Photoreduction of the Non Heme Iron in Photosystem II Studied by FTIR Difference Spectroscopy

Abstract

In photosystem II (PS II) as in the bacterial photosynthetic reaction center (RC), two quinones QA and QB constitute the primary and secondary electron acceptors. The three-dimensional structure of the bacterial RC showed that a non-heme iron (Fe) is located between QA and QB. This Fe is liganded to four histidine residues, the side-chain of a glutamate residue provides the fifth and sixth ligands (1). In PS II, the Fe is conserved and four histidine residues of the apoprotein could constitute its ligands. There is no equivalent in PS II of the glutamate present in the bacterial RC. A number of experiments showed that a bicarbonate ion interacts with the protein in the surounding of Fe and could possibly be a Fe ligand (2). This bicarbonate can be extracted or exchanged by formate; this procedure affects electron transfer between QA and QB (3). In PS II, the Fe can be oxidized by ferricyanide (4). In this study, we aimed to determine and characterize the Fe ligands in PS II by light-induced FTIR difference spectroscopy. Therefore, we monitored the photoreduction of Fe in PS II preparations where it is oxidized by ferricyanide in the dark. We used PS II membranes trypsinized at pH 6.2 where a very large fraction of Fe is accessible to ferricyanide. In the FTIR difference spectrum, we can follow the changes of interaction between Fe and the protein moeity (notably histidine residues) as well as IR signals from the bicarbonate ion.