Redox potentials of primary electron acceptor quinone molecule (Q _A)^- and conserved energetics of photosystem II in cyanobacteria with chlorophyll a and chlorophyll d

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 (Q_A), i.e., E_m(Q_A/Q _A^-), of PS II and the energy difference between [P680·Phe a^-·Q_A] and [P680·Phe a·Q_A^-], i.e., ΔG_PhQ. The E _m(Q_A/Q_A^-) 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 E_m(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 ΔG_PhQ 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 Q_A^- and Phe a^- were relatively shifted depending on the special pair, indicating a common strategy for electron transfer in oxygenic photosynthetic organisms.