Analysis of the Interaction of Water with the Manganese Cluster of Photosystem II Using Isotopically Labeled Water

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• 作者：Sandra Turconi ; Dugald J. MacLachlan ; Peter J. Bratt ; Jonathan H. A. Nugent ; and Michael C. W. Evans
• 刊名：Biochemistry
• 出版年：1997
• 出版时间：January 28, 1997
• 年：1997
• 卷：36
• 期：4
• 页码：879 - 885
• 全文大小：240K
• 年卷期：v.36,no.4(January 28, 1997)
• ISSN：1520-4995

文摘

The association of water with the Mn of the wateroxidizing complex was investigated usingH₂¹⁷O- and²H₂O-reconstituted lyophilized photosystem IIparticles. The pulsed electron paramagneticresonance (EPR) technique of electron spin echo envelope modulation(ESEEM) was used to investigatethe interaction of the magnetic ²H and ¹⁷Onuclei with the paramagnetic S₂ state of the Mn complexandother photosystem II components. ESEEM offers a much more specificand sensitive detection of thistype of interaction than continuous wave (CW) EPR. Unlike earlierreports using CW EPR, theseexperiments did not detect any interaction of water with the multilineEPR signal from the S₂ state of theMn complex. No signals indicating specific interaction of either Hor O with the multiline signal weredetected. Signals due to ²H and ¹⁷O weredetected only at the Larmour frequency, indicatingnonspecific"distant ENDOR" effects. A weak interaction with¹⁷O was detected both in S₁, when the Mn isEPRsilent, and in S₂, but only on the high-field side ofg = 2. This interaction may be with theRieskeiron-sulfur center in the cytochromeb₆f complex. The results werethe same whether the multiline signalwas generated by 200 K illumination of dark-frozen samples, or by roomtemperature illumination in thepresence of the inhibitor 3-(3,4-dichlorophenyl)-1,1-dimethylurea(DCMU). Illumination at roomtemperature in the presence of an electron acceptor to allow multipleturnovers of the system with cyclingof the S states did not result in the appearance of any newinteractions. These results appear to excludeclose (less than 6 Å) binding of water to the Mn center giving rise tothe multiline signal, and also toexclude mechanisms in which water oxidation involves the breaking andre-formation of the µ-oxo bridgesof the Mn complex. They cannot, however, exclude models in whichwater binding to the manganesecomplex and direct oxidation by the manganese complex occur in thehigher S states, or are catalyzed byone bis(µ-oxo) Mn dimer while oxidizing equivalents areaccumulated in the S₂ state by a secondbis(µ-oxo) Mn dimer.