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

The secondary plastoquinone (PQ) electron acceptor Q in photosystem II (PSII) undergoes a two-step photoreaction through electron transfer from the primary PQ electron acceptor Q, converting into plastoquinol (PQH). However, the detailed mechanism of the Q reactions remains elusive. Here, we investigated the reaction mechanism of Q 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 to Q, 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 site and an external PQ traveling to the Q site through channels, respectively, followed by immediate electron transfer. The resultant Q is suggested to be in equilibrium with QH, 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 QH and the protonation of Q followed by electron transfer, respectively, forming QH, which then immediately accepts a proton from D1-H215 at the proximal oxygen to become QH. 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 PQH and its replacement with PQ. The present results highlight the importance of time-resolved infrared spectroscopy in elucidating the mechanism of Q reactions in PSII.