PARENT SESSION

Symposium S5A Type II reaction centres: Structure
Wednesday September 1st, 2004 10:20 AM-12:20 PM Room 210A
Chair: Dave Britt
Co-Chair: Rick Debus

Mn₄Ca cluster of the water-oxidation enzyme studied by polarized X-RAY spectroscopy of PS II single crystals. Junko Yano^{1, 3}, Jan Kern², Kenneth Sauer^{1, 3}, Johannes Messinger⁴, Athina Zouni², Vittal Yachandra^*,1, ¹ Lawrence Berkeley National Laboratory, Berkeley, USA³ University of California at Berkeley, Berkeley, USA² Technische Universitat Berlin, Berlin, Germany⁴ Max-Planck Institut fur Bioanorganische Chemie, Mulheim, Germany

ABSTRACT- Oriented Photosystem II (PS II) membranes and single crystals of PS II have been studied by polarized X-ray spectroscopy (X-ray dichroism). Both the K-edge X-ray absorption spectra and the extended X-ray absorption fine structure (EXAFS) exhibit pronounced dichroism, depending on sample orientation relative to the direction of the electric vector of the polarized X-ray beam. Analysis of the EXAFS dichroism provides information concerning the orientation of vectors joining specific pairs of atoms in the Mn₄Ca cluster relative to the orientation axes. Both Mn-Mn and Mn-Ca vectors have been investigated. Single crystals of PS II isolated from thermophilic cyanobacteria have been studied by X-ray diffraction, and analyses at resolutions between 3.5 and 3.8 have been published. These analyses have localized electron density associated with the Mn₄Ca cluster within the large complex of PS II proteins, but the limited resolution is short of what is needed to place individual metal atoms precisely in the cluster. Previous EXAFS studies of spinach PS II have identified Mn-O(N), Mn-Mn and Mn-Ca vectors and have provided a basis for the formulation of structural models consistent with the data. The evidence supports the presence of three Mn-Mn vectors (di--oxo bridged) at 2.7-2.8 , one or two Mn-Mn vectors at 3.3 and one or two Mn-Ca vectors at 3.4 . New studies using oriented single crystals of PS II from cyanobacteria show pronounced dichroism in both the 2.7 and 3.3 peaks depending on crystal orientation relative to the electric vector of the polarized X-rays. Analysis of the dichroism is complicated by the crystal symmetry; each of the four PS II dimers per crystallographic unit contains two Mn₄Ca clusters related by a non-crystallographic C₂ axis. Nevertheless, the dichroism of Fourier peak II (Mn-Mn vectors at ∼2.7 ) shows clear evidence for the presence of at least two Mn-Mn distances. Fourier peak III (Mn-Mn and Mn-Ca vectors at ∼3.3 ) shows very pronounced dichroism. We have used the X-ray dichroism to evaluate each of the three predictions of the Mn cluster geometry based on the published analyses of X-ray diffraction. Models for the Mn₄Ca complex consistent with the dichroism of the single-crystal EXAFS data will be presented.

KEY WORDS: Mn Complex, PS II Single Crystals, Oxygen evolution, X-ray Spectroscopy