Photosystem II (PSII) is the water splitting enzyme of photosynthesis. Its appearance during evolution dramatically changed the chemical composition of our planet and set in motion an unprecedented ...explosion in biological activity. Powered by sunlight, PSII supplies biology with the 'hydrogen' needed to convert carbon dioxide into organic molecules. The questions now are can we continue to exploit this photosynthetic process through increased use of biomass as an energy source and, more importantly, can we address the energy/CO2 problem by developing new photochemical technologies which mimic the natural system? (Critical review, 82 references).
The oxygen in our atmosphere is derived from and maintained by the water-splitting process of photosynthesis. The enzyme that facilitates this reaction and therefore underpins virtually all life on ...our planet is known as photosystem II (PSII). It is a multisubunit enzyme embedded in the lipid environment of the thylakoid membranes of plants, algae, and cyanobacteria. Powered by light, PSII catalyzes the chemically and thermodynamically demanding reaction of water splitting. In so doing, it releases molecular oxygen into the atmosphere and provides the reducing equivalents required for the conversion of carbon dioxide into the organic molecules of life. Recently, a fully refined structure of an isolated 700 kDa cyanobacterial dimeric PSII complex was elucidated by X-ray crystallography, which gave organizational and structural details of the 19 subunits (16 intrinsic and 3 extrinsic) that make up each monomer and provided information about the position and protein environments of the many different cofactors it binds. The water-splitting site was revealed as a cluster of four Mn ions and a Ca ion surrounded by amino acid side chains, of which six or seven form direct ligands to the metals. The metal cluster was originally modeled as a cubane-like structure composed of three Mn ions and the Ca2+ linked by oxo bonds and the fourth Mn attached to the cubane via one of its O atoms. New data from X-ray diffraction and X-ray spectroscopy suggest some alternative arrangements. Nevertheless, all of the models are sufficiently similar to provide a basis for discussing the chemistry by which PSII splits water and makes oxygen.
Biological solar energy Barber, James
Philosophical transactions of the Royal Society of London. Series A: Mathematical, physical, and engineering sciences,
04/2007, Letnik:
365, Številka:
1853
Journal Article
Recenzirano
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.
The biological energy cycle of our planet is driven by photosynthesis whereby sunlight is absorbed by chlorophyll and other accessory pigments. The excitation energy is then efficiently transferred ...to a reaction centre where charge separation occurs in a few picoseconds. In the case of photosystem II (PSII), the energy of the charge transfer state is used to split water into oxygen and reducing equivalents. This is accomplished by the relatively low energy content of four photons of visible light. PSII is a large multi-subunit membrane protein complex embedded in the lipid environment of the thylakoid membranes of plants, algae and cyanobacteria. Four high energy electrons, together with four protons (4H+), are used to reduce plastoquinone (PQ), the terminal electron acceptor of PSII, to plastoquinol (PQH2). PQH2 passes its reducing equivalents to an electron transfer chain which feeds into photosystem I (PSI) where they gain additional reducing potential from a second light reaction which is necessary to drive CO2 reduction. The catalytic centre of PSII consists of a cluster of four Mn ions and a Ca2+ linked by oxo bonds. In addition, there are seven amino acid ligands. In this Article, I discuss the structure of this metal cluster, its stability and the probability that an acid-base (nucleophilic-electrophilic) mechanism catalyses the water splitting reaction on the surface of the metal-cluster. Evidence for this mechanism is presented from studies on water splitting catalysts consisting of organo-complexes of ruthenium and manganese and also by comparison with the enzymology of carbon monoxide dehydrogenase (CODH). Finally the relevance of our understanding of PSII is discussed in terms of artificial photosynthesis with emphasis on inorganic water splitting catalysts as oxygen generating photoelectrodes.
Demand for energy is projected to increase at least twofold by mid-century relative to the present global consumption because of predicted population and economic growth. This demand could be met, in ...principle, from fossil energy resources, particularly coal. However, the cumulative nature of carbon dioxide (CO2) emissions demands that stabilizing the atmospheric CO2 levels to just twice their pre-anthropogenic values by mid-century will be extremely challenging, requiring invention, development and deployment of schemes for carbon-neutral energy production on a scale commensurate with, or larger than, the entire present-day energy supply from all sources combined. Among renewable and exploitable energy resources, nuclear fusion energy or solar energy are by far the largest. However, in both cases, technological breakthroughs are required with nuclear fusion being very difficult, if not impossible on the scale required. On the other hand, 1 h of sunlight falling on our planet is equivalent to all the energy consumed by humans in an entire year. If solar energy is to be a major primary energy source, then it must be stored and despatched on demand to the end user. An especially attractive approach is to store solar energy in the form of chemical bonds as occurs in natural photosynthesis. However, a technology is needed which has a year-round average conversion efficiency significantly higher than currently available by natural photosynthesis so as to reduce land-area requirements and to be independent of food production. Therefore, the scientific challenge is to construct an ‘artificial leaf’ able to efficiently capture and convert solar energy and then store it in the form of chemical bonds of a high-energy density fuel such as hydrogen while at the same time producing oxygen from water. Realistically, the efficiency target for such a technology must be 10 per cent or better. Here, we review the molecular details of the energy capturing reactions of natural photosynthesis, particularly the water-splitting reaction of photosystem II and the hydrogen-generating reaction of hydrogenases. We then follow on to describe how these two reactions are being mimicked in physico-chemical-based catalytic or electrocatalytic systems with the challenge of creating a large-scale robust and efficient artificial leaf technology.
We have previously reported that radiotherapy (RT) added to androgen-deprivation therapy (ADT) improves survival in men with locally advanced prostate cancer. Here, we report the prespecified final ...analysis of this randomized trial.
NCIC Clinical Trials Group PR.3/Medical Research Council PR07/Intergroup T94-0110 was a randomized controlled trial of patients with locally advanced prostate cancer. Patients with T3-4, N0/Nx, M0 prostate cancer or T1-2 disease with either prostate-specific antigen (PSA) of more than 40 μg/L or PSA of 20 to 40 μg/L plus Gleason score of 8 to 10 were randomly assigned to lifelong ADT alone or to ADT+RT. The RT dose was 64 to 69 Gy in 35 to 39 fractions to the prostate and pelvis or prostate alone. Overall survival was compared using a log-rank test stratified for prespecified variables.
One thousand two hundred five patients were randomly assigned between 1995 and 2005, 602 to ADT alone and 603 to ADT+RT. At a median follow-up time of 8 years, 465 patients had died, including 199 patients from prostate cancer. Overall survival was significantly improved in the patients allocated to ADT+RT (hazard ratio HR, 0.70; 95% CI, 0.57 to 0.85; P < .001). Deaths from prostate cancer were significantly reduced by the addition of RT to ADT (HR, 0.46; 95% CI, 0.34 to 0.61; P < .001). Patients on ADT+RT reported a higher frequency of adverse events related to bowel toxicity, but only two of 589 patients had grade 3 or greater diarrhea at 24 months after RT.
This analysis demonstrates that the previously reported benefit in survival is maintained at a median follow-up of 8 years and firmly establishes the role of RT in the treatment of men with locally advanced prostate cancer.
The United Nations Climate Change Conference (COP21) held in Paris in 2015 and the follow-up conferences in Marrakesh (COP22) and very recently in Bonn (COP23) have established an unprecedented ...international agreement that during this century human society must break from its reliance on energy from fossil fuels to energy sources, which do not release greenhouse gases, particularly carbon dioxide. This is a very hard call. It will not only involve improving more efficient use of energy but also the establishment of new technologies and infrastructures for its generation and distribution. Among renewable energy resources able to tackle these challenges significantly are nuclear or those derived from solar radiation. However in the case of nuclear fusion, a technological breakthrough is required while nuclear fission is limited in the long run by the finite supply of uranium fuel. Here I focus on solar energy which is already contributing to the challenge of COP21. In particular, I discuss its capture and storage through the splitting of water to produce oxygen and hydrogen reducing equivalents, which is essentially the route taken by biology about 3 billion years ago. In so doing I will describe how Nature goes about achieving this fundamental energy converting reaction as a back drop to efforts to achieve the same goal using non-biological photo-electrochemical technology.
Students' ability to integrate learning across contexts is a critical outcome for higher education. Often the most powerful learning experiences that students report from their college years are ...those that prompt integration of learning, yet it remains an outcome that few educators explicitly work towards or specify as a course objective. Given that students will be more successful in college (and in life) if they can integrate their learning, James Barber offers a guide for college educators on how to promote students' integration of learning, and help them connect knowledge and insights across contexts, whether in-class or out-of-class, in co-curricular activities, or across courses and disciplinary boundaries. The opening chapters lay the foundation for the book, defining what integration of learning is, how to promote it and students' capacities for reflection; and introduce the author's research-based Integration of Learning (IOL) model.The second section of the book provides practical, real-world strategies for facilitating integration of learning that college educators can use right away in multiple learning contexts. James Barber describes practices that readers can integrate as appropriate in their classes or activities, under chapters respectively devoted to Mentoring, Writing as Praxis, Juxtaposition, Hands-On Experiences, and Diversity and Identity. The author concludes by outlining how to apply IOL to a multiplicity of settings, such as a major, a single course, programming for a student organization, or other co-curricular experience; as well as offering guidance on assessing and documenting students' mastery of this outcome.This book is addressed to a wide range of educators engaged with college student learning, from faculty to student affairs administrators, athletic coaches, internship supervisors, or anyone concerned with student development.
Parkinson's disease (PD) is a neurodegenerative movement disorder, which affects approximately 1–2% of the population over 60years of age. Current treatments for PD are symptomatic, and the pathology ...of the disease continues to progresses over time until palliative care is required. Mitochondria are key players in the pathology of PD. Genetic and post mortem studies have shown a large number of mitochondrial abnormalities in the substantia nigra pars compacta (SNc) of the parkinsonian brain. Furthermore, physiologically, mitochondria of nigral neurons are constantly under unusually high levels of metabolic stress because of the excitatory properties and architecture of these neurons. The protein deacetylase, Sirtuin 3 (SIRT3) reduces the impact subcellular stresses on mitochondria, by stabilising the electron transport chain (ETC), and reducing oxidative stress. We hypothesised that viral overexpression of myc-tagged SIRT3 (SIRT3-myc) would slow the progression of PD pathology, by enhancing the functional capacity of mitochondria. For this study, SIRT3-myc was administered both before and after viral induction of parkinsonism with the AAV-expressing mutant (A53T) α-synuclein. SIRT3-myc corrected behavioural abnormalities, as well as changes in striatal dopamine turnover. SIRT3-myc also prevented degeneration of dopaminergic neurons in the SNc. These effects were apparent, even when SIRT3-myc was transduced after the induction of parkinsonism, at a time point when cell stress and behavioural abnormalities are already observed. Furthermore, in an isolated mitochondria nigral homogenate prepared from parkinsonian SIRT3–myc infected animals, SIRT3 targeted the mitochondria, to reduce protein acetylation levels. Our results demonstrate that transduction of SIRT3 has the potential to be an effective disease-modifying strategy for patients with PD. This study also provides potential mechanisms for the protective effects of SIRT3-myc.
Photosynthetic generation of oxygen Barber, James
Philosophical transactions of the Royal Society of London. Series B. Biological sciences,
08/2008, Letnik:
363, Številka:
1504
Journal Article
Recenzirano
Odprti dostop
The oxygen in the atmosphere is derived from light-driven oxidation of water at a catalytic centre contained within a multi-subunit enzyme known as photosystem II (PSII). PSII is located in the ...photosynthetic membranes of plants, algae and cyanobacteria and its oxygen-evolving centre (OEC) consists of four manganese ions and a calcium ion surrounded by a highly conserved protein environment. Recently, the structure of PSII was elucidated by X-ray crystallography thus revealing details of the molecular architecture of the OEC. This structural information, coupled with an extensive knowledge base derived from a wide range of biophysical, biochemical and molecular biological studies, has provided a framework for understanding the chemistry of photosynthetic oxygen generation as well as opening up debate about its evolutionary origin.
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