PsbA3-PSII, in the PsbA3/H198Q mutant in which the Soret band of P_D1 is blue shifted and in the PsbA3/T179H mutant. Upon Tyr_Z^•Q_A^•- formation the Soret band of P_D1 is red shifted and the Soret band of Chl_D1 is blue shifted. In contrast, only P_D1 undergoes a detectable S-state dependent electrochromism. Thirdly, the time resolved S-state dependent electrochromism attributed to P_D1 is biphasic for all the S-state transitions except for S₁ to S₂, and shows that: i) the proton release in S₀ to S₁ occurs after the electron transfer and ii) the proton release and the electron transfer kinetics in S₂ to S₃, in T. elongatus, are significantly faster than often considered. The nature of S₂Tyr_Z^• is discussed in view of the models in the literature involving intermediate states in the S₂ to S₃ transition.

Evaluation of syncope association with α1-adrenoceptor blockers in males using the FAERS database: impact of concomitant hypertension

Previous studies have revealed an association between the administration of α₁-adrenoceptor blockers (α1Bs) and episodes of syncope in patients with benign prostatic hyperplasia (BPH). The objective of the present study was to evaluate the association between α1Bs and syncope in BPH patients with hypertension using two different pharmacoepidemiological indices. Using the US Food and Drug Administration Adverse Event Reporting System, we analyzed the whole dataset and subsets for specific indications, including hypertension, diabetes, and dyslipidemia, for males older than 40 years. The drugs of interest were alfuzosin, doxazosin, and terazosin as non-selective α1Bs and silodosin and tamsulosin as selective α1Bs. The reporting odds ratio (ROR) and information component (IC) were used for signal detection. The association between the non-selective α1Bs and syncope was observed for all the items examined. The results obtained using the whole dataset, as well as the diabetes and dyslipidemia subsets, were same for the selective and non-selective α1Bs in terms of the association with syncope, while no association with syncope was observed for both silodosin [ROR: 1.09, 95% confidence interval (CI): 0.61-1.93; IC: 0.10, 95% CI: -0.72-0.92] and tamsulosin (ROR: 1.08, 95% CI: 0.90-1.30; IC: 0.10, 95% CI: -0.17-0.37) in patients with hypertension. The data suggested that α1Bs, even those with receptor subtype selectivity, were associated with syncope. Thus, careful attention should be paid when prescribing α1Bs, especially to patients who do not take medications for hypertension.

Consequences of structural modifications in cytochrome b559 on the electron acceptor side of Photosystem II

Cytb₅₅₉ in Photosystem II is a heterodimeric b-type cytochrome. The subunits, PsbE and PsbF, consist each in a membrane α-helix. Mutants were previously designed and studied in Thermosynechococcus elongatus (Sugiura et al., Biochim Biophys Acta 1847:276-285, 2015) either in which an axial histidine ligand of the haem-iron was substituted for a methionine, the PsbE/H23M mutant in which the haem was lacking, or in which the haem environment was modified, the PsbE/Y19F and PsbE/T26P mutants. All these mutants remained active showing that the haem has no structural role provided that PsbE and PsbF subunits are present. Here, we have carried on the characterization of these mutants. The following results were obtained: (i) the Y19F mutation hardly affect the E_m of Cytb₅₅₉, whereas the T26P mutation converts the haem into a form with a E_m much below 0 mV (so low that it is likely not reducible by Q_B^-) even in an active enzyme; (ii) in the PsbE/H23M mutant, and to a less extent in PsbE/T26P mutant, the electron transfer efficiency from Q_A^- to Q_B is decreased; (iii) the lower E_m of the Q_A/Q_A^- couple in the PsbE/H23M mutant correlates with a higher production of singlet oxygen; (iv) the superoxide and/or hydroperoxide formation was not increased in the PsbE/H23M mutant lacking the haem, whereas it was significantly larger in the PsbE/T26P. These data are discussed in view of the literature to discriminate between structural and redox roles for the haem of Cytb₅₅₉ in the production of reactive oxygen species.

New insights on ChlD1 function in Photosystem II from site-directed mutants of D1/T179 in Thermosynechococcus elongatus

The monomeric chlorophyll, Chl_D1, which is located between the P_D1P_D2 chlorophyll pair and the pheophytin, Pheo_D1, is the longest wavelength chlorophyll in the heart of Photosystem II and is thought to be the primary electron donor. Its central Mg²⁺ is liganded to a water molecule that is H-bonded to D1/T179. Here, two site-directed mutants, D1/T179H and D1/T179V, were made in the thermophilic cyanobacterium, Thermosynechococcus elongatus, and characterized by a range of biophysical techniques. The Mn₄CaO₅ cluster in the water-splitting site is fully active in both mutants. Changes in thermoluminescence indicate that i) radiative recombination occurs via the repopulation of *Chl_D1 itself; ii) non-radiative charge recombination reactions appeared to be faster in the T179H-PSII; and iii) the properties of P_D1P_D2 were unaffected by this mutation, and consequently iv) the immediate precursor state of the radiative excited state is the Chl_D1⁺Pheo_D1^- radical pair. Chlorophyll bleaching due to high intensity illumination correlated with the amount of ¹O₂ generated. Comparison of the bleaching spectra with the electrochromic shifts attributed to Chl_D1 upon Q_A^- formation, indicates that in the T179H-PSII and in the WT*3-PSII, the Chl_D1 itself is the chlorophyll that is first damaged by ¹O₂, whereas in the T179V-PSII a more red chlorophyll is damaged, the identity of which is discussed. Thus, Chl_D1 appears to be one of the primary damage site in recombination-mediated photoinhibition. Finally, changes in the absorption of Chl_D1 very likely contribute to the well-known electrochromic shifts observed at ~430 nm during the S-state cycle.

Dimer-monomer equilibrium of human HSP27 is influenced by the in-cell macromolecular crowding environment and is controlled by fatty acids and heat

Small heat shock protein 27 (HSP27) is an essential element of the proteostasis network in human cells. The HSP27 monomer coexists with the dimer, which can bind unfolded client proteins. Here, we evaluated the in-cell dimer-monomer equilibrium and its relevance to the binding of client proteins in a normal human vascular endothelial cell line. When cells were treated with a membrane-permeable crosslinker, the protein existed primarily as a free monomer (27 kDa) with a markedly smaller percentage of dimer (54 kDa), hetero-conjugates, and minor smear-like bands. When the protein was crosslinked in a cell-free lysate, two of the hetero-conjugates that were crosslinked in live cells were also detected, but the dimer and other complexes were absent. However, when cells were pretreated with fatty acid (FA) and/or heat (42.5 °C), dissociation of the dimer was selectively prevented and two types of covalently linked dimers were increased. These changes occurred most prominently in cells treated with docosahexaenoic acid (DHA) and heat, which appeared to intensify the heat resistance of the cell. Both the formation of covalently linked dimers and heat resistance were prevented by N-acetylcysteine. By contrast, nearly all of the free monomers in the lysate converted to disulfide bond-linked dimers by a simple, long incubation at 4 °C. These results strongly suggest that the monomer-dimer equilibrium of HSP27 was inversed between the in-cell and cell-free systems. Temperature- and amphiphile-regulated dimerization was restricted probably due to the low hydration of the in-cell crowding environment.

Crystal structure and redox properties of a novel cyanobacterial heme protein with a His/Cys heme axial ligation and a Per-Arnt-Sim (PAS)-like domain

Photosystem II catalyzes light-induced water oxidation leading to the generation of dioxygen indispensable for sustaining aerobic life on Earth. The Photosystem II reaction center is composed of D1 and D2 proteins encoded by psbA and psbD genes, respectively. In cyanobacteria, different psbA genes are present in the genome. The thermophilic cyanobacterium Thermosynechococcus elongatus contains three psbA genes: psbA1, psbA2, and psbA3, and a new c-type heme protein, Tll0287, was found to be expressed in a strain expressing the psbA2 gene only, but the structure and function of Tll0287 are unknown. Here we solved the crystal structure of Tll0287 at a 2.0 Å resolution. The overall structure of Tll0287 was found to be similar to some kinases and sensor proteins with a Per-Arnt-Sim-like domain rather than to other c-type cytochromes. The fifth and sixth axial ligands for the heme were Cys and His, instead of the His/Met or His/His ligand pairs observed for most of the c-type hemes. The redox potential, E_½, of Tll0287 was -255 ± 20 mV versus normal hydrogen electrode at pH values above 7.5. Below this pH value, the E_½ increased by ≈57 mV/pH unit at 15 °C, suggesting the involvement of a protonatable group with a pK_red = 7.2 ± 0.3. Possible functions of Tll0287 as a redox sensor under microaerobic conditions or a cytochrome subunit of an H₂S-oxidizing system are discussed in view of the environmental conditions in which psbA2 is expressed, as well as phylogenetic analysis, structural, and sequence homologies.

Serum β-cryptoxanthin and β-carotene derived from Satsuma mandarin and brachial-ankle pulse wave velocity: The Mikkabi cohort study

BACKGROUND AND AIMS

Findings of observational studies suggest cardioprotective effects of antioxidant vitamins and carotenoids. However, recent meta-analyses failed to show the beneficial effects of supplemental intake of antioxidants on cardiovascular disease (CVD). We aimed to assess the association between CVD risk and β-cryptoxanthin in Japan, where Satsuma mandarin, a major source of β-cryptoxanthin, is widely consumed.

METHODS AND RESULTS

This was part of the Mikkabi cohort study. Surveys were conducted at baseline, in 2003 and 2005, and on follow-up in 2006, 2009, and 2013. We examined brachial-ankle pulse wave velocity (baPWV) with a high cut-off value set at 18.3 m s(-1). Hazard ratios (HR) and 95% confidence intervals for high baPWV were estimated using a Cox proportional hazards model with adjustment for potential confounders. A total of 635 participants with baPWV of less than 18.3 m s(-1) at baseline were included in the analysis. During the follow-up period of 57,921 person-months, 99 subjects developed high baPWV. After multivariate adjustment, the HR for high baPWV in the highest tertile compared with the lowest tertile was significantly low for β-cryptoxanthin, β-carotene, and total carotenoids. Serum concentrations of β-cryptoxanthin and β-carotene were higher in people who ate Satsuma mandarin frequently. Compared with <1/d intake of Satsuma mandarin, 3-4/d was associated with a low risk of high PWV.

CONCLUSION

This study indicated that β-cryptoxanthin and β-carotene derived from Satsuma mandarin are candidate micronutrients for preventing arteriosclerosis development. Further longitudinal and interventional studies will be required to validate the effect on CVD.

Electron transfer pathways from the S2-states to the S3-states either after a Ca2+/Sr2+ or a Cl-/I- exchange in Photosystem II from Thermosynechococcus elongatus

The site for water oxidation in Photosystem II (PSII) goes through five sequential oxidation states (S0 to S4) before O2 is evolved. It consists of a Mn4CaO5-cluster close to a redox-active tyrosine residue (YZ). Cl- is also required for enzyme activity. By using EPR spectroscopy it has been shown that both Ca2+/Sr2+ exchange and Cl-/I- exchange perturb the proportions of centers showing high (S=5/2) and low spin (S=1/2) forms of the S2-state. The S3-state was also found to be heterogeneous with: i) a S=3 form that is detectable by EPR and not sensitive to near-infrared light; and ii) a form that is not EPR visible but in which Mn photochemistry occurs resulting in the formation of a (S2YZ)' split EPR signal upon near-infrared illumination. In Sr/Cl-PSII, the high spin (S=5/2) form of S2 shows a marked heterogeneity with a g=4.3 form generated at low temperature that converts to a relaxed form at g=4.9 at higher temperatures. The high spin g=4.9 form can then progress to the EPR detectable form of S3 at temperatures as low as 180K whereas the low spin (S=1/2) S2-state can only advance to the S3 state at temperatures≥235 K. Both of the two S2 configurations and the two S3 configurations are each shown to be in equilibrium at ≥235 K but not at 198 K. Since both S2 configurations are formed at 198 K, they likely arise from two specific populations of S1. The existence of heterogeneous populations in S1, S2 and S3 states may be related to the structural flexibility associated with the positioning of the oxygen O5 within the cluster highlighted in computational approaches and which has been linked to substrate exchange. These data are discussed in the context of recent in silico studies of the electron transfer pathways between the S2-state(s) and the S3-state(s).

Longitudinal change in the physical performance of older adults in the community

The primary purposes of this study were 1) to confirm age-related deterioration of physical performance in older adults longitudinally, and 2) to predict future functional status and mortality by initial level of physical performances. The subjects were 517 older adults examined both in 1992 and 1996 in the Tokyo Metropolitan Institute of Gerontology, Longitudinal Interdisciplinary Study on Aging. The same battery consisting of muscle strength, balance, walking, and manual speed was administered to the subjects in the baseline and follow-up examinations. A significant longitudinal decline was observed in all physical performances except for grip strength. The age-related decline accelerated with aging for preferred walking velocity. Inter-subject variability in walking velocity significantly increased for 4 years period. Maximum walking velocity was a common predictor for functional status and mortality. The results suggest that physical performance measures, especially maximum walking velocity, is a valid means for physical therapy to evaluate physical functioning of community-living older persons.

Assembly of oxygen-evolving Photosystem II efficiently occurs with the apo-Cytb559 but the holo-Cytb559 accelerates the recovery of a functional enzyme upon photoinhibition

Cytb559 in Photosystem II is a heterodimeric b-type cytochrome. The subunits, PsbE and PsbF, consist each in a membrane α-helix. Roles for Cytb559 remain elusive. In Thermosynechococcus elongatus, taking advantage of the robustness of the PSII variant with PsbA3 as the D1 subunit (WT*3), 4 mutants were designed hoping to get mutants nevertheless the obligatory phototrophy of this cyanobacterium. In two of them, an axial histidine ligand of the haem-iron was substituted for either a methionine, PsbE/H23M, which could be potentially a ligand or for an alanine, PsbE/H23A, which cannot. In the other mutants, PsbE/Y19F and PsbE/T26P, the environment around PsbE/H23 was expected to be modified. From EPR, MALDI-TOF and O2 evolution activity measurements, the following results were obtained: Whereas the PsbE/H23M and PsbE/H23A mutants assemble only an apo-Cytb559 the steady-state level of active PSII was comparable to that in WT*3. The lack of the haem or, in PsbE/T26P, conversion of the high-potential into a lower potential form, slowed-down the recovery rate of the O2 activity after high-light illumination but did not affect the photoinhibition rate. This resulted in the following order for the steady-state level of active PSII centers under high-light conditions: PsbE/H23M≈PsbE/H23A<< PsbE/Y19F≤PsbE/T26P≤WT*3. These data show i) that the haem has no structural role provided that PsbE and PsbF are present, ii) a lack of correlation between the rate of photoinhibition and the Em of the haem and iii) that the holo-Cytb559 favors the recovery of a functional enzyme upon photoinhibition.

Some Photosystem II properties depending on the D1 protein variants in Thermosynechococcus elongatus

Cyanobacteria have multiple psbA genes encoding PsbA, the D1 reaction center protein of the Photosystem II complex which bears together with PsbD, the D2 protein, most of the cofactors involved in electron transfer reactions. The thermophilic cyanobacterium Thermosynechococcus elongatus has three psbA genes differently expressed depending on the environmental conditions. Among the 344 residues constituting each of the 3 possible PsbA variants there are 21 substitutions between PsbA1 and PsbA3, 31 between PsbA1 and PsbA2 and 27 between PsbA2 and PsbA3. In this review, we summarize the changes already identified in the properties of the redox cofactors depending on the D1 variant constituting Photosystem II in T. elongatus. This article is part of a special issue entitled: photosynthesis research for sustainability: keys to produce clean energy.

Crystal structure at 1.5Å resolution of the PsbV2 cytochrome from the cyanobacterium Thermosynechococcus elongatus

PsbV2 is a c-type cytochrome present in a very low abundance in the thermophilic cyanobacterium Thermosynechococcus elongatus. We purified this cytochrome and solved its crystal structure at a resolution of 1.5Å. The protein existed as a dimer in the crystal, and has an overall structure similar to other c-type cytochromes like Cytc6 and Cytc550, for example. However, the 5th and 6th heme iron axial ligands were found to be His51 and Cys101, respectively, in contrast to the more common bis-His or His/Met ligands found in most cytochromes. Although a few other c-type cytochromes were suggested to have this axial coordination, this is the first crystal structure reported for a c-type heme with this unusual His/Cys axial coordination. Previous spectroscopic characterizations of PsbV2 are discussed in relation to its structural properties.

Charge recombination in S(n)Tyr(Z)(•)Q(A)(-•) radical pairs in D1 protein variants of Photosystem II: long range electron transfer in the Marcus inverted region

Charge recombination in the light-induced radical pair SnTyrZ(•)QA(-•) in Photosystem II (PSII) from Thermosynechococcus elongatus has been studied at cryogenic temperatures by time-resolved EPR for different configurations of PSII that are expected to affect the driving force of the reaction (oxidation states S0, S1, or S2 of the Mn4CaO5 cluster; PsbA1, PsbA2, or PsbA3 as D1 protein). The kinetics were independent of temperature in the studied range from 4.2 to 50 K and were not affected by exchange of H2O for D2O, consistent with single-step electron tunneling over the distance of ∼32 Å without any repopulation through Boltzmann equilibration of intermediates lying higher in energy. In PsbA1-PSII, the charge recombinations in the radical pairs SnTyrZ(•)QA(-•) (ket = 3.4 × 10(-3) s(-1) for S1) were slower than in PsbA3-PSII despite an expected lower driving force owing to a downshifted Em(QA/QA(-•)) in PsbA1-PSII. Conversely, the reaction was slower in the presence of S2 than in the presence of S1, despite an expected larger driving force due to an upshifted Em(TyrZ(•)/TyrZ) in S2. These observations indicate that the charge recombination occurs in the Marcus inverted region. Assuming that the driving force of the reaction (-ΔG(0) ≈ 1.2 eV at room temperature for S1) does not vary strongly with temperature, the data indicate an optimal electron transfer rate (for a hypothetical -ΔG(0) = λ) substantially faster than would be predicted from extrapolation of room temperature intraprotein ET rates over shorter distances. Possible origins of this deviation are discussed, including a possible enhancement of the electronic coupling of TyrZ(•) and QA(-•) by aromatic cofactors located in between. Observed similar S1TyrZ(•)QA(-•) charge recombinations in PsbA2-PSII and PsbA3-PSII predict that Em(QA/QA(-•)) in PsbA2-PSII is similar to that in PsbA3-PSII.