PARENT SESSION

Symposium S2A Type I reaction centres
Monday August 30th, 2004 2:40 PM-4:40 PM Room 511D
Chair: John Golbeck
Co-Chair: Kevin Redding

Control of function by protein-cofactor interaction in the electron transfer chain of Photosystem I. Dietmar Stehlik^*,1, Yulia Pushkar¹, Irina Karyagina¹, Sarah Brown², Art van der Est², John Golbeck³, ¹ Freie Universität Berlin, Berlin, Germany² Brock University, Ontario, Canada³ Penn State University, PA, United States

ABSTRACT- The 3-dimensional structure of cyanobacterial PS I (1BJ0) at 2.5 resolution initiated a series of structure- and mutant-based studies aiming at a molecular understanding of biological function. Although directionality of electron transfer (ET) along the two branches of the pseudo-C₂-symmetric PS I core structure is an unsolved issue, a common result has emerged: under physiological conditions, the dominant ET pathway proceeds along the A-branch and is studied by TR EPR selectively. The functionally relevant charge-separated intermediate P₇₀₀A_n radicalion pair (RP) states (A_n stands for the series of sequential electron acceptors in PS I) is observed directly. The correlated photoaccumulated A_n radical anion is studied alternatively. Recent results, with a variety of multi-frequency and multiple-resonance TR EPR techniques applied to a wide range of specifically modified PS I centers, are reported to back up some key conclusions concerning the influence of specific protein cofactor interactions in type I and II RC: (a) In the functional A₁-A site of PS I only one quinone C-O group is strongly H-bonded by a specific NH backbone group. The proton hyperfine tensor in the H-bond is directly measured. Evidence comes also from the asymmetric spin density distribution over the quinone ring, now measured at all relevant ring positions. The PS I case of only one dominant H-bond for A₁-A provides a proper starting point for a quantitative analysis, also for the more complicated yet more extensively characterized Q_A site in type II RC with two strong but asymmetric H-bonds. (b)-stacking of quinone with a specific tryptophan residue is stronger in PS I than type II RC. (c) Quinone replacement is used to shift the redox potential in the A₁ site. The more negative redox potential of anthraquinone (AQ) versus native phylloquinone (PhyQ) leads to correlated kinetic changes. A₁ to F_x ET speeds up. The preceding A₀ to A₁ step slows down. Simultaneous observation of the kinetics of two consecutive ET steps is detected by TR EPR. Similar effects due to modified A₀ to A₁ ET kinetics have been observed for mutants of the A₀ ligand methionin. (d) A_n redox potential contributions due to electrostatic interaction with specific residues are predicted in recent numerical studies. Point mutants have allowed experimental checks.

KEY WORDS: Photosystem I, protein-cofactor interaction, Electron transfer, EPR