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pH Dependent Competition between YZ and YD in Photosystem II Probed by Illumination at 5 K
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Photochemistry and Molecular Science, Molecular Biomimetics.
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Photochemistry and Molecular Science, Molecular Biomimetics.
2007 (English)In: Biochemistry, ISSN 0006-2960, E-ISSN 1520-4995, Vol. 46, no 26, 7865-7874 p.Article in journal (Refereed) Published
Abstract [en]

The photosystem II (PSII) reaction center contains two redox active tyrosines, YZ and YD, situated on the D1 and D2 proteins, respectively. By illumination at 5 K, oxidation of YZ in oxygen-evolving PSII can be observed as induction of the Split S1 EPR signal from YZ* in magnetic interaction with the CaMn4 cluster, whereas oxidation of YD can be observed as the formation of the free radical EPR signal from YD*. We have followed the light induced induction at 5 K of the Split S1 signal between pH 4-8.5. The formation of the signal, that is, the oxidation of YZ, is pH independent and efficient between pH 5.5 and 8.5. At low pH, the split signal formation decreases with pKa approximately 4.7-4.9. In samples with chemically pre-reduced YD, the pH dependent competition between YZ and YD was studied. Only YZ was oxidized below pH 7.2, but at pH above 7.2, the oxidation of YD became possible, and the formation of the Split S1 signal diminished. The onset of YD oxidation occurred with pKa approximately 8.0, while the Split S1 signal decreased with pKa approximately 7.9 demonstrating that the two tyrosines compete in this pH interval. The results reflect the formation and breaking of hydrogen bonds between YZ and D1-His190 (HisZ) and YD and D2-His190 (HisD), respectively. The oxidation of respective tyrosine at 5 K demands that the hydrogen bond is well-defined; otherwise, the low-temperature oxidation is not possible. The results are discussed in the framework of recent literature data and with respect to the different oxidation kinetics of YZ and YD.

Place, publisher, year, edition, pages
2007. Vol. 46, no 26, 7865-7874 p.
Keyword [en]
Cold, Electron Spin Resonance Spectroscopy, Hydrogen-Ion Concentration, Models, Chemical, Oxidation-Reduction, Photosystem II Protein Complex/*physiology/radiation effects, Spinacia oleracea, Tyrosine/*metabolism
National Category
Chemical Sciences
Identifiers
URN: urn:nbn:se:uu:diva-11297DOI: 10.1021/bi700377gISI: 000247486800018PubMedID: 17559194OAI: oai:DiVA.org:uu-11297DiVA: diva2:39065
Available from: 2007-08-28 Created: 2007-08-28 Last updated: 2017-12-11Bibliographically approved
In thesis
1. EPR Studies of Photosystem II: Characterizing Water Oxidizing Intermediates at Cryogenic Temperatures
Open this publication in new window or tab >>EPR Studies of Photosystem II: Characterizing Water Oxidizing Intermediates at Cryogenic Temperatures
2009 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The principles of natures own light-driven water splitting catalyst, Photosystem II (PSII), can in the future inspire us to use water as electron and proton source to generate light-driven H2 production. To mimic this challenging step, it is important to understand how the enzyme system can oxidize water. The mechanism of light-driven water oxidation in PSII is in this thesis addressed by EPR spectroscopy. P680+ is a strong oxidant formed by light-oxidation of the chlorophyll species P680 positioned in the center of PSII. The redox active tyrosine-Z (YZ) can reduce P680+ and the YZ radical is formed. This transient radical is further reduced by the CaMn4-cluster, which is the binding site of the substrate water molecules. In a cyclic process called the S-cycle, this catalytic cluster accumulates four oxidizing equivalents to evolve one molecule of O2 and to oxidize two molecules of water. We can induce the YZ radical at cryogenic temperatures in the different oxidation states of the catalytic S-cycle and observe this in metalloradical EPR signals. These metalloradical EPR signals are here characterized and used to deduce mechanistic information from the intact PSII. The "double nature" of these spin-spin interaction signals, so called split EPR signals, makes them excellent probes to both YZ oxidation and, when YZ is present, also to the S-states of the CaMn4-cluster. The metalloradical EPR signals presented here, form a way to study the transient YZ radical in active PSII that has not been depleted of the catalytic metal cluster. This depleting method that has often been used in the past to study YZ is not representing studies of a mechanistically relevant material. The previously suggested disorder around YZ and accessibility to the bulk can be artifactual properties induced in the mechanistically defect PSII. On the contrary, our observation that proton coupled electron transfer from YZ to the light induced P680+ can occur in a high yield at cryogenic temperatures, suggests a well ordered catalytic site in the protein positioned for optimal performance. The optimized positioning of the redox components found in PSII might be a feature also important to build in an efficient water oxidizing catalyst.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2009. 72 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 621
National Category
Biochemistry and Molecular Biology
Identifiers
urn:nbn:se:uu:diva-99109 (URN)978-91-554-7458-4 (ISBN)
Public defence
2009-04-24, Häggsalen, Ångströmslabortoriet, Lägerhyddsvägen 1, Uppsala, 13:00 (English)
Opponent
Supervisors
Available from: 2009-04-03 Created: 2009-03-09 Last updated: 2010-01-13Bibliographically approved

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Styring, Stenbjörn

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