Electron Transfer in Cyanobacterial Photosystem I

Xu, Wu; Chitnis, Parag R.; Valieva, Alfia I.; Est, Art van der; Brettel, Klaus; Guergova-Kuras, Mariana; Pushkar, Yulia; Zech, Stephan G.; Stehlik, D.; Shen, Gaozhong; Zybailov, Boris; Golbeck, John H.

doi:10.1074/jbc.m302965200

Cited by 102 publications

(54 citation statements)

References 34 publications

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“…The absorption changes associated with reoxidation of PhQ are described by two exponential decay components (18,19). The fact that point mutations near PhQ A or PhQ B resulted in changes in the rates of either the slower ( Ϸ 200 ns) or faster ( Ϸ 20 ns) components, respectively (3,5), are most easily interpreted in terms of a bidirectional model, in which ET occurs with a given probability in either the A-or B-branch, resulting in reduction of PhQ A or PhQ B , respectively. This model predicts that the amplitudes of the two components are determined by the relative use of each branch, and the observation that mutations in neither PhQ-binding site observably changed these amplitudes argues in favor of this interpretation.…”

Section: Resultsmentioning

confidence: 99%

“…The two mutations would have to shift the equilibrium without noticeably changing either PhQ reoxidation rate. Conversely, mutations of the Trps adjacent to the PhQs, PsaA-Trp-697 and PsaB-Trp-677, would have to affect the rate constants in such a way that only one of the observed rates is altered without changing the relative amplitudes of the two phases (3,5).…”

Section: Discussionmentioning

confidence: 99%

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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: 99%

Section: Discussionmentioning

confidence: 99%

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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“…The spectra are assigned to P 700 ϩ A 1 Ϫ (E/A/E pattern) and P 700 ϩ FeS Ϫ (emissive spectrum) and have been extracted from the time/field data sets by fitting the individual transients. The parameters used in the fitting procedure are given in the accompanying paper (24 quinone bound to PsaA (Q K -A) contributes to these spectra, whereas the phylloquinone bound to PsaB (Q K -B) does not. Because the observed changes in the properties of the quinone are small, it is reasonable to extrapolate from this result to the conclusion that the electron cycle between P 700 and A 1 measured by EPR at low temperature occurs along the PsaA branch.…”

Section: Discussionmentioning

confidence: 99%

“…Therefore, it is important to compare time-resolved optical and EPR results for the same mutant samples and try to interpret the results in a unified manner. In the accompanying paper (24) we present such data and discuss the kinetics of electron transfer in greater detail.…”

Section: Discussionmentioning

confidence: 99%

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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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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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Electron Transfer in Cyanobacterial Photosystem I

Cited by 102 publications

References 34 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

Electron Transfer in Cyanobacterial Photosystem I

Metal ions in cyanobacterial photosystem I

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