PARENT SESSION
Posters P5A Type II reaction centres : Structure. Abstracts (289-312)

FTIR studies of the water oxidizing complex in Photosystem II. Richard Debus^*,1, Lee Walker¹, Melodie Strickler¹, Warwick Hillier², ¹ Department of Biochemistry, Riverside, CA, USA²

ABSTRACT- In Photosystem II, numerous vibrational modes of amino acid residues change as the (Mn)4 cluster is oxidized through the S state cycle. These residues either ligate the (Mn)4 cluster, are coupled to the cluster through hydrogen bonds, interact electrostatically with the cluster, or have side chains whose protonation states change as the (Mn)4 cluster is oxidized. Identifying the vibrational modes that change during the individual S State transitions is crucial to understanding the mechanism of water oxidation and provides information that complements that obtained from X-ray crystallography. Our efforts involve isotopic labeling and site-directed mutagenesis of the unicellular cyanobacterium, Synechocystis sp. PCC 6803. Our recent efforts have been directed at Asp170, Glu189, His332, Glu333, and Asp342 of the D1 polypeptide, at the C-terminus of the D1 polypeptide at Ala344, and at Glu354 of CP43. The latter residue was identified as a ligand of the (Mn)4 cluster on the basis of a recent X-ray structural model [K. N. Ferreira, T. M. Iverson, K. Maghlaoui, J. Barber, and S. Iwata (2004) Science 303, 1831-1838]. On the basis of our work, we recently obtained evidence that the C-terminal carboxylate group of D1-Ala344 is a unidentate ligand of the Mn ion that undergoes oxidation during the S1 to S2 transition [H.-A. Chu, W. Hiller, and R. J. Debus (2004) Biochemistry 43, 3152-3166].

KEY WORDS: photosystem II, FTIR, water oxidizing complex, manganese