Rapid-Scan Fourier Transform Infrared Monitoring of the Photoactivation Process in Cyanobacterial Photosystem II.

The catalytic site of photosynthetic water oxidation, the MnCaO cluster, in photosystem II (PSII) is known to be formed by a light-induced process called photoactivation. However, details of its molecular mechanism remain unresolved. In this study, we monitored the photoactivation process in cyanobacterial PSII using rapid-scan, time-resolved Fourier transform infrared (FTIR) spectroscopy. The Mn/Mn FTIR difference spectra of PSII, in which D1-D170 was specifically C labeled, and PSII from the D1-D170A, D1-E189A, and D1-D342A mutants provide strong evidence that the initial Mn is coordinated by D1-D170 and D1-E189. Protein conformational changes and relocation of photo-oxidized Mn in the dark rearrangement process were detected as slow-phase signals in the amide I and carboxylate regions, whereas similar signals were not observed in D1-E189A PSII. It is thus proposed that relocation of Mn via D1-E189 induces the conformational changes of the proteins to form proper Mn binding sites in the mature protein conformation.