Reaction Mechanism of the Terminal Plastoquinone Q_B in Photosystem II as Revealed by Time-Resolved Infrared Spectroscopy

The secondary plastoquinone (PQ) electron acceptor Q_B in photosystem II (PSII) undergoes a two-step photoreaction through electron transfer from the primary PQ electron acceptor Q_A, converting into plastoquinol (PQH₂). However, the detailed mechanism of the Q_B reactions remains elusive. Here, we investigated the reaction mechanism of Q_B in cyanobacterial PSII core complexes using two time-revolved infrared (TRIR) methods: dispersive-type TRIR spectroscopy and rapid-scan Fourier transform infrared spectroscopy. Upon the first flash, the ∼140 μs phase is attributed to electron transfer from Q_A^•- to Q_B, while the ∼2.2 and ∼440 ms phases are assigned to the binding of an internal PQ in a nearby cavity to the vacant Q_B site and an external PQ traveling to the Q_B site through channels, respectively, followed by immediate electron transfer. The resultant Q_B^•- is suggested to be in equilibrium with Q_BH^•, which is protonated at the distal oxygen. Upon the second flash, the ∼130 μs and ∼3.3 ms phases are attributed to electron transfer to Q_BH^• and the protonation of Q_B^•- followed by electron transfer, respectively, forming Q_BH^-, which then immediately accepts a proton from D1-H215 at the proximal oxygen to become Q_BH₂. The resultant D1-H215 anion is reprotonated in ∼22 ms via a pathway involving the bicarbonate ligand. The final ∼490 ms phase may reflect the release of PQH₂ and its replacement with PQ. The present results highlight the importance of time-resolved infrared spectroscopy in elucidating the mechanism of Q_B reactions in PSII.