Redox Potential of the Primary Plastoquinone Electron Acceptor QA in Photosystem II from Thermosynechococcus elongatus Determined by Spectroelectrochemistry

The redox potential of the primary plastoquinone electron acceptor Q(A), E-m(Q(A)/Q(A)(-)), in an oxygen-evolging photosystem (PS) 11 complex from a thermophilic cyanobacterium Thermosynechococcus elongatus was determined to be - 140 +/- 2 mV vs. SHE by thin-layer cell spectroclectrochemistry for the first time. The E-m(Q(A)/Q(A)(-)) value obtained here together with the recently determined redox potential of pheophytin (Phe) a [Kato et at. (2009) Proc. Natl. Acad. Sci. U.S.A. 106, 17365-17370] yields -330 to -370 mV for the free energy change by electron transfer from Phe a(-) to Q(A) and provides a renewed picture for the energetics oil the electron acceptor side in PS II.