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pH dependence of the four individual transitions in the catalytic S-cycle during photosynthetic oxygen evolution.

Bernat, Gabor LU ; Morvaridi, Susan F LU ; Feyziyev, Yashar LU and Styring, Stenbjörn LU (2002) In Biochemistry 41(18). p.5830-5843
Abstract
We have investigated the pH dependence for each individual redox transition in the S-cycle of the oxygen evolving complex (OEC) of photosystem II by electron paramagnetic resonance (EPR) spectroscopy. In the experiments, OEC is advanced to the appropriate S-state at normal pH. Then, the pH is rapidly changed, and a new flash is given. The ability to advance to the next S-state in the cycle at different pHs is determined by measurements of the decrease or increase of characteristic EPR signals from the OEC in different S-states. In some cases the measured EPR signals are very small (this holds especially for the S0 ML signal at pH >7.5 and pH <4.8). Therefore, we refrain from providing error limits for the determined pK's. Our results... (More)
We have investigated the pH dependence for each individual redox transition in the S-cycle of the oxygen evolving complex (OEC) of photosystem II by electron paramagnetic resonance (EPR) spectroscopy. In the experiments, OEC is advanced to the appropriate S-state at normal pH. Then, the pH is rapidly changed, and a new flash is given. The ability to advance to the next S-state in the cycle at different pHs is determined by measurements of the decrease or increase of characteristic EPR signals from the OEC in different S-states. In some cases the measured EPR signals are very small (this holds especially for the S0 ML signal at pH >7.5 and pH <4.8). Therefore, we refrain from providing error limits for the determined pK's. Our results indicate that the S1 --> S2 transition is independent of pH between 4.1 and 8.4. All other S-transitions are blocked at low pH. In the acidic region, the pK's for the inhibition of the S2 --> S3, the S3 --> [S4] --> S0, and the S0 --> S1 transitions are about 4.0, 4.5, and 4.7, respectively. The similarity of these pK values indicates that the inhibition of the steady-state oxygen evolution in the acidic range, which occurs with pK approximately 4.8, is a consequence of similar pH blocks in three of the redox steps involved in the oxygen evolution. In the alkaline region, we report a clear pH block in the S3 --> [S4] --> S0 transition with a pK of about 8.0. Our study also indicates the existence of a pH block at very high pH (pK approximately 9.4) in the S2 --> S3 transition. The S0 --> S1 transition is not affected, at least up to pH 9.0. This suggests that the inhibition of the steady-state oxygen evolution, which occurs with a pK of 8.0, is dominated by the inhibition of the S3 --> [S4] --> S0 transition. Our results are obtained in the presence of 5% methanol (v/v). However, it is unlikely that the determined pK's are affected by the presence of methanol since our results also show that the pH dependence of the steady-state oxygen evolution is not affected by methanol. The results in the alkaline region are in good agreement with a model, which suggests that the redox potential of Y(Z*)/Y(Z) is directly affected by high pH. At high pH the Y(Z*)/Y(Z) potential becomes lower than that of S2/S1 and S3/S2. The acidic block, with a pK of 4-5 in three S-transitions, implies that the inhibition mechanism is similar, and we suggest that it reflects protonation of a carboxylic side chain in the proton relay that expels protons from the OEC. (Less)
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author
organization
publishing date
type
Contribution to journal
publication status
published
subject
keywords
Catalysis, Hydrogen-Ion Concentration, Oxidation-Reduction, Oxygen : metabolism, Photosynthesis, Photosynthetic Reaction Center, Plant : chemistry, Plant : metabolism, Protons, Spinach : metabolism, Electron Spin Resonance Spectroscopy
in
Biochemistry
volume
41
issue
18
pages
5830 - 5843
publisher
The American Chemical Society
external identifiers
  • wos:000175365300015
  • pmid:11980487
  • scopus:0037035522
ISSN
0006-2960
DOI
10.1021/bi011691u
language
English
LU publication?
yes
id
b66be7ce-d57f-4f42-9b49-ee8fb072f0d5 (old id 107903)
date added to LUP
2007-07-04 13:40:04
date last changed
2017-03-05 03:36:00
@article{b66be7ce-d57f-4f42-9b49-ee8fb072f0d5,
  abstract     = {We have investigated the pH dependence for each individual redox transition in the S-cycle of the oxygen evolving complex (OEC) of photosystem II by electron paramagnetic resonance (EPR) spectroscopy. In the experiments, OEC is advanced to the appropriate S-state at normal pH. Then, the pH is rapidly changed, and a new flash is given. The ability to advance to the next S-state in the cycle at different pHs is determined by measurements of the decrease or increase of characteristic EPR signals from the OEC in different S-states. In some cases the measured EPR signals are very small (this holds especially for the S0 ML signal at pH &gt;7.5 and pH &lt;4.8). Therefore, we refrain from providing error limits for the determined pK's. Our results indicate that the S1 --&gt; S2 transition is independent of pH between 4.1 and 8.4. All other S-transitions are blocked at low pH. In the acidic region, the pK's for the inhibition of the S2 --&gt; S3, the S3 --&gt; [S4] --&gt; S0, and the S0 --&gt; S1 transitions are about 4.0, 4.5, and 4.7, respectively. The similarity of these pK values indicates that the inhibition of the steady-state oxygen evolution in the acidic range, which occurs with pK approximately 4.8, is a consequence of similar pH blocks in three of the redox steps involved in the oxygen evolution. In the alkaline region, we report a clear pH block in the S3 --&gt; [S4] --&gt; S0 transition with a pK of about 8.0. Our study also indicates the existence of a pH block at very high pH (pK approximately 9.4) in the S2 --&gt; S3 transition. The S0 --&gt; S1 transition is not affected, at least up to pH 9.0. This suggests that the inhibition of the steady-state oxygen evolution, which occurs with a pK of 8.0, is dominated by the inhibition of the S3 --&gt; [S4] --&gt; S0 transition. Our results are obtained in the presence of 5% methanol (v/v). However, it is unlikely that the determined pK's are affected by the presence of methanol since our results also show that the pH dependence of the steady-state oxygen evolution is not affected by methanol. The results in the alkaline region are in good agreement with a model, which suggests that the redox potential of Y(Z*)/Y(Z) is directly affected by high pH. At high pH the Y(Z*)/Y(Z) potential becomes lower than that of S2/S1 and S3/S2. The acidic block, with a pK of 4-5 in three S-transitions, implies that the inhibition mechanism is similar, and we suggest that it reflects protonation of a carboxylic side chain in the proton relay that expels protons from the OEC.},
  author       = {Bernat, Gabor and Morvaridi, Susan F and Feyziyev, Yashar and Styring, Stenbjörn},
  issn         = {0006-2960},
  keyword      = {Catalysis,Hydrogen-Ion Concentration,Oxidation-Reduction,Oxygen : metabolism,Photosynthesis,Photosynthetic Reaction Center,Plant : chemistry,Plant : metabolism,Protons,Spinach : metabolism,Electron Spin Resonance Spectroscopy},
  language     = {eng},
  number       = {18},
  pages        = {5830--5843},
  publisher    = {The American Chemical Society},
  series       = {Biochemistry},
  title        = {pH dependence of the four individual transitions in the catalytic S-cycle during photosynthetic oxygen evolution.},
  url          = {http://dx.doi.org/10.1021/bi011691u},
  volume       = {41},
  year         = {2002},
}