Mutations in Both Sides of the Photosystem I Reaction Center Identify the Phylloquinone Observed by Electron Paramagnetic Resonance Spectroscopy

Boudreaux, Brent; MacMillan, Fraser; Teutloff, Christian; Agalarov, Rufat; Gu, Feifei; Grimaldi, Stéphane; Bittl, Robert; Brettel, Klaus; Redding, Kevin

doi:10.1074/jbc.m102327200

Cited by 69 publications

(85 citation statements)

References 49 publications

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“…Again, the wild-type and the Trp 677 PsaB and Ser 672 PsaB mutants yield the same spectra. This agrees with an assessment that those electron transfer steps detected by EPR spectroscopy in the eukaryotic organism C. reinhardtii also involve cofactors associated with the PsaA side (15,22). It is more difficult to assess from EPR data whether any electron transfer occurs on the cofactors associated with the PsaB side, and the accompanying paper (24) will address the biphasic kinetics of electron transfer in the point mutants in detail.…”

Section: Discussionmentioning

confidence: 49%

Electron Transfer in Cyanobacterial Photosystem I

Chitnis

Valieva

et al. 2003

Journal of Biological Chemistry

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The Photosystem I (PS I) reaction center contains two branches of nearly symmetric cofactors bound to the PsaA and PsaB heterodimer. From the x-ray crystal structure it is known that Trp A 1؊ radical pair show that none of the mutations causes a significant change in the orientation of the measured phylloquinone. Pulsed ENDOR spectra reveal that the W697F PsaA mutation leads to about a 5% increase in the hyperfine coupling of the methyl group on the phylloquinone ring, whereas the S692C PsaA mutation causes a similar decrease in this coupling. The changes in the methyl hyperfine coupling are also reflected in the transient EPR spectra of P 700 ؉ A 1 ؊ and the CW EPR spectra of photoaccumulated A 1 ؊ . We conclude that: (i) the transient EPR spectra at room temperature are predominantly from radical pairs in the PsaA branch of cofactors; (ii) at low temperature the electron cycle involving P 700 and A 1 similarly occurs along the PsaA branch of cofactors; and (iii) mutation of amino acids in close contact with the PsaA side quinone leads to changes in the spin density distribution of the reduced quinone observed by EPR. Photosynthetic reaction centers (RCs)1 are classified into two general types depending on the identity and function of the terminal electron acceptors. Those RCs that incorporate ironsulfur clusters are classified as "Type I," and those that incorporate a mobile (secondary) quinone are classified as "Type II." Type I RCs include Photosystem I (PS I) of cyanobacteria and plants and those in heliobacteria and green sulfur bacteria. Type II RCs include Photosystem II of cyanobacteria and plants and those in green non-sulfur bacteria and purple bacteria. Despite the difference in the identity of the terminal electron acceptors, Type I and Type II RCs share a common motif in terms of polypeptide arrangement and cofactor composition (1).The primary cofactors are bound to proteins that are present as dimers in the membrane. This results in a set of electron transfer cofactors that are arranged (pseudo)symmetrically (2). In PS I, these cofactors include a special pair of chlorophyll a/aЈ molecules as the primary donor, two bridging chlorophyll a molecules, and two chlorophyll a molecules, at least one of which functions as the primary acceptor (3). In the purple bacterial reaction center, the cofactors include a special pair of bacteriochlorophyll a molecules as the primary donor, two bridging bacteriochlorophyll a molecules, and two pheophytin molecules, one of which functions as the primary acceptor (4). PS I and the purple bacterial reaction center contain two quinones; in the latter, one quinone is rather immobile (Q A ) and the other is mobile (Q B ), whereas in PS I both quinones (Q K -A and Q K -B) are, to the best of our knowledge, immobile in their normal function.In Type II reaction centers, a single turnover results in the reduction of Q B , to a semiquinone and a second turnover results in the further reduction (and protonation) of Q B to a hydroquinone. The stability of Q B Ϫ requires that t...

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Section: Discussionmentioning

confidence: 49%

Electron Transfer in Cyanobacterial Photosystem I

Chitnis

Valieva

et al. 2003

Journal of Biological Chemistry

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“…PCC 6803 (this work), and this is the same quinone that is observed by EPR in photoaccumulation experiments (29) and in low temperature spin-polarized EPR and electron nuclear double resonance experiments (2,34).…”

Section: Discussionmentioning

confidence: 57%

Electron Transfer in Cyanobacterial Photosystem I

Chitnis

Valieva

et al. 2003

Journal of Biological Chemistry

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102

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The directionality of electron transfer in Photosystem I (PS I) is investigated using site-directed mutations in the phylloquinone (Q K ) and F X binding regions of Synnechocystis sp. PCC 6803. The kinetics of forward electron transfer from the secondary acceptor A 1 (phylloquinone) were measured in mutants using time-resolved optical difference spectroscopy and transient EPR spectroscopy. In whole cells and PS I complexes of the wildtype both techniques reveal a major, slow kinetic component of Ϸ 300 ns while optical data resolve an additional minor kinetic component of Ϸ 10 ns. Whole cells and PS I complexes from the W697F PsaA and S692C PsaA mutants show a significant slowing of the slow kinetic component, whereas the W677F PsaB and S672C PsaB mutants show a less significant slowing of the fast kinetic component. Transient EPR measurements at 260 K show that the slow phase is ϳ3 times slower than at room temperature. Simulations of the early time behavior of the spin polarization pattern of P 700 ؉ A 1 ؊ , in which the decay rate of the pattern is assumed to be negligibly small, reproduce the observed EPR spectra at 260 K during the first 100 ns following laser excitation. Thus any spin polarization from P 700 ؉ F X ؊ in this time window is very weak. From this it is concluded that the relative amplitude of the fast phase is negligible at 260 K or its rate is much less temperature-dependent than that of the slow component. Together, the results demonstrate that the slow kinetic phase results from electron transfer from Q K -A to F X and that this accounts for at least 70% of the electrons. Although the assignment of the fast kinetic phase remains uncertain, it is not strongly temperature dependent and it represents a minor fraction of the electrons being transferred. All of the results point toward asymmetry in electron transfer, and indicate that forward transfer in cyanobacterial PS I is predominantly along the PsaA branch.

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“…In the crystal structure, the inter-planar distances between the indole ring and the quinone ring vary between 3.0 and 3.5 Å (1, 67) or 3.7 Å in ESEEM studies (72). These tryptophans were point-mutated to Phe, His, or Leu, and the changes of kinetics in the ET process were investigated by UV-visible absorption spectroscopy (7) and EPR studies (8,9,73).…”

Section: Role Of Tryptophans Near the A 1 Binding Sites-trp-a697mentioning

confidence: 99%