Bidirectional Electron Transfer in Photosystem I:  Accumulation of A0- in A-Side or B-Side Mutants of the Axial Ligand to Chlorophyll A0

Ramesh, V. M.; Gibasiewicz, Krzysztof; Lin, Su; Bingham, Scott E.; Webber, Andrew N.

doi:10.1021/bi0354177

Cited by 85 publications

(75 citation statements)

References 28 publications

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“…Experiments at room temperature using longer delays permitted observation of primary charge separation and secondary ET. Both YF mutants exhibited trapping times (Ϸ25 ps) and DAS (data not shown) indistinguishable from those of the WT (8), indicating that the rates of energy transfer and initial charge separation are not strongly affected. Thus, neither mutation produced detectable long-range perturbations of core antenna Chls, nor did they inhibit primary charge separation within the RC.…”

Section: Resultsmentioning

confidence: 92%

“…In both WT and PsaA-Y696F PS1, the main feature is the P 700 ϩ bleaching band (Fig. 6A); there is no significant accumulation of a negative band at Ϸ680 nm, attributed to the A 0 Ϫ intermediate Chl (8,22). However, there are indications of such a band in the PsaB-Y676F mutant.…”

Section: Resultsmentioning

confidence: 93%

“…Cells were grown in Tris-acetate-phosphate (TAP) medium (37) at 25°C. Thylakoid membranes were purified by using sucrose gradients (38), and PS1 particles were prepared by anion exchange chromatography (8).…”

Section: Methodsmentioning

confidence: 99%

“…1). Site-directed mutations of the Met serving as axial ligand to each ec3 Chl have been described in Chlamydomonas (6)(7)(8) and Synechocystis (9,10). Ultrafast pump-probe data from the C. reinhardtii mutants indicate a slowing or blockage of ET from the affected ec3 Chl in mutants of both PsaA-Met-688 § § and PsaB-Met-668 (8); along with pulsed EPR data from the same mutants (7,12), these data implied the use of both branches.…”

mentioning

confidence: 98%

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Directing electron transfer within Photosystem I by breaking H-bonds in the cofactor branches

Est

Lucas

et al. 2006

Proc. Natl. Acad. Sci. U.S.A.

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101

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Photosystem I has two branches of cofactors down which lightdriven electron transfer (ET) could potentially proceed, each consisting of a pair of chlorophylls (Chls) and a phylloquinone (PhQ). Forward ET from PhQ to the next ET cofactor (F X) is described by two kinetic components with decay times of Ϸ20 and Ϸ200 ns, which have been proposed to represent ET from PhQ B and PhQA, respectively. Immediately preceding each quinone is a Chl (ec3), which receives a H-bond from a nearby tyrosine. To decrease the reduction potential of each of these Chls, and thus modify the relative yield of ET within the targeted branch, this H-bond was removed by conversion of each Tyr to Phe in the green alga Chlamydomonas reinhardtii. Together, transient optical absorption spectroscopy performed in vivo and transient electron paramagnetic resonance data from thylakoid membranes showed that the mutations affect the relative amplitudes, but not the lifetimes, of the two kinetic components representing ET from PhQ to F X. The mutation near ec3 A increases the fraction of the faster component at the expense of the slower component, with the opposite effect seen in the ec3B mutant. We interpret this result as a decrease in the relative use of the targeted branch. This finding suggests that in Photosystem I, unlike type II reaction centers, the relative efficiency of the two branches is extremely sensitive to the energetics of the embedded redox cofactors.Chlamydomonas ͉ directionality ͉ photosynthetic reaction center ͉ pump-probe spectroscopy ͉ transient EPR P hotosynthetic reaction centers (RCs) are the membrane proteins responsible for the capture and storage of light energy in photosynthetic organisms. As with all of the RCs, Photosystem I (PS1) has a C2 symmetrical structure with two virtually identical branches of redox cofactors extending across the membrane (see Fig. 1). The x-ray crystal structure of PS1 from Thermosynechococcus elongatus (1) has allowed identification of amino acid residues interacting with the electron transfer (ET) cofactors, permitting the use of site-directed mutagenesis to investigate to what extent ET occurs in the two branches. Many of these studies (2-10) point toward the possibility that ET takes place in both branches of cofactors (A-branch and Bbranch; see Fig. 1). The data consistently show that the major fraction occurs in the A-branch and that the slow phase of ET from phylloquinone (PhQ) to F x is associated with this branch. The involvement of the B-branch is less clear, but there is mounting evidence to support the assignment of the fast component of PhQ to F x ET to this branch. If ET does indeed occur in both branches in PS1, this behavior would be remarkably different from the type II RCs. In purple bacterial RCs, for example, it is well known that initial ET is biased almost exclusively toward the A-branch, probably because the B-branch quinone is a mobile electron and proton carrier in this type of RC. The strong biasing of the ET in the type II RCs makes it difficult to investigate the fac...

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Section: Resultsmentioning

confidence: 92%

Section: Resultsmentioning

confidence: 93%

Section: Methodsmentioning

confidence: 99%

mentioning

confidence: 98%

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Directing electron transfer within Photosystem I by breaking H-bonds in the cofactor branches

Est

Lucas

et al. 2006

Proc. Natl. Acad. Sci. U.S.A.

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101

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“…An intermediate value of 25.0 Å was obtained in the WT membranes, which was offered as evidence for bidirectional electron transfer [26]. Ultrafast transient absorbance measurements showed that the A 0 -minus A 0 difference signal accumulates in both the M688H PsaA and M668H PsaB mutants, indicating that electron transfer from A 0 Á -to A 1 is blocked or slowed on the appropriate branch [30]. However, a recent spin-polarized EPR study shows a transient EPR spectrum, probably due to P 700 Á ?…”

Section: Discussionmentioning

confidence: 88%

Alteration of the Axial Met Ligand to Electron Acceptor A0 in Photosystem I: Effect on the Generation of P 700 ·+ A 1 ·− Radical Pairs as Studied by W-band Transient EPR

et al. 2009

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Photosystem I (PS I) mutants from the cyanobacterium Synechocystis sp. PCC 6803 bearing point mutations to the axial ligands of A 0A (M688N PsaA ) and A 0B (M668N PsaB ) were studied by high-field W-band electron paramagnetic resonance (EPR) spectroscopy. It was found that the EPR observables of PS I from the M668N PsaB mutant were virtual identical to that of the wild type (WT), and are clearly distinct from the M688N PsaA mutant. In particular, the P 700 Á ? decay kinetics in the M688N PsaA mutant is significantly slower than in the WT or the M668N PsaB mutant. The analysis of the out-of-phase electron-electron dipolar electron spin echo envelope modulation shows that in the M668N PsaB mutant, the estimated distance of 26.0 ± 0.3 Å agrees well with the 25.8 Å distance for the P 700 Á ? A 1A Á -radical pair measured in the X-ray crystal structure. In the M688N PsaA mutant, two populations are found with estimated distances of 26.0 ± 0.3 and 25.0 ± 0.3 Å in a ratio of 0.7-0.3, which agree well with the 25.8 Å distance for the P 700 Á ? A 1A Á -radical

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Metal ions in cyanobacterial photosystem I

Biesiadka

Loll

2004

Handbook of Metalloproteins

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Photosystems I (PSI) and photosystem II (PSII) are large multi‐subunit protein–cofactor complexes located in the photosynthetic thylakoid membrane and acting in series. They are fueled by sunlight, PSII oxidizing water to atmospheric O 2 , H + , and electrons that are delivered at the lumenal side of the membrane. The electrons are transferred to PSI where their reducing potential is increased sufficiently to finally produce reduced nicotinamide adenine dinucleotide phosphate (NADPH) at oxidized form of nicotinamide adenine dinucleotide phosphate (NADP + ) reductase while the H + form a gradient across the membrane that drives adenosine triphosphate (ATP‐)synthetase and both, NADPH and ATP, convert CO 2 to carbohydrates in the Calvin cycle. PSI is composed of 12 different protein subunits of which 9 are embedded in the thylakoid membrane and 3 are located on the cytoplasmic (stromal) side of the membrane. Sunlight is absorbed by a large internal antenna composed of 90 chlorophyll a (Chl a ), and the photons are guided to the electron transfer chain (ETC) formed by two symmetry‐related branches where the ‘special pair P 700′ located close to the lumenal side of the membrane is excited and expels an electron to form the cation radical P700 + . The electron is guided from P700 to the stromal side along the ETC consisting of pairs of accessory Chl a , of the primary electron acceptors (Chl a ), of the secondary acceptors phylloquinone (PQ), to the terminal acceptors formed by three Fe 4 S 4 clusters F X , F A , F B . Since each Chl a coordinates one Mg 2+ , PSI harbors a plethora of Mg and Fe cations and one structural Ca 2+ . We describe the coordination of these cations and their function in the photosynthetic process carried out by PSI.

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Bidirectional Electron Transfer in Photosystem I: Accumulation of A₀^- in A-Side or B-Side Mutants of the Axial Ligand to Chlorophyll A₀

Cited by 85 publications

References 28 publications

Directing electron transfer within Photosystem I by breaking H-bonds in the cofactor branches

Directing electron transfer within Photosystem I by breaking H-bonds in the cofactor branches

Alteration of the Axial Met Ligand to Electron Acceptor A0 in Photosystem I: Effect on the Generation of P 700 ·+ A 1 ·− Radical Pairs as Studied by W-band Transient EPR

Metal ions in cyanobacterial photosystem I

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