Reconstitution of the primary quinone electron acceptor Q_A in photosystem II core complexes

Abstract

The primary and secondary quinone electron acceptors, Q_A and Q_B, in photosystem II (PSII) both consist of a plastoquinone (PQ) molecule. Whereas Q_B is readily exchanged, Q_A is tightly bound and thought to be unexchangeable without decomposition of protein subunits. In this study, we have performed reconstitution of Q_A with PQ retaining the protein structure of the PSII complex. Q_A is depleted from PSII by treatment with dithionite and benzyl viologen, and then excess PQ was added to the sample. Fourier transform infrared difference spectra upon Q_A photo-reduction showed that more than 70% of Q_A was reconstituted with a proper interaction at the Q_A site. This result was confirmed by reconstitution of ¹³C-labelled PQ, showing downshifts of signals of PQ. Reconstitution of Q_A in PSII will be a useful tool to investigate its structure in the protein and the mechanism for controlling the redox potential.