Bioenergetic studies on the quinone electron acceptors of photosystem II

Abstract

Photosystem II (PSII) is a membrane-bound protein complex found in plants, algae and cyanobacteria that converts light into chemical energy. Despite extensive research, many energetic and mechanistic questions of PSII remain unresolved. Here the energetics and kinetics of the electron-acceptor side of PSII from Thermosynechococcus elongatus were investigated using biophysical approaches. Based on data from electron paramagnetic resonance and thermoluminescence measurements, the two midpoint potentials of the terminal electron acceptor, QB, were measured (Em(QB/QB•−) = 92 mV; Em(QB•−/QBH2) = 43 mV). It was found that i) QB•− is significantly stabilized, contradicting the recent literature, ii) the energy-gap between QA and QB is larger than previously assumed (235 mV instead of ≈ 80 mV), contradicting the older literature, and iii) the release of QBH2 into the pool is thermodynamically favourable, ( ≈ 50 meV). No significant shift of the QB midpoint potentials in response to the loss of the Mn4O5Ca cluster was found. These findings allow for a better understanding of charge separation and the energetics of PSII. Isolated PSII from T. elongatus is used in many structural and functional studies but the electron acceptor side kinetics of this organism are poorly defined. Using absorption spectroscopy, the kinetics which were previously treated as a single “fast phase”, were resolved as follows: QA•−→ Fe 3+ (t1/2 = 50 µs); QA•−→QB(t1/2 = 350 µs); QA•−→ QB•− (t1/2 = 1.3 ms). Furthermore, the kinetic data analysis developed in this work allowed the proportions of these reactions to be determined under a range of conditions. It was found that in long dark-adapted samples up to 50% of the non-heme iron was oxidized and this oxidation was inhibited when bicarbonate was present. These data will be useful for future research on PSII and help understanding the mechanism of electron transfer on the acceptor side.