Redox titration of electron acceptor Q and the plastoquinone pool in Photosystem II

The primary photochemical quencher Q and the secondary electron acceptor pool in Photosystem II have been titrated. We used particles of Scenedesmus mutant No. 8 that lack System I and allowed the system to equilibrate with external redox mediators in darkness prior to measurement of the fluorescence rise curve. The titration of Q, as indicated by the dark level of F_i, occurs in two discrete steps. The high-potential component (Q_h) has a midpoint potential of +68 mV (pH 7.2) and accounts for ∼67% of Q. The pH sensitivity of the midpoint potential is -60 mV, indicating the involvement of 1 H⁺ e. The low-potential component (Q₁) accounts for the remaining 33% of Q and shows a midpoint potential near-300 mV (pH 7.2). The plastoquinone pool, assayed as the half-time of the fluorescence rise curve, titrates as a single component with a midpoint potential 30-40 mV more oxidizing than that of Q_h, i.e., at 106 mV (pH 7.2). The E_m shows a pH sensitivity of -60 mV/pH unit, indicating the involvement of 1 H⁺ e. The observation that all 12-14 electron equivalents in the pool titrate as a single component indicates that the heterogeneity otherwise observed in the secondary acceptor system is a kinetic rather than a thermodynamic property. Illumination causes peculiar, and as yet unclarified, changes of both Q and the secondary pool under anaerobic conditions that are reversed by oxygen.