Photosystem II: the reaction center of oxygenic photosynthesis.

Photosystem II (PSII) uses light energy to split water into chemical products that power the planet. The stripped protons contribute to a membrane electrochemical potential before combining with the stripped electrons to make chemical bonds and releasing O2 for powering respiratory metabolisms. In this review, we provide an overview of the kinetics and thermodynamics of water oxidation that highlights the conserved performance of PSIIs across species. We discuss recent advances in our understanding of the site of water oxidation based upon the improved (1.9-Å resolution) atomic structure of the Mn4CaO5 water-oxidizing complex (WOC) within cyanobacterial PSII. We combine these insights with recent knowledge gained from studies of the biogenesis and assembly of the WOC (called photoassembly) to arrive at a proposed chemical mechanism for water oxidation.

Revisiting the Preparation and Catalytic Performance of a Phosphine-Modified Co(II) Hydroformylation Precatalyst.

In light of recent conflicting reports regarding the hydroformylation catalytic activity derived from cationic Co(II) precatalysts of the form [Co(acac)(bis(phosphine))]BF4, the synthetic procedures and characterization of [Co(acac)(dppBz)]BF4, 1, are evaluated. Leveraging calibrated ESI-TOF MS methodologies, substantial quantities of Co(acac)2(dppBz), 2, were observed within samples of 1. The source of the impurity, 2, is determined to derive from incomplete protonolysis of the Co(acac)2 precursor and ligand scrambling occurring during the synthesis of 1. Revised synthetic procedures using lower temperature conditions and longer reaction times afford analytically pure samples of 1 based on ESI-TOF MS and NMR spectroscopic analysis. Complex 1 is demonstrated to act as a hydroformylation precatalyst for the conversion of 1-hexene to 1-heptanal under relatively mild conditions at 51.7 bar and 140 °C. The presence of impurity 2 is shown to dramatically decrease the catalytic performance derived from 1.

Bicarbonate is a key regulator but not a substrate for O2 evolution in Photosystem II.

Photosystem II (PSII) uses light energy to oxidize water and to reduce plastoquinone in the photosynthetic electron transport chain. O2 is produced as a byproduct. While most members of the PSII research community agree that O2 originates from water molecules, alternative hypotheses involving bicarbonate persist in the literature. In this perspective, we provide an overview of the important roles of bicarbonate in regulating PSII activity and assembly. Further, we emphasize that biochemistry, spectroscopy, and structural biology experiments have all failed to detect bicarbonate near the active site of O2 evolution. While thermodynamic arguments for oxygen-centered bicarbonate oxidation are valid, the claim that bicarbonate is a substrate for photosynthetic O2 evolution is challenged.

Structure of a monomeric photosystem II core complex from a cyanobacterium acclimated to far-red light reveals the functions of chlorophylls d and f.

Far-red light (FRL) photoacclimation in cyanobacteria provides a selective growth advantage for some terrestrial cyanobacteria by expanding the range of photosynthetically active radiation to include far-red/near-infrared light (700-800 nm). During this photoacclimation process, photosystem II (PSII), the water:plastoquinone photooxidoreductase involved in oxygenic photosynthesis, is modified. The resulting FRL-PSII is comprised of FRL-specific core subunits and binds chlorophyll (Chl) d and Chl f molecules in place of several of the Chl a molecules found when cells are grown in visible light. These new Chls effectively lower the energy canonically thought to define the "red limit" for light required to drive photochemical catalysis of water oxidation. Changes to the architecture of FRL-PSII were previously unknown, and the positions of Chl d and Chl f molecules had only been proposed from indirect evidence. Here, we describe the 2.25 Å resolution cryo-EM structure of a monomeric FRL-PSII core complex from Synechococcus sp. PCC 7335 cells that were acclimated to FRL. We identify one Chl d molecule in the ChlD1 position of the electron transfer chain and four Chl f molecules in the core antenna. We also make observations that enhance our understanding of PSII biogenesis, especially on the acceptor side of the complex where a bicarbonate molecule is replaced by a glutamate side chain in the absence of the assembly factor Psb28. In conclusion, these results provide a structural basis for the lower energy limit required to drive water oxidation, which is the gateway for most solar energy utilization on earth.

Cationic Cobalt(II) Bisphosphine Hydroformylation Catalysis: In Situ Spectroscopic and Reaction Studies.

[HCo(CO)x(bisphosphine)](BF4), x = 1-3, is a highly active hydroformylation catalyst system, especially for internal branched alkenes. In situ infrared spectroscopy (IR), electron paramagnetic resonance (EPR), and nuclear magnetic resonance studies support the proposed catalyst formulation. IR studies reveal the formation of a dicationic Co(I) paramagnetic CO-bridged dimer, [Co2(µ-CO)2(CO)(bisphosphine)2]2+, at lower temperatures formed from the reaction of two catalyst complexes via the elimination of H2. DFT studies indicate a dimer structure with square-pyramidal and tetrahedral cobalt centers. This monomer-dimer equilibrium is analogous to that seen for HCo(CO)4, reacting to eliminate H2 and form Co2(CO)8. EPR studies on the catalyst show a high-spin (S = 3/2) Co(II) complex. Reaction studies are presented that support the cationic Co(II) bisphosphine catalyst as the catalyst species present in this system and minimize the possible role of neutral Co(I) species, HCo(CO)4 or HCo(CO)3(phosphine), as catalysts.

A low-cost and realistic noisy light system for studying photosynthesis.

Unlike the light conditions commonly used to grow photosynthetic organisms in the research laboratory, the light intensity in real environments is dynamic. A simple and low-cost system is described in which a commercial dimmable LED panel is controlled to simulate a sinusoidal function representing daylight hours and overlaid with stochastic shading events. The output closely resembles light intensity measurements on Earth's surface on partly cloudy days or in lower levels of plant canopies. This tool may be useful to researchers studying photosynthetic acclimation responses.

Chloride facilitates Mn(III) formation during photoassembly of the Photosystem II oxygen-evolving complex.

The Mn4Ca oxygen-evolving complex (OEC) in Photosystem II (PSII) is assembled in situ from free Mn2+, Ca2+, and water. In an early light-driven step, Mn2+ in a protein high-affinity site is oxidized to Mn3+. Using dual-mode electron paramagnetic resonance spectroscopy, we observed that Mn3+ accumulation increases as chloride concentration increases in spinach PSII membranes depleted of all extrinsic subunits. At physiologically relevant pH values, this effect requires the presence of calcium. When combined with pH studies, we conclude that the first Mn2+ oxidation event in OEC assembly requires a deprotonation that is facilitated by chloride.

Photosystem II oxygen-evolving complex photoassembly displays an inverse H/D solvent isotope effect under chloride-limiting conditions.

Photosystem II (PSII) performs the solar-driven oxidation of water used to fuel oxygenic photosynthesis. The active site of water oxidation is the oxygen-evolving complex (OEC), a Mn4CaO5 cluster. PSII requires degradation of key subunits and reassembly of the OEC as frequently as every 20 to 40 min. The metals for the OEC are assembled within the PSII protein environment via a series of binding events and photochemically induced oxidation events, but the full mechanism is unknown. A role of proton release in this mechanism is suggested here by the observation that the yield of in vitro OEC photoassembly is higher in deuterated water, D2O, compared with H2O when chloride is limiting. In kinetic studies, OEC photoassembly shows a significant lag phase in H2O at limiting chloride concentrations with an apparent H/D solvent isotope effect of 0.14 ± 0.05. The growth phase of OEC photoassembly shows an H/D solvent isotope effect of 1.5 ± 0.2. We analyzed the protonation states of the OEC protein environment using classical Multiconformer Continuum Electrostatics. Combining experiments and simulations leads to a model in which protons are lost from amino acid that will serve as OEC ligands as metals are bound. Chloride and D2O increase the proton affinities of key amino acid residues. These residues tune the binding affinity of Mn2+/3+ and facilitate the deprotonation of water to form a proposed µ-hydroxo bridged Mn2+Mn3+ intermediate.

Thylakoid localized bestrophin-like proteins are essential for the CO2 concentrating mechanism of Chlamydomonas reinhardtii.

The green alga Chlamydomonas reinhardtii possesses a CO2 concentrating mechanism (CCM) that helps in successful acclimation to low CO2 conditions. Current models of the CCM postulate that a series of ion transporters bring HCO3- from outside the cell to the thylakoid lumen, where the carbonic anhydrase 3 (CAH3) dehydrates accumulated HCO3- to CO2, raising the CO2 concentration for Ribulose bisphosphate carboxylase/oxygenase (Rubisco). Previously, HCO3- transporters have been identified at both the plasma membrane and the chloroplast envelope, but the transporter thought to be on the thylakoid membrane has not been identified. Three paralogous genes (BST1, BST2, and BST3) belonging to the bestrophin family have been found to be up-regulated in low CO2 conditions, and their expression is controlled by CIA5, a transcription factor that controls many CCM genes. YFP fusions demonstrate that all 3 proteins are located on the thylakoid membrane, and interactome studies indicate that they might associate with chloroplast CCM components. A single mutant defective in BST3 has near-normal growth on low CO2, indicating that the 3 bestrophin-like proteins may have redundant functions. Therefore, an RNA interference (RNAi) approach was adopted to reduce the expression of all 3 genes at once. RNAi mutants with reduced expression of BST1-3 were unable to grow at low CO2 concentrations, exhibited a reduced affinity to inorganic carbon (Ci) compared with the wild-type cells, and showed reduced Ci uptake. We propose that these bestrophin-like proteins are essential components of the CCM that deliver HCO3- accumulated in the chloroplast stroma to CAH3 inside the thylakoid lumen.

Binding of dinitrogen to an iron-sulfur-carbon site.

Nitrogenases are the enzymes by which certain microorganisms convert atmospheric dinitrogen (N2) to ammonia, thereby providing essential nitrogen atoms for higher organisms. The most common nitrogenases reduce atmospheric N2 at the FeMo cofactor, a sulfur-rich iron-molybdenum cluster (FeMoco). The central iron sites that are coordinated to sulfur and carbon atoms in FeMoco have been proposed to be the substrate binding sites, on the basis of kinetic and spectroscopic studies. In the resting state, the central iron sites each have bonds to three sulfur atoms and one carbon atom. Addition of electrons to the resting state causes the FeMoco to react with N2, but the geometry and bonding environment of N2-bound species remain unknown. Here we describe a synthetic complex with a sulfur-rich coordination sphere that, upon reduction, breaks an Fe-S bond and binds N2. The product is the first synthetic Fe-N2 complex in which iron has bonds to sulfur and carbon atoms, providing a model for N2 coordination in the FeMoco. Our results demonstrate that breaking an Fe-S bond is a chemically reasonable route to N2 binding in the FeMoco, and show structural and spectroscopic details for weakened N2 on a sulfur-rich iron site.

Endothelial Cell Autonomous Role of Akt1: Regulation of Vascular Tone and Ischemia-Induced Arteriogenesis.

OBJECTIVE: The importance of PI3K/Akt signaling in the vasculature has been demonstrated in several models, as global loss of Akt1 results in impaired postnatal ischemia- and VEGF-induced angiogenesis. The ubiquitous expression of Akt1, however, raises the possibility of cell-type-dependent Akt1-driven actions, thereby necessitating tissue-specific characterization. APPROACH AND RESULTS: Herein, we used an inducible, endothelial-specific Akt1-deleted adult mouse model (Akt1iECKO) to characterize the endothelial cell autonomous functions of Akt1 in the vascular system. Endothelial-targeted ablation of Akt1 reduces eNOS (endothelial nitric oxide synthase) phosphorylation and promotes both increased vascular contractility in isolated vessels and elevated diastolic blood pressures throughout the diurnal cycle in vivo. Furthermore, Akt1iECKO mice subject to the hindlimb ischemia model display impaired blood flow and decreased arteriogenesis. CONCLUSIONS: Endothelial Akt1 signaling is necessary for ischemic resolution post-injury and likely reflects the consequence of NO insufficiency critical for vascular repair.

A [3Fe-4S] cluster is required for tRNA thiolation in archaea and eukaryotes.

The sulfur-containing nucleosides in transfer RNA (tRNAs) are present in all three domains of life; they have critical functions for accurate and efficient translation, such as tRNA structure stabilization and proper codon recognition. The tRNA modification enzymes ThiI (in bacteria and archaea) and Ncs6 (in archaea and eukaryotic cytosols) catalyze the formation of 4-thiouridine (s4U) and 2-thiouridine (s2U), respectively. The ThiI homologs were proposed to transfer sulfur via cysteine persulfide enzyme adducts, whereas the reaction mechanism of Ncs6 remains unknown. Here we show that ThiI from the archaeon Methanococcus maripaludis contains a [3Fe-4S] cluster that is essential for its tRNA thiolation activity. Furthermore, the archaeal and eukaryotic Ncs6 homologs as well as phosphoseryl-tRNA (Sep-tRNA):Cys-tRNA synthase (SepCysS), which catalyzes the Sep-tRNA to Cys-tRNA conversion in methanogens, also possess a [3Fe-4S] cluster similar to the methanogenic archaeal ThiI. These results suggest that the diverse tRNA thiolation processes in archaea and eukaryotic cytosols share a common mechanism dependent on a [3Fe-4S] cluster for sulfur transfer.

Progress Toward a Molecular Mechanism of Water Oxidation in Photosystem II.

The active site of photosynthetic water oxidation is the oxygen-evolving complex (OEC) in the photosystem II (PSII) reaction center. The OEC is a Mn4CaO5 cluster embedded in the PSII protein matrix, and it cycles through redox intermediates known as Si states (i = 0-4). Significant progress has been made in understanding the inorganic and physical chemistry of states S0-S3 through experiment and theory. The chemical steps from S3 to S0 are more poorly understood, however, because the identity of the substrate water molecules and the mechanism of O-O bond formation are not well established. In this review, we highlight both the consensuses and the remaining challenges of PSII research.

Uncoupling Caveolae From Intracellular Signaling In Vivo.

RATIONALE: Caveolin-1 (Cav-1) negatively regulates endothelial nitric oxide (NO) synthase-derived NO production, and this has been mapped to several residues on Cav-1, including F92. Herein, we reasoned that endothelial expression of an F92ACav-1 transgene would let us decipher the mechanisms and relationships between caveolae structure and intracellular signaling. OBJECTIVE: This study was designed to separate caveolae formation from its downstream signaling effects. METHODS AND RESULTS: An endothelial-specific doxycycline-regulated mouse model for the expression of Cav-1-F92A was developed. Blood pressure by telemetry and nitric oxide bioavailability by electron paramagnetic resonance and phosphorylation of vasodilator-stimulated phosphoprotein were determined. Caveolae integrity in the presence of Cav-1-F92A was measured by stabilization of caveolin-2, sucrose gradient, and electron microscopy. Histological analysis of heart and lung, echocardiography, and signaling were performed. CONCLUSIONS: This study shows that mutant Cav-1-F92A forms caveolae structures similar to WT but leads to increases in NO bioavailability in vivo, thereby demonstrating that caveolae formation and downstream signaling events occur through independent mechanisms.

Iron and Cobalt Diazoalkane Complexes Supported by ß-Diketiminate Ligands: A Synthetic, Spectroscopic, and Computational Investigation.

Diazoalkanes are interesting redox-active ligands and also precursors to carbene fragments. We describe a systematic study of the binding and electronic structure of diphenyldiazomethane complexes of ß-diketiminate supported iron and cobalt, which span a range of formal d-electron counts of 7-9. In end-on diazoalkane complexes of formally monovalent three-coordinate transition metals, the electronic structures are best described as having the metal in the +2 oxidation state with an antiferromagnetically coupled radical anion diazoalkane as shown by crystallography, spectroscopy, and computations. A formally zerovalent cobalt complex has different structures depending on whether potassium binds; potassium binding gives transfer of two electrons into the η2-diazoalkane, but the removal of the potassium with crown ether leads to a form with only one electron transferred into an η1-diazoalkane. These results demonstrate the influence of potassium binding and metal oxidation state on the charge localization in the diazoalkane complexes. Interestingly, none of these reduced complexes yield carbene fragments, but the new cobalt(II) complex LtBuCoPF6 (LtBu = bulky ß-diketiminate) does catalyze the formation of an azine from its cognate diazoalkane, suggesting N2 loss and transient carbene formation.

Slow Equilibration between Spectroscopically Distinct Trap States in Reduced TiO2 Nanoparticles.

Understanding the nature of charge carriers in nanoscale titanium dioxide is important for its use in solar energy conversion, photocatalysis, and other applications. UV-irradiation of aqueous, colloidal TiO2 nanoparticles in the presence of methanol gives highly reduced suspensions. Two distinct types of electron traps were observed and characterized by EPR and optical spectroscopies. The relative populations of the states depend on temperature, indicating a small energy difference, ΔH° = 3.0 ± 0.6 kcal/mol (130 ± 30 meV). Interconversion between the electron traps occurs slowly over the course of minutes to hours within the temperature range studied here, 0-50 °C. The slow time scale implies that interconversion involves changes in structure or stoichiometry, not just the movement of electrons. This occurrence of slow structural modification with changes in trap state occupancy is likely a general feature of reduced TiO2 systems at thermodynamic equilibria or photostationary states and should be considered in the design of TiO2-containing devices.

Mechanistic Study of an Improved Ni Precatalyst for Suzuki-Miyaura Reactions of Aryl Sulfamates: Understanding the Role of Ni(I) Species.

Nickel precatalysts are potentially a more sustainable alternative to traditional palladium precatalysts for the Suzuki-Miyaura coupling reaction. Currently, there is significant interest in Suzuki-Miyaura coupling reactions involving readily accessible phenolic derivatives such as aryl sulfamates, as the sulfamate moiety can act as a directing group for the prefunctionalization of the aromatic backbone of the electrophile prior to cross-coupling. By evaluating complexes in the Ni(0), (I), and (II) oxidation states we report a precatalyst, (dppf)Ni(o-tolyl)(Cl) (dppf = 1,1'-bis(diphenylphosphino)ferrocene), for Suzuki-Miyaura coupling reactions involving aryl sulfamates and boronic acids, which operates at a significantly lower catalyst loading and at milder reaction conditions than other reported systems. In some cases it can even function at room temperature. Mechanistic studies on precatalyst activation and the speciation of nickel during catalysis reveal that Ni(I) species are formed in the catalytic reaction via two different pathways: (i) the precatalyst (dppf)Ni(o-tolyl)(Cl) undergoes comproportionation with the active Ni(0) species; and (ii) the catalytic intermediate (dppf)Ni(Ar)(sulfamate) (Ar = aryl) undergoes comproportionation with the active Ni(0) species. In both cases the formation of Ni(I) is detrimental to catalysis, which is proposed to proceed via a Ni(0)/Ni(II) cycle. DFT calculations are used to support experimental observations and provide insight about the elementary steps involved in reactions directly on the catalytic cycle, as well as off-cycle processes. Our mechanistic investigation provides guidelines for designing even more active nickel catalysts.

S3 State of the O2-Evolving Complex of Photosystem II: Insights from QM/MM, EXAFS, and Femtosecond X-ray Diffraction.

The oxygen-evolving complex (OEC) of photosystem II has been studied in the S3 state by electron paramagnetic resonance, extended X-ray absorption fine structure (EXAFS), and femtosecond X-ray diffraction (XRD). However, the actual structure of the OEC in the S3 state has yet to be established. Here, we apply hybrid quantum mechanics/molecular mechanics methods and propose a structural model that is consistent with EXAFS and XRD. The model supports binding of water ligands to the cluster in the S2 → S3 transition through a carousel rearrangement around Mn4, inspired by studies of ammonia binding.

Ammonia Binding in the Second Coordination Sphere of the Oxygen-Evolving Complex of Photosystem II.

Ammonia binds to two sites in the oxygen-evolving complex (OEC) of Photosystem II (PSII). The first is as a terminal ligand to Mn in the S2 state, and the second is at a site outside the OEC that is competitive with chloride. Binding of ammonia in this latter secondary site results in the S2 state S = (5)/2 spin isomer being favored over the S = (1)/2 spin isomer. Using electron paramagnetic resonance spectroscopy, we find that ammonia binds to the secondary site in wild-type Synechocystis sp. PCC 6803 PSII, but not in D2-K317A mutated PSII that does not bind chloride. By combining these results with quantum mechanics/molecular mechanics calculations, we propose that ammonia binds in the secondary site in competition with D1-D61 as a hydrogen bond acceptor to the OEC terminal water ligand, W1. Implications for the mechanism of ammonia binding via its primary site directly to Mn4 in the OEC are discussed.

Metabolic and photosynthetic consequences of blocking starch biosynthesis in the green alga Chlamydomonas reinhardtii sta6 mutant.

Upon nutrient deprivation, microalgae partition photosynthate into starch and lipids at the expense of protein synthesis and growth. We investigated the role of starch biosynthesis with respect to photosynthetic growth and carbon partitioning in the Chlamydomonas reinhardtii starchless mutant, sta6, which lacks ADP-glucose pyrophosphorylase. This mutant is unable to convert glucose-1-phosphate to ADP-glucose, the precursor of starch biosynthesis. During nutrient-replete culturing, sta6 does not re-direct metabolism to make more proteins or lipids, and accumulates 20% less biomass. The underlying molecular basis for the decreased biomass phenotype was identified using LC-MS metabolomics studies and flux methods. Above a threshold light intensity, photosynthetic electron transport rates (water → CO2) decrease in sta6 due to attenuated rates of NADPH re-oxidation, without affecting photosystems I or II (no change in isolated photosynthetic electron transport). We observed large accumulations of carbon metabolites that are precursors for the biosynthesis of lipids, amino acids and sugars/starch, indicating system-wide consequences of slower NADPH re-oxidation. Attenuated carbon fixation resulted in imbalances in both redox and adenylate energy. The pool sizes of both pyridine and adenylate nucleotides in sta6 increased substantially to compensate for the slower rate of turnover. Mitochondrial respiration partially relieved the reductant stress; however, prolonged high-light exposure caused accelerated photoinhibition. Thus, starch biosynthesis in Chlamydomonas plays a critical role as a principal carbon sink influencing cellular energy balance however, disrupting starch biosynthesis does not redirect resources to other bioproducts (lipids or proteins) during nutrient-replete culturing, resulting in cells that are susceptible to photochemical damage caused by redox stress.