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 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
Crystal structure of cyanobacterial photosystem II at 3.2  resolution. Jan Kern*,1, Jacek Biesiadka2, Bernhard Loll2, Athina Zouni1, Klaus-Dieter Irrgang1, Wolfram Saenger2, 1 Max-Volmer-Laboratories for Biophysical Chemistry, Berlin, Germany2 Institute of Crystallography, Free University of Berlin, Berlin, Germany
ABSTRACT- Dimeric Photosystem II (PSII) was prepared from Thermosynechococcus elongatus. On the basis of these samples crystals which are fully active in water oxidation were recently obtained. They diffract in the best direction to 2.9 resolution leading to a 3D structure at 3.2 resolution. Several loop regions of the principal protein subunits are now defined that were not interpretable at lower 3.8 resolution, resulting in a more complete model. The head groups of the cofactors of the electron transfer chain and of the antennae have been modeled, coordinating and hydrogen bonding amino acids identified and the nature of the binding pockets derived. The orientations of these cofactors resemble those of the reaction center from anoxygenic purple bacteria, but differences in hydrogen bonding and protein environment modulate their properties and provide the unique high redox potential of the primary donor P680 of PSII. The protein environment around the manganese cluster, the redox active TyrZ and the non-haem Fe2+ have been described. One carotene could be identified with certainty in our electron density, connecting cyt b559, ChlzD2 and ChlD2. We could place four Mn-ions in a 3+1 like arrangement in the density attributed to the Mn-cluster. We can not exclude that one of the four Mn-ions in our model is a Ca2+, and interpretation in terms of a possible Mn-Ca heteronuclear cluster is possible and supported by our diffraction data measured at and beyond the Mn-edge.
KEY WORDS: structure, photosystem II, Mn-cluster, Thermosynechococcus elongatus
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