Disassembly and Reassembly of the Manganese Complex of Photosystem II

Light is essential for photosynthetic oxygen evolution not only as a source of energy to oxidize water, but also as a stimulus to activate the system for oxygen evolution. The water oxidizing complex of chloroplast photosystem II (PSII) is located on the lumenal side of the thylakoid membrane, where it performs the decomposition of water into molecular oxygen, protons and electrons. The active site is composed of a Mn4Ca cluster that can accumulate four oxidizing redox equivalents. These are ‘neutralized’ through the oxidation of two water molecules. The precise organization of the Mn4Ca cluster and its association with the PSII polypeptides are still under debate. The present work includes the results of recent experimental studies on disassembly and reassembly of the Manganese complex (Mn complex) in PSII and their biological implications. Combining a variety of biochemical and biophysical methods enabled me to examine in detail the intermediate states of both processes. By employing a trigger event characterized by a stepwise increase in temperature (temperature jump to 47°C) it was possible to initiate a relatively slow and stepwise disassembly of the Mn complex of PSII, involving several distinct intermediates. By combining oxygen polarography, X-ray absorption spectroscopy (XAS) at the Mn K-edge, electron paramagnetic resonance (EPR), atomic absorption spectroscopy (AAS) and recombination chlorophyll fluorescence techniques, it was shown that the heat-induced disassembly process involves at least three steps. (1) The first most rapid step occurs within ~5 min and is coupled to the release of the extrinsic protein with 18 kDa molecular weight from PSII; circumstantial evidence suggested that concomitantly the affinity of the essential Ca ion which is bound at the Mn complex is lowered, leading to the observed inactivation of oxygen evolution. (2) In a second step, two Mn ions are reduced to Mn and released from their binding sites into the bulk within ~15 min; the remaining binuclear complex contains two Mn ions connected by a di-μ-oxo bridge. (3) The third step occurs only within hours and leads to the reduction and liberation of the remaining two Mn ions as Mn whereby apo-PSII with an empty Mn/Ca site is formed. The second part of this work includes the results obtained on the photoassembly process of the Mn4Ca complex. The complex spontaneously is assembled in the light; a process denoted as photoactivation. Recovery of oxygen evolution activity after photoactivation was monitored by O2-polarography and delayed chlorophyll fluorescence measurements, starting from various states as obtained by the previous heat treatment. Oxidation state and