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 QA, Em(QA/QA−), in an oxygen-evolving photosystem (PS) II complex from a thermophilic cyanobacterium Thermosynechococcus elongatus was determined to be −140 ± 2 mV vs. SHE by thin-layer cell spectroelectrochemistry for the first time. The Em(QA/QA−) value obtained here together with the recently determined redox potential of pheophytin (Phe) a [Kato et al. (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 QA and provides a renewed picture for the energetics on the electron acceptor side in PS II.