Evidence for the Involvement of Loosely Bound Plastosemiquinones in Superoxide Anion Radical Production in Photosystem II

Abstract

Recent evidence has indicated the presence of novel plastoquinone-binding sites, QC and QD, in photosystem II (PSII). Here, we investigated the potential involvement of loosely bound plastosemiquinones in superoxide anion radical (O2 •−) formation in spinach PSII membranes using electron paramagnetic resonance (EPR) spin-trapping spectroscopy. Illumination of PSII membranes in the presence of the spin trap EMPO (5-(ethoxycarbonyl)-5-methyl-1-pyrroline N-oxide) resulted in the formation of O2 •−, which was monitored by the appearance of EMPO-OOH adduct EPR signal. Addition of exogenous short-chain plastoquinone to PSII membranes markedly enhanced the EMPO-OOH adduct EPR signal. Both in the unsupplemented and plastoquinone-supplemented PSII membranes, the EMPO-OOH adduct EPR signal was suppressed by 50% when the urea-type herbicide DCMU (3-(3,4-dichlorophenyl)-1,1-dimethylurea) was bound at the QB site. However, the EMPO-OOH adduct EPR signal was enhanced by binding of the phenolic-type herbicide dinoseb (2,4-dinitro-6-sec-butylphenol) at the QD site. Both in the unsupplemented and plastoquinone-supplemented PSII membranes, DCMU and dinoseb inhibited photoreduction of the high-potential form of cytochrome b 559 (cyt b 559). Based on these results, we propose that O2 •− is formed via the reduction of molecular oxygen by plastosemiquinones formed through one-electron reduction of plastoquinone at the QB site and one-electron oxidation of plastoquinol by cyt b 559 at the QC site. On the contrary, the involvement of a plastosemiquinone formed via the one-electron oxidation of plastoquinol by cyt b 559 at the QD site seems to be ambiguous. In spite of the fact that the existence of QC and QD sites is not generally accepted yet, the present study provided more spectroscopic data on the potential functional role of these new plastoquinone-binding sites.

Oxygen and ROS in Photosynthesis

“…To explain O 2 reduction via an apparently thermodynamically unlikely reaction, it has been suggested that the A possible alternative solution is that cyt b559 mediates the formation of semiquinones at Q D and Q C sites. Experimental evidence of the reduction in O 2 by a loosely bound plastosemiquinone anion radical (PQ •− ) at the Q C site was provided by Yadav et al [250]. The authors showed that PQ •− can be formed by a one electron reduction in PQ at the Q B site and one electron oxidation of PQH 2 by cyt b559 at the Q C site.…”

Production of Reactive Oxygen Species by Photosystem II as a Response to Light and Temperature Stress

“…Pheophytin (Pheo D1 •– ), tightly bound plastosemiquinone (Q A •– ), loosely bound plastosemiquinones (Q B •– or Q C •– ), free PQ (PQ •– ), and ferrous iron of LP form of cyt b 559 were proposed to serve as electron donors to O 2 (Ananyev et al, 1994; Cleland and Grace, 1999; Pospíšil et al, 2004, 2006; Yadav et al, 2014). As Pheo D1 •– has highly negative redox potential, the reduction of O 2 by Pheo D1 •– is thermodynamically feasible; however, its short lifetime makes the diffusion limited reduction of O 2 less reasonable.…”

“…The detection of O 2 •– in isolated thylakoids by a voltammetric method showed O 2 •– production by the tightly bound plastosemiquinone Q A •– (Cleland and Grace, 1999). Experimental evidence has been recently provided on the reduction of O 2 by the loosely bound plastosemiquinones (Yadav et al, 2014). The authors demonstrated that plastosemiquinone is formed by the one-electron reduction of plastoquinone at the Q B site and the one-electron oxidation of plastoquinol by cyt b 559 at the Q C site.…”

Detection of hydrogen peroxide in Photosystem II (PSII) using catalytic amperometric biosensor

“…Formation of O results from non-enzymatic and enzymatic one-electron reductions of molecular oxygen. Pheophytin (Pheo •− ) (Ananyev et al, 1994 ; Pospíšil et al, 2004 ), tightly bound plastosemiquinone (Q ), (Cleland and Grace, 1999 ; Pospíšil et al, 2004 ), loosely bound plastosemiquinone (Q ), (Zhang et al, 2003 ; Yadav et al, 2014 ), and free plastosemiquinone (PQ •− ) (Mubarakshina and Ivanov, 2010 ) maintains non-enzymatic reduction of molecular oxygen to O . Heme iron of low-potential (LP) form of cyt b 559 reduces molecular oxygen to O in the enzymatic reaction pathway (Pospíšil et al, 2006 ; Pospišil, 2011 ).…”