Rapid-Scan Time-Resolved ATR-FTIR Study on the Photoassembly of the Water-Oxidizing Mn₄CaO₅ Cluster in Photosystem II

Abstract

The catalytic center of photosynthetic water oxidation, the Mn4CaO5 cluster, is assembled in photosystem II (PSII) through a light-driven process called photoactivation, whose mechanism remains elusive. Here, we used rapid-scan time-resolved Fourier transform infrared (FTIR) spectroscopy combined with the attenuated total reflection (ATR) technique to monitor the photoactivation process. Rapid-scan ATR-FTIR spectra of apo-PSII with Mn2+ upon flash illumination showed spectral features typical of carboxylate stretching vibrations, which were attributed to two carboxylate ligands, D1-D170 and D1-E189, by quantum chemical calculations. The FTIR signal decayed with a time constant of ∼0.7 s, showing that the subsequent “dark rearrangement” step occurred with a low quantum yield and Mn3+ ions were mostly released during this decay. Simulation of the kinetic process provided a slow intrinsic rate of the dark rearrangement, which was attributed to a large protein conformational change. The photoassembly mechanism of the Mn4CaO5 cluster is proposed based on these findings.

Structure of a monomeric photosystem II core complex from a cyanobacterium acclimated to far-red light reveals the functions of chlorophylls d and f

“…Based on these observations, we suggest that the unique conformation of the cation-binding loop of PsbC(2) observed herein is only achieved when Psb27 is unbound to the PsbC(2) lumenal domain, and may, therefore, represent an unproductive PSII state. This supports the hypothesis that Psb27 stabilizes the lumenal domain of PsbC(2) during PSII assembly ( 53 , 75 , 76 , 77 , 78 , 79 ). As is the case for the acceptor-side perturbations described previously, further data are needed to determine whether the lumenal domain of this core complex is representative of characteristics observed during PSII maturation in vivo .…”

“…The map quality of the OEC-binding site is somewhat poor, likely because of heterogeneity and flexibility in that region. Some previous cryo-EM maps of PSII lacking the OEC contain an ESP signal proposed to arise from a cation bound between the carboxylate side chains of PsbA-Asp170 † and PsbA-Asp189 ( 27 , 28 , 30 ) that have been suggested to comprise the high-affinity Mn-binding site ( 29 , 53 ) involved in the initial steps of photoactivation ( 26 ). Although we observe an ESP signal between PsbA3-Asp170 and PsbA3-Glu189 in the apo-FRL-PSII structure, it corresponds better to two water molecules and is modeled as such.…”

Mimicking the Oxygen-Evolving Center in Photosynthesis

“…Considering the observed thermodynamical stability of the fully carboxylic ligand coordinated Mn 3 CaO 4 -cluster (5) observed (Chen et al, 2022), we propose that a similar Mn 3 CaO 4cluster could be present during the synthesis of the OEC in PSII. If it was the case, a mono-nuclear Mn ion (similar to that in 6) would be necessary to be incorporated into the Mn 3 CaO 4 -cluster, followed by structural rearrangements to form the intact OEC, as has been suggested recently (Gisriel et al, 2020;Sato et al, 2021).…”

ATR-FTIR Spectroelectrochemical Study on the Mechanism of the pH Dependence of the Redox Potential of the Non-Heme Iron in Photosystem II

“…PSII membranes were prepared from spinach according to the method reported previousely . Sample treatments and FTIR measurements using an ATR apparatus (DuraSampl IR II, Smiths Detection) were carried out with the procedure analogous to that described previously. ,− An aliquot (5 μL) of the PSII suspension (1 mg of Chl/mL), which was washed with Milli-Q water by centrifugation, was deposited on an ATR silicon prism (3 mm in diameter) and then dried with N 2 gas. Washing with Milli-Q water was necessary to facilitate adsorption of the PSII membranes on the prism by eliminating ions.…”