| Abstract: | In oxygenic photosynthesis, water is oxidized at a protein-cofactor complex comprising four Mn atoms and, presumably, one calcium. Using multilayers of Photosystem II membrane particles, we investigated the time course of the disassembly of the Mn complex initiated by a temperature jump from 25°C to 47°C and terminated by rapid cooling after distinct heating periods. We monitored polarographically the oxygen-evolution activity, the amount of the YDox radical and of released Mn2+ by EPR spectroscopy, and the structure of the Mn complex by x-ray absorption spectroscopy (XAS, EXAFS). Using a novel approach to analyze time-resolved EXAFS data, we identify three distinct phases of the disassembly process: (1) Loss of the oxygen-evolution activity and reduction of YDox occur simultaneously (k1 = 1.0 min−1). EXAFS spectra reveal the concomitant loss of an absorber-backscatterer interaction between heavy atoms separated by ~3.3 Å, possibly related to Ca release. (2) Subsequently, two Mn(III) or Mn(IV) ions seemingly separated by ~2.7 Å in the native complex are reduced to Mn(II) and released (k2 = 0.18 min−1). The x-ray absorption spectroscopy data is highly suggestive that the two unreleased Mn ions form a di-μ-oxo bridged Mn(III)2 complex. (3) Finally, the tightly-bound Mn2(μ-O)2 unit is slowly reduced and released (k3 = 0.014 min−1). |
