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

• Published in: Biochemistry (Easton) Vol. 63; no. 21; pp. 2778 - 2792
• Main Authors: Kato, Yuki, Ito, Honami, Noguchi, Takumi
• Format: Journal Article
• Language: English
• Published: American Chemical Society 05.11.2024
• ISSN: 0006-2960; 1520-4995; 1520-4995
• DOI: 10.1021/acs.biochem.4c00509

The secondary plastoquinone (PQ) electron acceptor QB in photosystem II (PSII) undergoes a two-step photoreaction through electron transfer from the primary PQ electron acceptor QA, converting into plastoquinol (PQH2). However, the detailed mechanism of the QB reactions remains elusive. Here, we investigated the reaction mechanism of QB 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 QA •– to QB, while the ∼2.2 and ∼440 ms phases are assigned to the binding of an internal PQ in a nearby cavity to the vacant QB site and an external PQ traveling to the QB site through channels, respectively, followed by immediate electron transfer. The resultant QB •– is suggested to be in equilibrium with QBH•, 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 QBH• and the protonation of QB •– followed by electron transfer, respectively, forming QBH–, which then immediately accepts a proton from D1-H215 at the proximal oxygen to become QBH2. 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 QB reactions in PSII.