Creation of a 3Mn/1Fe Cluster in the Oxygen-Evolving Complex of Photosystem II and Investigation of its Functional Activity

Michael Seibert, B. K..Semin, L. Davletshina, A. Rubin

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Abstract

Extraction of Mn cations from the oxygen-evolving complex (OEC) of Ca-depleted PSII membranes (PSII[-Ca,4Mn]) by reductants like hydroquinone (H2Q) occurs with lower efficiency at acidic pH (2Mn/reaction center [RC] are extracted at pH 5.7) than at neutral pH (3Mn/RC are extracted at pH 6.5) [Semin et al. Photosynth. Res. 125 (2015) 95]. Fe(II) also extracts Mn cations from PSII(-Ca,4Mn), but only 2Mn/RC at pH 6.5, forming a heteronuclear 2Mn/2Fe cluster [Semin and Seibert, J. Bioenerg. Biomembr. 48 (2016) 227]. Here we investigated the efficiency of Mn extraction by Fe(II) at acidic pH and found that Fe(II) cations can extract only 1Mn/RC from PSII(-Ca,4Mn) membranes at pH 5.7, forming a 3Mn/1Fe cluster. Also we found that the presence of Fe cations in a heteronuclear cluster (2Mn/2Fe) increases the resistance of the remaining Mn cations to H2Q action, since H2Q can extract Mn cations from homonuclear Mn clusters of PSII(-Ca,4Mn) and PSII(-Ca,2Mn) membranes but not from the heteronuclear cluster in PSII(-Ca,2Mn,2Fe) membranes. H2Q also cannot extract Mn from PSII membranes obtained by incubation of PSII(-Ca,4Mn) membranes with Fe(II) cations at pH 5.7, which suggests the formation of a heteronuclear 3Mn/1Fe cluster in the OEC. Functional activity of PSII with a 3Mn/1Fe cluster was investigated. PSII preparations with a 3Mn/1Fe cluster in the OEC are able to photoreduce the exogenous electron acceptor 2,6-dichlorophenolindophenol, possibly due to incomplete oxidation of water molecules as is the case with PSII(-Ca,2Mn,2Fe) samples. However, in the contrast to PSII(-Ca,2Mn,2Fe) samples PSII(-Ca,3Mn,1Fe) membranes can evolve O2 at a low rate in the presence of exogenous Ca2 + (at about 27% of the rate of O2 evolution in native PSII membranes). The explanation for this phenomenon (either water splitting and production of molecular O2 by the 3Mn/1Fe cluster or apparent O2 evolution due to minor contamination of PSII(3Mn,1Fe) samples with PSII(-Ca,4Mn) membranes) is discussed.

Original languageAmerican English
Pages (from-to)192-200
Number of pages9
JournalJournal of Photochemistry and Photobiology B: Biology
Volume178
DOIs
StatePublished - 2018

Bibliographical note

Publisher Copyright:
© 2017

NREL Publication Number

  • NREL/JA-2700-68047

Keywords

  • Calcium
  • Iron
  • Manganese
  • Oxygen-evolving complex
  • Photosystem II
  • Water-splitting

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