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The S0 state of the water oxidizing complex in photosystem II: pH dependence of the EPR Split signal, induction and mechanistic implications
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.
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.
2009 (English)In: Biochemistry, ISSN 0006-2960, E-ISSN 1520-4995, Vol. 48, no 40, 9393-9404 p.Article in journal (Refereed) Published
Abstract [en]

Water oxidation in photosystem II is catalyzed by the CaMn4 cluster.   The electrons extracted from the CaMn4 cluster are transferred to   P-680(+) via the redox-active tyrosine residue D1-Tyr161 (Y-Z). The   oxidation of Y-Z is coupled to a deprotonation creating the neutral   radical Y-Z(center dot). Light-induced oxidation of Y-Z is possible   down to extreme temperatures. This call be observed as a split EPR   signal from Y-Z(center dot) in a magnetic interaction with the CaMn4   cluster, offering a way to probe for Y-Z oxidation in active PSII. Here   we have used the split S-0 EPR signal to study the mechanism of Y-Z   oxidation at 5 K in the S-0 state. The state of the hydrogen bond   between Y-Z and its proposed hydrogen bond partner D1-His190 is   investigated by varying the pH. The split S-0 EPR signal was induced by   illumination at 5 K between pH 3.9 and pH 9.0. Maximum signal intensity   was observed between pH 6 and pH 7. On both the acidic and alkaline   sides the signal intensity decreased with the apparent pK(a)s (pK(app))   similar to 4.8 and similar to 7.9, respectively. The illumination   protocol used to induce the split S-0 EPR signal also induces a mixed   radical signal in the g similar to 2 region. One part of this signal   decays with similar kinetics as the split S-0 EPR signal (similar to 3   min, at 5 K) and is easily distinguished from a stable radical   originating from Car/Chi. We suggest that this fast-decaying radical   originates from Y-Z(center dot). The pH dependence of the light-induced   fast-decaying radical was measured in the same pH range. as for the   split S-0 EPR signal. The pK(app) for the light-induced fast-decaying   radical was identical at acidic pH (similar to 4.8). At alkaline pH the   behavior was more complex. Between pH 6.6 and pH 7.7 the signal   decreased with pK(app) similar to 7.2. However, above pH 7.7 the   induction of the radical species was pH independent. We compare our   results with the pH dependence of the split S-1 EPR signal induced at 5   K and the S-0 -> S-1 and S-1 -> S-2 transitions at room temperature.   The result allows mechanistic conclusions concerning differences   between the hydrogen bond pattern around Y-Z in the S-0 and S-1 states.

Place, publisher, year, edition, pages
Easton: American Chemical Society (ACS), 2009. Vol. 48, no 40, 9393-9404 p.
National Category
Chemical Sciences
Identifiers
URN: urn:nbn:se:uu:diva-99342DOI: 10.1021/bi901177wISI: 000270459100010OAI: oai:DiVA.org:uu-99342DiVA: diva2:207650
Available from: 2009-03-12 Created: 2009-03-12 Last updated: 2017-12-13Bibliographically 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
2. Trapping Tyrosine Z: Exploring the Relay between Photochemistry and Water Oxidation in Photosystem II
Open this publication in new window or tab >>Trapping Tyrosine Z: Exploring the Relay between Photochemistry and Water Oxidation in Photosystem II
2012 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Photosystem II is unique! It remains the only enzyme that can oxidize water using light as energy input. Water oxidation in photosystem II is catalyzed by the CaMn4 cluster. The electrons extracted from the CaMn4 cluster are transferred to P680+ via the tyrosine residue D1-Tyr161 (YZ). Favorable oxidation of YZ is coupled to a proton transfer along a hydrogen bond to the nearby D1-His190 residue, resulting in the neutral radical YZ. By illuminating photosystem II at cryogenic temperatures, YZ can be trapped in a stable state. Magnetic interaction between this radical and the CaMn4 cluster gives rise to a split electron paramagnetic resonance (EPR) signal with characteristics that depend on the oxidation state (S state) of the cluster.

The mechanism by which the split EPR signals are formed is different depending on the S state. In the S0 and S1 states, split signal induction proceeds via a P680+-centered mechanism, whereas in the S2 and S3 states, our results show that split induction stems from a Mn-centered mechanism. This S state-dependent pattern of split EPR signal induction can be correlated to the charge of the CaMn4 cluster in the S state in question and has prompted us to propose a general model for the induction mechanism across the different S states. At the heart of this model is the stability or otherwise of the YZ–(D1-His190)+ pair during cryogenic illumination. The model is closely related to the sequence of electron and proton transfers from the cluster during the S cycle.

Furthermore, the important hydrogen bond between YZ and D1-His190 has been investigated by following the split EPR signal formation in the different S states as a function of pH. All split EPR signals investigated decrease in intensity with a pKa of ~4-5. This pKa can be correlated to a titration event that disrupts the essential hydrogen bond, possibly by a direct protonation of D1-His190.  This has important consequences for the function of the CaMn4 cluster as this critical YZ–D1-His190 hydrogen bond steers a multitude of reactions at the cluster.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2012. 74 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 943
Keyword
Photosystem II, Tyrosine Z, EPR, proton-coupled electron transfer, hydrogen bond, pH
National Category
Biochemistry and Molecular Biology Biophysics
Identifiers
urn:nbn:se:uu:diva-173575 (URN)978-91-554-8390-6 (ISBN)
Public defence
2012-06-15, Polhemsalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 10:15 (English)
Opponent
Supervisors
Available from: 2012-05-25 Created: 2012-04-27 Last updated: 2012-08-01Bibliographically approved

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Sjöholm, JohannesMamedov, FikretStyring, Stenbjörn

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