Investigations of the water oxidation complex in PS II
Shim, Hyunsuk
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https://hdl.handle.net/2142/21339
Description
Title
Investigations of the water oxidation complex in PS II
Author(s)
Shim, Hyunsuk
Issue Date
1992
Doctoral Committee Chair(s)
Debrunner, Peter G.
Department of Study
Biophysics
Discipline
Biophysics
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Biology, Botany
Biophysics, General
Biology, Plant Physiology
Language
eng
Abstract
In this thesis an attempt was made to take the water oxidizing complex apart in a controlled way and to reconstitute it to its functional form.
The mechanism of photosynthetic water oxidation has been probed by the use of the substrate analogue NH$\sb2$OH in 1 M NaCl treated PSII membranes lacking the 17 and 23 kD extrinsic proteins. A plot of the Mn released versus (NH$\sb2$OH) shows a sigmoidal shape. The results were interpreted in terms of a cooperativity model. The plot of Mn release versus oxygen evolving activity shows that all 4 Mn in the reaction center are essential for active oxygen evolution.
1 M CaCl$\sb2$ treated PSII membranes, which lack all 3 extrinsic polypeptides (17, 23, and 33 kD) have low oxygen evolving activity in spite of the full complement of 4 Mn per reaction center. If the light intensity is sufficiently low, 1 M CaCl$\sb2$ treated PSII evolve the same number of oxygen molecules per photon as 1 M NaCl treated PSII. Therefore, removal of the 33 kD polypeptide did not inactivate the Mn center and all Mn centers are intact. When the light intensity is high, there is possible alternative electron donors to P$\sb{680}\sp{+}$ in 1 M CaCl$\sb2$ treated PSII (e.g., Chl). As a result, the fluorescence and the oxygen evolving activity are low. I analyzed the fluorescence data of the S$\sb1$ $\to$ S$\sb2$ transition in DCMU-treated samples. The transition time of CaCl$\sb2$ PSII is 1.4 times longer than that of NaCl PSII. This slow-down of the S$\sb1$ $\to$ S$\sb2$transition rate is not the main reason for slow donor side and there are other reports that indicate the slow-down of the S$\sb3$ $\to$ S$\sb0$ transition rate. It is likely that other S state transitions, including the dark reaction, are slowed down as well.
I reconstituted 37% of the oxygen evolving activity with 40% Mn concentration in the reconstituted sample. Therefore, about 40% of the centers have all 4 Mn while the other 60% of the centers have no Mn. It is very plausible that Mn rebinding also involves cooperativity. Other divalent transition metals like Fe$\sp{2+}$ and Co$\sp{2+}$ apparently compete for the Mn binding sites and may form mixed metal clusters.
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