Direct determination of the oxidation reduction potential of the iron—quinone electron acceptor (Q) in photosystem II in Chlamydomonas reinhardtii

Abstract

The redox properties of the electron acceptors of the photosystem II reaction centre are complex and it has proved difficult to obtain a definitive value for the potential of the quinone electron acceptor Q. Q was originally defined as a component which quenched fluorescence from the light-harvesting chlorophyll associated with photosystem II, reduction of Q chemically or by continuous illumination resulting in increased fluorescence [I]. The extent of this variable fluorescence at different potentials has been used to determine the redox potential of Q. These experiments identify two different quenchers: QL with &,7.0 =-250 mV and QH with &,7.0 = 0 mV (see [2] for review). The precise E,,, varies with the organism used and also differences in experimental technique. Direct detection of the electron acceptor is difficult; an absorption change at 550 nm due to an electrochromic absorption shift is an indicator of the redox state of Q. Originally observed at 77 K, it can also be seen as a kinetic change at room temperature. Redox potential determinations for Csso vary between +50 mV to -50 mV [2]. In chloroplasts the potential is pH dependent. A recent measurement for a Chlamydomonas reinhardtii preparation similar to that used here indicated a potential of 0 mV which was pH independent between pH 5.0 and 8.0 [3]. Q can also be detected optically by the absorption change of the quinone at 320 nm. The change is however small and difficult to measure and its redox properties have not been determined. We have recently shown that in preparations from a mutant of C. reinhardtii lacking photosystern I an EPR signal characteristic of an ironquinone complex similar to those found in purple bacteria can be detected and identified as an electron acceptor of photosystem II [4]. We have now titrated this EPR signal to obtain a direct measurement of the redox potential of this quinone electron acceptor of photosystem II, and to determine whether it is QL or QH.