In a previous study, we measured the redox potential of the primary electron acceptor pheophytin (Phe) a of photosystem (PS) II in the chlorophyll d-dominated cyanobacterium Acaryochloris marina and a chlorophyll a-containing cyanobacterium, Synechocystis. We obtained the midpoint redox potential (E m) values of -478 mV for A. marina and -536 mV for Synechocystis. In this study, we measured the redox potentials of the primary electron acceptor quinone molecule (QA), i.e., Em(QA/Q A-), of PS II and the energy difference between [P680·Phe a-·QA] and [P680·Phe a·QA-], i.e., ΔGPhQ. The E m(QA/QA-) of A. marina was determined to be +64 mV without the Mn cluster and was estimated to be -66 to -86 mV with a Mn-depletion shift (130-150 mV), as observed with other organisms. The Em(Phe a/Phe a-) in Synechocystis was measured to be -525 mV with the Mn cluster, which is consistent with our previous report. The Mn-depleted downshift of the potential was measured to be approximately -77 mV in Synechocystis, and this value was applied to A. marina (-478 mV); the E m(Phe a/Phe a-) was estimated to be approximately -401 mV. These values gave rise to a ΔGPhQ of -325 mV for A. marina and -383 mV for Synechocystis. In the two cyanobacteria, the energetics in PS II were conserved, even though the potentials of QA- and Phe a- were relatively shifted depending on the special pair, indicating a common strategy for electron transfer in oxygenic photosynthetic organisms.
|Number of pages||5|
|Journal||Proceedings of the National Academy of Sciences of the United States of America|
|Publication status||Published - May 10 2011|
- Photochemical reaction
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