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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

Sjöholm, Johannes ; Havelius, Kajsa G.V. LU ; Mamedov, Fikret LU and Styring, Stenbjörn LU (2009) In Biochemistry 48(40). p.9393-9404
Abstract

Water oxidation in photosystem II is catalyzed by the CaMn4 cluster. The electrons extracted from the CaMn4 cluster are transferred to P680+ via the redox-active tyrosine residue D1-Tyr161 (YZ). The oxidation of YZ is coupled to a deprotonation creating the neutral radical YZ•. Light-induced oxidation of YZ is possible down to extreme temperatures. This can be observed as a split EPR signal from YZ • in a magnetic interaction with the CaMn4 cluster, offering a way to probe for YZ oxidation in active PSII. Here we have used the split S0 EPR signal to study the mechanism of YZ... (More)

Water oxidation in photosystem II is catalyzed by the CaMn4 cluster. The electrons extracted from the CaMn4 cluster are transferred to P680+ via the redox-active tyrosine residue D1-Tyr161 (YZ). The oxidation of YZ is coupled to a deprotonation creating the neutral radical YZ•. Light-induced oxidation of YZ is possible down to extreme temperatures. This can be observed as a split EPR signal from YZ • in a magnetic interaction with the CaMn4 cluster, offering a way to probe for YZ oxidation in active PSII. Here we have used the split S0 EPR signal to study the mechanism of YZ oxidation at 5 K in the S0 state. The state of the hydrogen bond between YZ and its proposed hydrogen bond partner D1-His190 is investigated by varying the pH. The split S0 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 pKas (pKapp) ∼4.8 and ∼7.9, respectively. The illumination protocol used to induce the split S0 EPR signal also induces a mixed radical signal in the g ∼ 2 region. One part of this signal decays with similar kinetics as the split S0 EPR signal (∼ 3 min, at 5 K) and is easily distinguished froma stable radical originating from Car/Chl. We suggest that this fast-decaying radical originates from YZ•. The pH dependence of the light-induced fast-decaying radical was measured in the same pH range as for the split S0 EPR signal. The pKapp for the light-induced fast-decaying radical was identical at acidic pH (∼4.8). At alkaline pH the behavior was more complex. Between pH 6.6 and pH 7.7 the signal decreased with pKapp ∼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 S1 EPR signal induced at 5 K and the S 0 → S1 and S1 → S2 transitions at room temperature. The result allows mechanistic conclusions concerning differences between the hydrogen bond pattern around YZ in the S0 and S1 states.

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publishing date
type
Contribution to journal
publication status
published
in
Biochemistry
volume
48
issue
40
pages
12 pages
publisher
The American Chemical Society (ACS)
external identifiers
  • pmid:19736946
  • scopus:70350140360
ISSN
0006-2960
DOI
10.1021/bi901177w
language
English
LU publication?
no
id
3f7d7aec-c75d-47c9-860d-12308f85ba69
date added to LUP
2020-01-15 10:33:49
date last changed
2024-01-02 04:08:28
@article{3f7d7aec-c75d-47c9-860d-12308f85ba69,
  abstract     = {{<p>Water oxidation in photosystem II is catalyzed by the CaMn<sub>4</sub> cluster. The electrons extracted from the CaMn<sub>4</sub> cluster are transferred to P<sub>680</sub><sup>+</sup> via the redox-active tyrosine residue D1-Tyr161 (Y<sub>Z</sub>). The oxidation of Y<sub>Z</sub> is coupled to a deprotonation creating the neutral radical Y<sub>Z</sub><sup>•</sup>. Light-induced oxidation of Y<sub>Z</sub> is possible down to extreme temperatures. This can be observed as a split EPR signal from Y<sub>Z</sub> <sup>•</sup> in a magnetic interaction with the CaMn<sub>4</sub> cluster, offering a way to probe for Y<sub>Z</sub> oxidation in active PSII. Here we have used the split S<sub>0</sub> EPR signal to study the mechanism of Y<sub>Z</sub> oxidation at 5 K in the S<sub>0</sub> state. The state of the hydrogen bond between Y<sub>Z</sub> and its proposed hydrogen bond partner D1-His190 is investigated by varying the pH. The split S<sub>0</sub> 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<sub>a</sub>s (pK<sub>app</sub>) ∼4.8 and ∼7.9, respectively. The illumination protocol used to induce the split S<sub>0</sub> EPR signal also induces a mixed radical signal in the g ∼ 2 region. One part of this signal decays with similar kinetics as the split S<sub>0</sub> EPR signal (∼ 3 min, at 5 K) and is easily distinguished froma stable radical originating from Car/Chl. We suggest that this fast-decaying radical originates from Y<sub>Z</sub><sup>•</sup>. The pH dependence of the light-induced fast-decaying radical was measured in the same pH range as for the split S<sub>0</sub> EPR signal. The pK<sub>app</sub> for the light-induced fast-decaying radical was identical at acidic pH (∼4.8). At alkaline pH the behavior was more complex. Between pH 6.6 and pH 7.7 the signal decreased with pK<sub>app</sub> ∼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<sub>1</sub> EPR signal induced at 5 K and the S <sub>0</sub> → S<sub>1</sub> and S<sub>1</sub> → S<sub>2</sub> transitions at room temperature. The result allows mechanistic conclusions concerning differences between the hydrogen bond pattern around Y<sub>Z</sub> in the S<sub>0</sub> and S<sub>1</sub> states.</p>}},
  author       = {{Sjöholm, Johannes and Havelius, Kajsa G.V. and Mamedov, Fikret and Styring, Stenbjörn}},
  issn         = {{0006-2960}},
  language     = {{eng}},
  month        = {{10}},
  number       = {{40}},
  pages        = {{9393--9404}},
  publisher    = {{The American Chemical Society (ACS)}},
  series       = {{Biochemistry}},
  title        = {{The S<sub>0</sub> state of the water oxidizing complex in photosystem II : pH dependence of the EPR split signal induction and mechanistic implications}},
  url          = {{http://dx.doi.org/10.1021/bi901177w}},
  doi          = {{10.1021/bi901177w}},
  volume       = {{48}},
  year         = {{2009}},
}