Abstract Details

What is the Role of Ca in Photosynthetic Water Oxidation: Polarized X-ray Absorption Spectroscopy of the Ca-depleted Oxygen Evolving Complex of Photosystem II
Abstract ID	BIO-09
Presenter	Thomas  Lohmiller
Presentation Type	Poster
Full Author List	T. F. Lohmiller (1) , X. Long (1) , Y. Pushkar (1) , K. Sauer (1,2) , J. Yano (1) , V. K. Yachandra (1)
Affiliations	(1) Physical Biosciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA, 94720 (2) Dept. of Chemistry, University of California, Berkeley, CA 94720
Category	Bio/Life Sciences
Abstract	An integral part of oxygenic photosynthesis is the photo-induced oxidation of water, which takes place at the membrane protein complex, photosystem II (PS II). Light energy absorbed by chlorophylls is used by the oxygen-evolving complex (OEC), a protein-bound cluster containing four Mn atoms and one Ca atom (Mn4Ca) connected by mono-m-oxo, di-m-oxo and/or hydroxo bridges [1], to catalyze the splitting of H2O into dioxygen, protons and reduction equivalents. During the catalytic reaction cycle, the complex undergoes successive transitions through five different redox states S0 to S4 before the spontaneous return to S0 from the transient state S4, resulting in the release of O2 [2]. The presence of the Ca2+ ion in the OEC is essential for the evolution of oxygen, possibly acting as a Lewis acid responsible for the deprotonation of H2O [4]. Despite its inability to complete the reaction cycle, Ca-depleted PS II in S1' state can be advanced to two other states S2' and S3' upon illumination, which show distinct EPR-signals [4, 5]. Knowledge about structural differences among the three consecutive Ca-depleted states as well as comparison to corresponding native states might help explain why the Ca-depleted OEC, which in contrast to the native form does not seem to be oxidized upon the S2' the S3' transition, cannot complete the catalytic cycle. X-ray absorption spectroscopy (XAS) requires a considerably lower x-ray dose than x-ray diffraction studies, during which the geometry of the metal site is disrupted by radiation damage [6]; enabling a detailed analysis of the OEC. Polarized EXAFS on PS II single crystals has led to a set of three possible similar high-resolution structures [7] of the Mn4Ca cluster. The role of the Ca cofactor has been investigated using Mn [8], Ca [9] and Sr EXAFS [10] on Ca-depleted, native and Sr-substituted PS II preparations. In this work, Ca-depleted and Ca-reconstituted PS II samples were prepared by treatment at pH 3 [11] as one-dimensionally ordered membrane layers for polarized XAS [12]. They were poised in the S1', S2' and S3' states and the native S1 state, respectively, and characterized according to their enzymatic oxygen evolution activity and distinct EPR signals. Polarized XAS was used to obtain a more detailed picture of the geometry of the Ca-depleted OEC, including angular information for internuclear vectors, and of the structural impact of the removal of the Ca2+ ion, compared to studies on solution samples [8]. Mn K-edge XANES and EXAFS spectra, collected at angles of 10° and 80° between the membrane normal of the samples and the E-vector of the incident x-ray beam, exhibit pronounced dichroism. Fits of the polarized EXAFS data provide numbers and distances for neighboring atoms of the four Mn atoms combined with angular information. The structural changes on Ca removal and the implications for the water oxidation reaction will be presented in the poster.
Footnotes	1. T. Wydrzynski, S. Satoh, Eds., Photosystem II: The Light-Driven Water:Plastoquinone Oxidoreductase (Springer, Dordrecht, Netherlands, 2005). 2. B. Kok, B. Forbush, M. McGloin, Photochem. Photobiol. 11, 457 (1970). 3. C. F. Yocum, Coord. Chem. Rev. 252, 296 (2008). 4. A. Boussac, J.-L. Zimmermann, A. W. Rutherford, Biochemistry 28, 8984 (1989). 5. M. Sivaraja, J. Tso, G. C. Dismukes, Biochemistry 28, 9459 (1989). 6. J. Yano et al., Proc. Natl. Acad. Sci. USA 102, 12047 (2005). 7. J. Yano et al., Science 314, 821 (2006). 8. M. J. Latimer, V. J. DeRose, V. K. Yachandra, K. Sauer, M. P. Klein, J. Phys. Chem. B 102, 8257 (1998). 9. R. M. Cinco et al., Biochemistry 41, 12928 (2002). 10. R. M. Cinco et al., Biochemistry 43, 13271 (2004). 11. T. Ono, Y. Inoue, FEBS Letters 227, 147 (1988). 12. A. W. Rutherford, Biochim. Biophys. Acta 807, 189 (1985).
Funding Acknowledgement	This work was supported by the NIH Grant GM 55302, and by the Director, Office of Science, Office of Basic Energy Sciences (OBES), Division of Chemical Sciences, Geosciences, and Biosciences of the Department of Energy (DOE) under Contract DE-AC02-05CH11231. This work was performed at SSRL, which is funded by the DOE Office of Basic Energy Science. The SSRL SMB Program is supported by the NIH National Center for Research Resources, Biomedical Technology Program and by the DOE Office of Biological and Environmental Research.