Self-Adaptable Quinone-Quinol Exchange Mechanism of Photosystem II.

Abstract

The step of plastoquinone (PQ) reduction to plastoquinol (PQH2) can regulate the photoreaction rate of photosystem II (PSII). To experimentally unravel the PQ-PQH2 exchange mechanism of PSII, we investigate the reaction kinetics of plant PSII membranes and the subunits-trimmed PSII core complexes with various PQ analogues and directly probe the reductions of PQ and other quinones by 257 nm resonance Raman scattering. Two phases of quinone concentration effect on the reaction rate originate from the quinone-quinol exchange mechanism. The results indicate that high concentrations of quinone, more than one movable quinone molecule per PSII reaction center, could trigger quinone-quinol exchange adapting to the unidirectional route: quinones enter through channel I and/or III, and quinols leave through channel II. A weak quinone binding site near QB probably plays a crucial role in pushing quinone-quinol exchange forward in the unidirectional route. Our work provides experimental proofs demonstrating a self-adaptable quinone-quinol exchange mechanism of PSII.