Electron Spin Resonance Spectroscopy; Metalloproteins; Molecular Biology; Photosynthesis
Photosystem II is the chloroplast membrane protein complex that catalyzes the light-induced oxidation of water to dioxygen in the first step of photosynthetic electron transport. We use spectroscopic, biophysical, and molecular biological methods to probe the structure and function of the redox centers, the kinetics and yields of electron-transfer reactions, and the chemistry of water oxidation in photosystem II. Our goal is to define how Nature has solved the difficult problem of efficient light-driven, four-electron oxidation of water to dioxygen. The studies on photosystem II provide insight into the design of artificial systems that split water. In conjunction with our biophysical studies of the natural photosynthetic system, we are also investigating inorganic models of the tetramanganese active site in photosystem II. The synergism between the inorganic and biological chemistry is an important aspect of this research and yielded the first inorganic manganese water-oxidation catalyst. We are also working to develop artificial processes that use solar energy for fuel production. Our aim is to use a bioinspired approach for solar fuel production based on our water-oxidation catalysts attached to nanostructured TiO2.
Specialized Terms: photosystem II; EPR spectroscopy; metalloproteins; artificial photosynthesis
Extensive Research Description
What is the molecular basis for energy transduction in plant photosynthesis? This is the question that has been the focus of our research program. The first step in the energy-transduction process is the light-induced charge-separation reaction that occurs in a membrane-protein complex called the photoreaction center. Thereafter, a series of rapid electron-transfer reactions serve to stabilize the charge-separated state. Later still, the photochemically-produced oxidant and reductant are consumed in the oxidation of water and the reduction of carbon dioxide. One of the primary targets of our research is the plant enzyme called photosystem II that catalyzes the oxidation of water to dioxygen at a site within the enzyme containing a tetranuclear manganese cluster. Photosystem II is one of two reaction center complexes that initiate the light-driven electron transfer reactions of plant photosynthesis. The long-term objective of our research is to develop an understanding at the molecular level of the conversion of light energy into chemical energy in plant photosynthesis. Toward this goal, we are pursuing several related lines of research. A major effort involves biophysical studies of the purified photosystem II complex itself. We use spectroscopic and biophysical methods to probe the structure of the redox centers, the kinetics and yields of electron-transfer reactions, and the chemistry of water oxidation in photosystem II. Our aim is to define how Nature has solved the difficult problem of the efficient light-driven, four-electron oxidation of water to O2. It is hoped that these studies will provide insight into the design of artificial systems that split water. Toward this goal, we are also investigating inorganic models of the manganese site in photosystem II. Because the model complexes are more easily characterized, the inorganic studies provide important information that can aid the interpretation of results from the biological system. On the other hand, the information from the biophysical studies better define the nature of the catalytic manganese complex that is to be modeled. The synergism between the inorganic and biological chemistry is an important aspect of this research. A third area of research is artificial photosynthesis. We are working to develop artificial processes that use solar energy for fuel production in collaboration with Professors Batista, Crabtree and Schmuttenmaer. Our aim is to use a bioinspired approach for solar fuel production based on our water-oxidation catalysts attached to nanostructured TiO2.
Biophysical Studies of Photosystem II
We use EPR, optical, and Raman spectroscopy, turnover measurements of oxygen evolution and site-directed mutagenesis to monitor the photochemical events and to obtain structural and mechanistic information on photosystem II. Photosystem II contains more than ten distinct redox-active centers. One of these is the tetranuclear manganese cluster that, together with a redox-active tyrosine (Yz), catalyzes the oxidation of water. We study photosystem II samples that are prepared and trapped at low temperature in each of the oxidation states ("S-states") of the manganese complex. The binding and reactions of substrates and inhibitors are also studied in order to define the structure and chemical properties of the manganese cluster as it proceeds through the catalytic cycle. For example, acetate competes with chloride for binding to the manganese cluster. Illumination of PSII treated with acetate inhibits the enzyme in a state in which a paramagnetic S-state (S2) of the manganese cluster interacts with oxidized Yz. We have investigated this interaction using EPR and used the spectroscopic results in conjunction with spectral simulations and molecular modeling to devise a proposed structure for the manganese cluster and its associated cofactors (calcium, chloride, and Yz). In order to control the turnover of photosystem II in the highly concentrated samples needed for spectroscopic studies, we have used herbicides that bind to photosystem II and block electron transfer. One recent extension of this approach involves tethering a redox-active center to the herbicide so that multi-electron turnover control can be achieved. Other projects involve the use of optical or fluorescence spectroscopy to study the electron-transfer reactions that occur during the water oxidation cycle. We have also used Raman spectroscopy to characterize the oxidized forms of some of the electron-transfer cofactors, such as chlorophyll and carotenoid cation radicals, and also the manganese complex itself. Photosystem II contains several redox centers, including cytochrome b559, that do not play a direct role in the reactions leading to water oxidation. The function of these species is currently not well understood, but they may be involved in protection of photosystem II from photodamage. By using a combination of spectroscopic measurements, functional assays and site-directed mutagenesis, we are working to reveal how these alternate electron donors function.
Inorganic Models of the Manganese Cluster in Photosystem II
The S states in the water oxidation cycle are different oxidation states of the tetranuclear manganese cluster. The chemistry of this manganese cluster involves high-valent manganese and H2O. The goal of inorganic modeling studies is to give an insight into the high-valent Mn chemistry in aqueous media that may be relevant to the photoactivated assembly of the tetranuclear manganese cluster, its structure and physical properties, and the mechanism of water oxidation. We have synthesized the first di-mu-oxo Mn-complex capable of catalytically forming O2 from O-atom transfer reagents such as oxone (HSO5-). EPR, low-temperature optical and Raman spectroscopy, magnetics and electrochemistry are some of the methods that we use to characterize the manganese complexes.
- Pokhrel, R., McConnell, I.L. and Brudvig, G.W. (2011) Chloride Regulation of Enzyme Turnover: Application to the Role of Chloride in Photosystem II. Biochemistry 50, 2725–2734.
- Moore, G.F. and Brudvig, G.W. (2011) Energy Conversion in Photosynthesis: A Paradigm for Solar Fuel Production. Annu. Rev. Condensed Matter Phys. 2, 303-327.
- Sproviero, E.M., Gascón, J.A., McEvoy, J.P., Brudvig, G.W. and Batista, V.S. (2008) Quantum Mechanics/Molecular Mechanics Study of the Catalytic Cycle of Water Splitting in Photosystem II. J. Am. Chem. Soc. 130, 3428-3442.
- Das, S., Incarvito, C.D., Crabtree, R.H., and Brudvig, G.W. (2006) Molecular recognition in the selective oxygenation of saturated C-H bonds by a dimanganese catalyst. Science 312:1119-1134.
- McEvoy, J.P. and Brudvig, G.W. (2006) Water-splitting chemistry of Photosystem II. Chem. Rev. 106:4455-4483.
- Tagore, R., Chen, H., Crabtree, R.H. and Brudvig, G.W. (2006) Determination of µ-oxo exchange rates in di-µ-oxo dimanganese complexes by electrospray ionization mass spectrometry. J. Am. Chem. Soc. 128, 9457-9465.
- Frank, H.A. and Brudvig, G.W. (2004) Redox functions of carotenoids in photosynthesis. Biochemistry 43, 8607-8615.