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

The catalytic site of photosynthetic water oxidation, the Mn 4 CaO 5 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 3+ /Mn 2+ FTIR difference spectra of PSII, in which D1-D170 was specifically 13 C labeled, and PSII from the D1-D170A, D1-E189A, and D1-D342A mutants provide strong evidence that the initial Mn 2+ is coordinated by D1-D170 and D1-E189. Protein conformational changes and relocation of photo-oxidized Mn 3+ 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 3+ via D1-E189 induces the conformational changes of the proteins to form proper Mn binding sites in the mature protein conformation.