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component Precision Index (DPI) ÉÂ¢ÄÓêÄ¨«½©Á½B¢B»ðÍÅÍÜ¸¿»ð²»µC»Ì X üñÜ­xðªèµCñÜ­x©çvZ³êé\¢öqÌUðs(x, Bavg)1.0(Ni/p)1/2C |1/3Rdmin(1)ÏªÖÆ·éÅ¬ 2 æ@ðKpµÄ´qÀWð¸§»µC´qÌÀWÆÆàÉèL¦â·xöqðÜÞ\¢p±±Å Bavg ÍS´qÉí½é½ÏÌ·xöqCNi Æ p Í[^Ìë·ð©ÏàéBµ©µÊíÌ^pN¿»ð»ê¼êªèlÆ\¢p[^ÌCC ÍtóÔÉ¨¯ÍÅÍ»ÌñÜªð\ªs\ªÅèCªè³êéñÜ_éñÜ­xªèÌ®S«CR Í R öqCdmin Íªð\ðÌÍ¸§»³êé\¢p[^ÌÌ 2 {öxÉÆ\·B®(1)É]¤ÆC¡ñÌ PSII Ì»\¢ðÍÌÀW ë · Í 0.11 Å Æ v Z ³ ê C ± ê Í PDB À W i ID F
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5.ucñ¾ÖqvÌ`ðµ½ Mn4CaO5NX^[¡ñÌ»\¢ðÍÉæè PSII Ì Mn4Ca NX^[
ÍCMn4CaO5 Ìg¬ð¿ucñ¾ÖqvÌ`ðµÄ¢
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Fig. 2

Structure of photosystem II dimer in a crystallographic
asymmetric unit. Two PSII monomers are related by a non­crystallographic two­fold axis. Water molecules surrounding
PSII dimer were depicted by red spheres.

6  úËõ Jan. 2013 Vol.26 No.1s[NÌ³©ç 4 ÂÌ MniMn1_Mn4jÆ 1 ÂÌ Ca
ð¾ÄÉæÊ·é±ÆªÅ«éBÎFÌdq§xªzÍ 7VO}xÅ`¢½ Fo_Fc ·t[G}ÅC5 ÂÌà®´qðÂÈ® 5 ÂÌIL\Ì_fiO1_O5CÔFjðà ¡ õ¬Ì

Fig. 4

Fig. 3ªðÉ_f­¶@\Ìð¾Éü¯Ä

Hydrogen­bond network relating to the proposal of proton­
coupled electron transfer. Mesh indicates the network from
Mn4CaO5 cluster in left to a bulk solution region out of PSII
in right. Small spheres included in the mesh indicate thewater molecules included in the hydrogen­bond network.

(a) Identication of atoms in Mn4CaO5 cluster, (b) atomic
distances and (c) ligation sphere around Mn4CaO5 cluster.

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8. Mn4Ca NX^[Ì X ü¹âèÀ±û@ðLqµ½Ú 3 ÌÅãÅq×½æ¤ÉC¡
ñÌ X üñÜ­xªèÅÍC900ÌñÜðBe·éÉ
Fig. 5Prediction of water­splitting and oxygen­evolving reaction
mechanism.

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È¨±±Éq×½ S0¨S3 ÜÅÌ½@\ÍCÅßäXÌ

8  úËõ Jan. 2013 Vol.26 No.1

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BL41XU Æ BL38B1 Ìr[CX^btÌûXÉ[­´Ó·éBðà ¡ õ¬Ì

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ÒÐî
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åãs§åw¡æ[¤@\É³ö
E­mail: nkamiyasci.osaka­cu.ac.jpêåF\¢¶¨»wÉX ü»wmªðn
1981 N ¼ Ã ® å w å w @  w ¤  È C¹CwmB1985N©ç»w¤¤õCåC¤õC2005N©çåãs§åwåw@w¤È³öðoÄC
2010Næè»EBìãbTåãs§åw¡æ[¤@\ÉÁCy³öE­mail: keikawasci.osaka­cu.ac.jp
êåFA¨¶wÉõ»wnÜÌ\¢¶¨wmªðn
2010NªRåw©RÈw¤ÈC¹C
wmB2010N©çú{wpU»ïÁÊ¤õðoÄC2011Næè»EB~¼×jåãs§åw¡æ[¤@\ÉÁCy³öE­mail: umenaocarina.osaka­cu.ac.jpêåF`¿ X ü»w
mªðn
2007Nåãåwåw@w¤ÈC¹C
wmB¯N©çåãs§åwåw@w¤Èm¤õC2010N©çåãåw`¿¤m¤õðoÄC2011Næè»EB¾

mªRåwåw@©RÈw¤ÈÉ³ö
E­mail: shencc.okayama­u.ac.jpêåF¶»wÉA¨¶wmªðn
1990Nåwåw@w¤ÈC¹C
wmB»w¤îbÈwÁÊ¤õC¤õCæC¤õðoÄC2003N
æè»EB

úËõ Jan. 2013 Vol.26 No.1  9

Towards elucidation of water­splitting andoxygen­evolving reaction mechanism inphotosysthesis
Nobuo KAMIYAThe OCU Advanced Research Institute for Natural Science and Technology(OCARINA)/Graduate School of Science, Osaka City University,
3_3_138 Sugimoto, Sumiyoshi, Osaka 558_8585, Japan

Keisuke KAWAKAMIThe OCU Advanced Research Institute for Natural Science and Technology(OCARINA), Osaka City University,
3_3_138 Sugimoto, Sumiyoshi, Osaka 558_8585, Japan

Yasufumi UMENAThe OCU Advanced Research Institute for Natural Science and Technology(OCARINA), Osaka City University,
3_3_138 Sugimoto, Sumiyoshi, Osaka 558_8585, Japan

Jian­Ren SHEN

Division of Bioscience, Graduate School of Natural Science and Technology/
Faculty of Science; Okayama University, Okayama 700_8530, Japan

Abstract

Crystal structure of photosystem II (PSII), splitting water molecules and evolving molecular oxyg­
en in photosynthesis, was resolved at a resolution of 1.9 Å. The crystal structures of PSII havebeen reported at 3.8_2.9 Å resolutions, however, structures of Mn4Ca­cluster, the oxygen­evolv­
ing center (OEC), and of ligands around OEC are not clearly resolved due to the limited resolu­
tions. Furthermore, water molecules including substrates of oxygen­evolving reaction and compo­
nents of hydrogen­bond network for proton transfer reaction are not uncovered. X­ray crystallo­graphic study at 1.9 Å resolution revealed the chemical composition of OEC as Mn4CaO5. The
most signicant feature of the Mn4CaO5 cluster is its distorted chair form. The longer distances
between one of the ve oxygen atoms, O5, and metal atoms suggested that O5 may exist as ahydroxide ion in the S1­state of Kok cycle, and may provide one of two substrates for the di­oxy­
gen bond formation. Since two water molecules binding to OEC are within the hydrogen­bond dis­
tances to O5, one of these two water molecules may provide another substrate. An extensivehydrogen­bond network was found between YZ (D1­Tyr161) and the Mn4CaO5 cluster, and from
YZ to a bulk solution region. These new ndings are expected to accelerate researches of naturaland articial photosynthesis.

10  úËõ Jan. 2013 Vol.26 No.1