Specific degradation of the D1 protein in photosystem II

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1) Various types of derivatives of phloroglucinol were synthesized in order to investigate the structure/function relationship on the D1 protein breakage activity. For the activity, OH-group at positions 2,4,6 on phenol ring is essential. Derivatives without a bulky hydrophobic side chain at position 3 showed very weak activity, indicating that the bulky group is also required for the activity. A derivative (G63) that has not nitro group at position 3 but has substituted group at position 6 has a unique activity that breaks the D1 protein to produce the same fragments as those found by photoinhibitory illumination. 2) Illumination of Ca-depleted PS II membranes sample retaining an abnormal S_2-state generated two EPR signals at g =2 region. One signal was ascribed to dipole interaction between a radical pair, and the other signal was ascribed to interaction between a radical and the Mn-cluster. A new high-spin EPR signal was detected when the latter signal was produced. 3) Spin-exchange structures of the S_2-state Mn-cluster were numerically evaluated. The calculation was performed by taking into account the magnetic properties of the S_2 multiline signal and the effective hyperfine constants determined by simulating oriented multiline spectrum. 4) Binding of Mn^<2+> to Mn-depleted PS II and electron donation from the bound Mn^<2+> to an oxidized Y_Z tyrosine were studied, and found that Mn-depleted membranes have only one unique binding site which has high affinity and high specificity for Mn^<2+>, and that Mn^<2+> bound to this site can be deliver an electron to Y_Z^+ with high efficiency.