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

Abstract

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.

Fig. 1.

Redox titration of Q_A/Q_A⁻ in PS II complexes with and without the Mn cluster in A. marina (A), Synechocystis (B), and spinach (C) PS II particles. ○, PS II complex with the Mn cluster; ● and ▲, PS II complex without a Mn cluster. NH₂OH and CHES indicate the hydroxyl amine treatment for the Mn depletion.

Fig. 2.

Redox titration of Phe a/Phe a⁻ in the PS II complexes in Synechocystis control (A), after CHES treatment (B), and restoration of the Mn cluster after CHES treatment (C).

Fig. 3.

Overall energetics of PS II in the two cyanobacteria A. marina and Synechocystis.