Structure and Reaction of the Secondary Quinone Electron Acceptor Q_B in Photosystem II as Studied by Fourier Transform Infrared Spectroscopy

Abstract

In PSII, the secondary quinone electron acceptor Q_B is reduced by the primary quinone Q_A, and after two-electron reduction, Q_B is protonated and released from the protein. The structural relevance of the Q_B binding site to this Q_B function still remains unclear. Here, we have studied the structure and reaction of Q_B using FTIR spectroscopy. FTIR difference spectra of S₂Q_B^-/S₁Q_B and of S₂/S₁ were measured using PSII preparations from spinach or the cyanobacterium T. elongatus, and a Q_B^-/Q_B spectrum was calculated as a double difference. In this spectrum, the CO stretching band of Q_B^- was observed at 1480 cm^-1, the frequency 2 cm^-1 higher than that of Q_A, indicating the difference in H-bonding interactions between Q_A^- and Q_B^-. In addition, a positive C=O band of a COOH group was observed at 1747 cm^-1, suggesting that a nearby Asp or Glu residue is partially protonated upon Q_B^- formation.