title: Exploring the electron transfer pathways in photosystem I by high-time-resolution electron paramagnetic resonance: observation of the B-side radical pair P700(+)A1B(-) in whole cells of the deuterated green alga Chlamydomonas reinhardtii at cryogenic temperatures.
abstract: Crystallographic models of photosystem I (PS I) highlight a symmetrical arrangement of the electron transfer cofactors which are organized in two parallel branches (A, B) relative to a pseudo-C2 symmetry axis that is perpendicular to the membrane plane. Here, we explore the electron transfer pathways of PS I in whole cells of the deuterated green alga Chlamydomonas reinhardtii using high-time-resolution electron paramagnetic resonance (EPR) at cryogenic temperatures. Particular emphasis is given to quantum oscillations detectable in the tertiary radical pairs P700(+)A1A(-) and P700(+)A1B(-) of the electron transfer chain. Results are presented first for the deuterated site-directed mutant PsaA-M684H in which electron transfer beyond the primary electron acceptor A0A on the PsaA branch of electron transfer is impaired. Analysis of the quantum oscillations, observed in a two-dimensional Q-band (34 GHz) EPR experiment, provides the geometry of the B-side radical pair. The orientation of the g tensor of P700(+) in an external reference system is adapted from a time-resolved multifrequency EPR study of deuterated and 15N-substituted cyanobacteria (Link, G.; Berthold, T.; Bechtold, M.; Weidner, J.-U.; Ohmes, E.; Tang, J.; Poluektov, O.; Utschig, L.; Schlesselman, S. L.; Thurnauer, M. C.; Kothe, G. J. Am. Chem. Soc. 2001, 123, 4211-4222). Thus, we obtain the three-dimensional structure of the B-side radical pair following photoexcitation of PS I in its native membrane. The new structure describes the position and orientation of the reduced B-side quinone A1B(-) on a nanosecond time scale after light-induced charge separation. Furthermore, we present results for deuterated wild-type cells of C. reinhardtii demonstrating that both radical pairs P700(+)A1A(-) and P700(+)A1B(-) participate in the electron transfer process according to a mole ratio of 0.71/0.29 in favor of P700(+)A1A(-). A detailed comparison reveals different orientations of A1A(-) and A1B(-) in their respective binding sites such that formation of a strong hydrogen bond from A1(-) to the protein backbone is possible only in the case of A1A(-). We suggest that this is relevant to the rates of forward electron transfer from A1A(-) or A1B(-) to the iron-sulfur center F(X), which differ by a factor of 10. Thus, the present study sheds new light on the orientation of the phylloquinone acceptors in their binding pockets in PS I and the effect this has on function.
authors.affiliation: Department of Physical Chemistry, University of Freiburg, Freiburg, Germany.
authors.forename: Thomas
authors.initials: T
authors.lastname: Berthold
authors.forename: Erika Donner
authors.initials: ED
authors.lastname: von Gromoff
authors.forename: Stefano
authors.initials: S
authors.lastname: Santabarbara
authors.forename: Patricia
authors.initials: P
authors.lastname: Stehle
authors.forename: Gerhard
authors.initials: G
authors.lastname: Link
authors.forename: Oleg G
authors.initials: OG
authors.lastname: Poluektov
authors.forename: Peter
authors.initials: P
authors.lastname: Heathcote
authors.forename: Christoph F
authors.initials: CF
authors.lastname: Beck
authors.forename: Marion C
authors.initials: MC
authors.lastname: Thurnauer
authors.forename: Gerd
authors.initials: G
authors.lastname: Kothe
mesh_terms: D016825:Chlamydomonas reinhardtii / Q000737:chemistry* / Q000166:cytology; D003080:Cold Temperature; D003903:Deuterium / Q000737:chemistry; D004578:Electron Spin Resonance Spectroscopy; D004579:Electron Transport; D008958:Models, Molecular; D045331:Photosystem I Protein Complex / Q000737:chemistry*; D010940:Plant Proteins / Q000737:chemistry*
chemical_list: D045331:Photosystem I Protein Complex; D010940:Plant Proteins; D003903:Deuterium