Calcium exchange and structural changes during the photosynthetic oxygen-evolving cycle
Antonio De Riso 1, David Jenson 1 and Bridgette A. Barry 1*
1 Georgia Institute of Technology
* To whom correspondence should be addressed. E-mail: bridgette.barry{at}chemistry.gatech.edu.
Submitted on April 18, 2006
Revised on May 9, 2006
Accepted on 1 June 2006
 |
Abstract |
|---|
Photosystem II (PSII) catalyzes the oxidation of water and reduction of plastoquinone in oxygenic photosynthesis. PSII contains an oxygen-evolving complex, which is located on the lumenal side of the PSII reaction center and which contains manganese, calcium, and chloride. Four sequential photo-oxidation reactions are required to generate oxygen. This process produces five Sn states, where n refers to the number of oxidizing equivalents stored. Calcium is required for oxygen production. Strontium is the only divalent cation that replaces calcium and maintains activity. In our previous FT-IR work, we assessed the effect of strontium substitution on substrate-limited PSII preparations, which were inhibited at the S3 to S0 transition. In this paper, we report reaction-induced FT-IR studies of hydrated PSII preparations, which undergo the full S state cycle. The observed difference FT-IR spectra reflect long-lived photo-induced conformational changes in the oxygen-evolving complex; strontium exchange identifies vibrational bands sensitive to substitutions at the calcium site. During the S1' to S2' transition, the data are consistent with an electrostatic or structural perturbation of the calcium site. During the S3' to S0' and S0' to S1' transitions, the data are consistent with a perturbation of a hydrogen bonding network, which contains calcium, water, and peptide carbonyl groups. To explain our data, persistent shifts in divalent cation coordination must occur when strontium is substituted for calcium. A modified S state model is proposed to explain these results and results in the literature.
Key Words:
amide, manganese, strontium, vibrational spectroscopy, water oxidation
