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Effects of pH on the S3 state of the oxygen evolving complex in photosystem II probed by EPR split signal induction

Sjöholm, Johannes ; Havelius, Kajsa G.V. LU ; Mamedov, Fikret LU and Styring, Stenbjörn LU (2010) In Biochemistry 49(45). p.9800-9808
Abstract

The electrons extracted from the CaMn4 cluster during water oxidation in photosystem II are transferred to P680+ via the redox-active tyrosine D1-Tyr161 (YZ). Upon YZ oxidation a proton moves in a hydrogen bond toward D1-His190 (HisZ). The deprotonation and reprotonation mechanism of YZ-OH/Y Z-O is of key importance for the catalytic turnover of photosystem II. By light illumination at liquid helium temperatures (∼5 K) YZ can be oxidized to its neutral radical, YZ•. This can be followed by the induction of a split EPR signal from YZ • in a magnetic interaction with the CaMn4 cluster,... (More)

The electrons extracted from the CaMn4 cluster during water oxidation in photosystem II are transferred to P680+ via the redox-active tyrosine D1-Tyr161 (YZ). Upon YZ oxidation a proton moves in a hydrogen bond toward D1-His190 (HisZ). The deprotonation and reprotonation mechanism of YZ-OH/Y Z-O is of key importance for the catalytic turnover of photosystem II. By light illumination at liquid helium temperatures (∼5 K) YZ can be oxidized to its neutral radical, YZ•. This can be followed by the induction of a split EPR signal from YZ • in a magnetic interaction with the CaMn4 cluster, offering a way to probe for YZ oxidation in active photosystem II. In the S3 state, light in the near-infrared region induces the split S3 EPR signal, S2?YZ•. Here we report on the pH dependence for the induction of S2?Y Z• between pH 4.0 and pH 8.7. At acidic pH the split S3 EPR signal decreases with the apparent pKa (pK app) ∼ 4.1. This can be correlated to a titration event that disrupts the essential H-bond in the YZ-HisZ motif. At alkaline pH, the split S3 EPR signal decreases with the pK app ∼ 7.5. The analysis of this pH dependence is complicated by the presence of an alkaline-induced split EPR signal (pKapp ∼ 8.3) promoted by a change in the redox potential of YZ. Our results allow dissection of the proton-coupled electron transfer reactions in the S 3 state and provide further evidence that the radical involved in the split EPR signals is indeed YZ•.

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organization
publishing date
type
Contribution to journal
publication status
published
in
Biochemistry
volume
49
issue
45
pages
9 pages
publisher
The American Chemical Society (ACS)
external identifiers
  • scopus:78149453539
  • pmid:20925430
ISSN
0006-2960
DOI
10.1021/bi101364t
language
English
LU publication?
no
id
c45f9026-27bf-4e67-b8d2-cfa3fa30456d
date added to LUP
2020-01-15 10:27:30
date last changed
2024-04-17 03:20:25
@article{c45f9026-27bf-4e67-b8d2-cfa3fa30456d,
  abstract     = {{<p>The electrons extracted from the CaMn<sub>4</sub> cluster during water oxidation in photosystem II are transferred to P<sub>680</sub><sup>+</sup> via the redox-active tyrosine D1-Tyr161 (Y<sub>Z</sub>). Upon Y<sub>Z</sub> oxidation a proton moves in a hydrogen bond toward D1-His190 (His<sub>Z</sub>). The deprotonation and reprotonation mechanism of Y<sub>Z</sub>-OH/Y <sub>Z</sub>-O is of key importance for the catalytic turnover of photosystem II. By light illumination at liquid helium temperatures (∼5 K) Y<sub>Z</sub> can be oxidized to its neutral radical, Y<sub>Z</sub><sup>•</sup>. This can be followed by the induction of 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 photosystem II. In the S<sub>3</sub> state, light in the near-infrared region induces the split S<sub>3</sub> EPR signal, S<sub>2</sub>?Y<sub>Z</sub><sup>•</sup>. Here we report on the pH dependence for the induction of S<sub>2</sub>?Y <sub>Z</sub><sup>•</sup> between pH 4.0 and pH 8.7. At acidic pH the split S<sub>3</sub> EPR signal decreases with the apparent pK<sub>a</sub> (pK <sub>app</sub>) ∼ 4.1. This can be correlated to a titration event that disrupts the essential H-bond in the Y<sub>Z</sub>-His<sub>Z</sub> motif. At alkaline pH, the split S<sub>3</sub> EPR signal decreases with the pK <sub>app</sub> ∼ 7.5. The analysis of this pH dependence is complicated by the presence of an alkaline-induced split EPR signal (pK<sub>app</sub> ∼ 8.3) promoted by a change in the redox potential of Y<sub>Z</sub>. Our results allow dissection of the proton-coupled electron transfer reactions in the S <sub>3</sub> state and provide further evidence that the radical involved in the split EPR signals is indeed Y<sub>Z</sub><sup>•</sup>.</p>}},
  author       = {{Sjöholm, Johannes and Havelius, Kajsa G.V. and Mamedov, Fikret and Styring, Stenbjörn}},
  issn         = {{0006-2960}},
  language     = {{eng}},
  month        = {{11}},
  number       = {{45}},
  pages        = {{9800--9808}},
  publisher    = {{The American Chemical Society (ACS)}},
  series       = {{Biochemistry}},
  title        = {{Effects of pH on the S<sub>3</sub> state of the oxygen evolving complex in photosystem II probed by EPR split signal induction}},
  url          = {{http://dx.doi.org/10.1021/bi101364t}},
  doi          = {{10.1021/bi101364t}},
  volume       = {{49}},
  year         = {{2010}},
}