Light-induced oxidation of the acceptor-side Fe(II) of Photosystem II by exogenous quinones acting through the Q B binding site. I. Quinones, kinetics and pH-dependence

Abstract

We have recently shown by optical, EPR and Mössbauer spectroscopy that the high spin Fe(II) of the quinone-iron acceptor complex of Photosystem II can be oxidized by ferricyanide to high-spin Fe(III). The midpoint potential of the Fe(III)/Fe(II) couple is 370 mV at pH 7.5 and shows an approximate pH-dependence of −60 mV/pH unit. The iron was identified as being responsible for the high potential Photosystem II acceptor known as Q 400, discovered by Ikegami and Katoh ((1975) Plant Cell Physiol. 14, 829–836) but until now not identified chemically. We establish here that Q A and the oxidized Fe(III) are linked in series, with Q A the first to be reduced in the primary charge separation of Photosystem II. At pH 7.5, an electron is then transferred from Q − A to Fe(III) with a t 1 2 of 25 μs, reforming Q A Fe(II). The Fe(II) can also be oxidized to Fe(III) in oxygen-evolving thylakoid membranes through a photoreduction-induced oxidation in the presence of exogenous quinones, where E m,7 ( Q − QH 2) > E m,7 ( Fe(III) Fe(II)) − 60 mV . Single turnover illumination of the Photosystem II reaction center at 200 K, followed by warming to 0°C, results in photoreduction of these quinones to the semiquinone form which in turn oxidizes the Fe(II) to Fe(III). A second turnover of the reaction center reduces Fe(III) back to Fe(II). These reactions, similar to those reported by Zimmermann and Rutherford (Zimmermann, J.L. and Rutherford, A.W. (1986) Biochim. Biophys. Acta 851, 416–423) at room temperature, in work largely done in parallel, are summarized below: ▪ where Q A and Q ex are the primary quinone acceptor of Photosystem II and exogenous quinone, respectively. Detection of Fe(III) at g = 8 by EPR spectroscopy shows this signal to oscillate with period two upon successive turnovers of the Photosystem II reaction center. Different exogenous quinones give different EPR spectra for Fe(III), indicating that these bind close to the Fe binding site and modify the symmetry of the Fe(III) environment. A study of the pH-dependence of the light-induced oxidation of the Fe(III) by phenyl- p-BQ shows a pH-optimum at 6–7. The decline at higher pH is consistent with a pH-dependence of −60 mV/pH unit and −120 mV/pH unit, respectively, for redox couples Fe(III)/Fe(II) and Q −/QH 2. The decline at lower pH was not foreseen and appears associated with a transformation of the quinone-iron environment from that showing a Q − AFe(II) EPR resonance of g = 1.9 at high pH to one at g = 1.84 below pH 6.5. The latter form appears not to support light-induced oxidation of the Fe(II) by exogenous quinones.