1 research outputs found
Exploring the Electron Transfer Pathways in Photosystem I by High-Time-Resolution Electron Paramagnetic Resonance: Observation of the B-Side Radical Pair P<sub>700</sub><sup>+</sup>A<sub>1B</sub><sup>–</sup> in Whole Cells of the Deuterated Green Alga Chlamydomonas reinhardtii at Cryogenic Temperatures
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-<i>C</i><sub>2</sub> 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
P<sub>700</sub><sup>+</sup>A<sub>1A</sub><sup>–</sup> and P<sub>700</sub><sup>+</sup>A<sub>1B</sub><sup>–</sup> 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 A<sub>0A</sub> 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 <b>g</b> tensor of P<sub>700</sub><sup>+</sup> in an external reference system is adapted from a time-resolved
multifrequency EPR study of deuterated and <sup>15</sup>N-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. <i>J. Am. Chem. Soc.</i> <b>2001</b>, <i>123</i>, 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 A<sub>1B</sub><sup>–</sup> 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
P<sub>700</sub><sup>+</sup>A<sub>1A</sub><sup>–</sup> and P<sub>700</sub><sup>+</sup>A<sub>1B</sub><sup>–</sup> participate
in the electron transfer process according to a mole ratio of 0.71/0.29
in favor of P<sub>700</sub><sup>+</sup>A<sub>1A</sub><sup>–</sup>. A detailed comparison reveals different orientations of A<sub>1A</sub><sup>–</sup> and A<sub>1B</sub><sup>–</sup> in their
respective binding sites such that formation of a strong hydrogen
bond from A<sub>1</sub><sup>–</sup> to the protein backbone is possible only in the case of A<sub>1A</sub><sup>–</sup>. We suggest
that this is relevant to the rates of forward electron transfer from
A<sub>1A</sub><sup>–</sup> or
A<sub>1B</sub><sup>–</sup> to
the iron–sulfur center F<sub>X</sub>, 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