Redox potentials of primary electron acceptor quinone molecule (Q _A ) ⁻ 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. ) 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.photosynthesis | photochemical reaction C hlorophylls (Chls) are key pigments for photosynthesis, and oxygenic photosynthetic organisms containing Chls are found all over the world-they sustain all terrestrial life through the primary production of organic molecules and the production of oxygen. Chl a and its derivatives, pheophytin (Phe) a and Chl a epimer (Chl a′), are used as the electron transfer components in photosystem (PS) II and PS I, respectively, except for in two taxonomic groups, marine cyanobacteria Prochlorococcus spp. containing divinyl-Chl a (1) and marine cyanobacteria Acaryochloris spp. containing Chl d (2). Consequently, experimental and theoretical analyses of the electron transfer system in oxygenic photosynthesis have been exclusively performed in Chl a-containing organisms, and the basic concept of the photosynthetic electron flow has been constructed on the basis of Chl a. However, our understanding of photosynthetic electron transfer is not yet complete because the reaction systems with divinyl-Chl a and Chl d have not been fully analyzed as yet.Since the discovery of Chl d in Acaryochloris marina in 1996 (3, 4), questions have arisen on the whether the electron transfer system in this organism is significantly different from those with Chl a. Interestingly, the functional "special pair" are Chl d molecules in the reaction centers of PS I and II in A. marina (5, 6); however, in PS II, Phe a is the primary electron acceptor. Chl d gains light energy at longer wavelengths and with a lowered redox potential, by ∼80 mV, compared with that of Chl a in Synechocystis sp. PCC 6803 (hereafter referred to as Synechocystis) and spinach (7-11). However, the overall ...

Bicarbonate-induced redox tuning in Photosystem II for regulation and protection

“…Relevant to this argument is a report in which Allakhverdiev et al (24) verified the values reported by Shibamoto et al (22) but used the method by Krieger et al (12). This finding, however, does not contradict the present explanation, because the work in ref.…”

“…Subsequently, some evidence was published indicating that, when titrations were done with the same mediators, the lower potential values were obtained, whereas in the absence of mediators, the higher potential values similar to those by Krieger and coworkers (12)(13)(14) were obtained (23). However, low potential values were also recently reported without the use of mediators (24). Since then, the situation has remained unresolved, leaving a doubt about the correct potential and consequently, a serious ambiguity at the heart of PSII bioenergetics.…”

“…This finding, however, does not contradict the present explanation, because the work in ref. 24 A oxidation (i.e., fluorescence decay as in Fig. 2).…”

Redox potential of the terminal quinone electron acceptor QBin photosystem II reveals the mechanism of electron transfer regulation

“…Specifically, they exchange D1 subunits originating from different psbA genes to change the hydrogen bond interactions of Pheo (16)(17)(18)(19)(20). On the other hand, it was found that impairment of the Mn 4 CaO 5 cluster led to a significant increase in the E m of Q A by ∼150 mV (21)(22)(23)(24)(25)(26)(27). This potential increase was thought to inhibit forward electron transfer to Q B to promote direct relaxation of Q A − without forming triplet-state Chl, a precursor of harmful singlet oxygen (2,5,14,15,17,23).…”

Novel chlorophylls and new directions in photosynthesis research

“…However, the red-shifted chlorophylls might encounter the energy losses during the subsequent photochemistry that critically affect the efficiency of electron-transfer, and thereby energy storage. A recent study investigating the redox potential of the Chl d special pairs in A. marina reported no differences for the redox potential level between Chl a-containing cyanobacteria and A. marina (Allakhverdiev et al 2010;Allakhverdiev et al 2011). For PSII reaction centre in A. marina, there is general agreement that Chl a is replaced by Chl d, at least at accessory sites (Chl D1 and Chl D2 ) Tomo et al 2007) and Pheo a (instead of Pheo d) is the primary acceptor of D1-side as in other oygenic photosynthetic organisms (Tomo et al 2007(Tomo et al , 2008.…”