Self-Adaptable Quinone-Quinol Exchange Mechanism of Photosystem II

摘要： The step of plastoquinone (PQ) reduction to plastoquinol (PQH2) can regulate the photo-reaction 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.