Observation of the Reduction and Reoxidation of the Primary Electron Acceptor in Photosystem I

Hastings, Gary; Kleinherenbrink, Frank A.M.; Lin, Su; McHugh, Thomas J.; Blankenship, Robert E.

doi:10.1021/bi00177a008

Cited by 106 publications

(180 citation statements)

References 42 publications

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“…However, if one posits that this event were biased to a unique branch to produce a specific P 700 ϩ A 0 Ϫ state, then one must explain the reduction of the phylloquinone on the other branch. A 1 is reduced faster than the resolution time of our instrument (5 ns), consistent with previous reports for A 0 reoxidation with a half time of a few tens of picoseconds (28). Electron transfer between the two phylloquinones, based on the distance between them (22.3 Å; ref.…”

Section: Discussionsupporting

confidence: 92%

Evidence for two active branches for electron transfer in photosystem I

Guergova-Kuras

Boudreaux

Joliot

et al. 2001

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

327

288

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All photosynthetic reaction centers share a common structural theme. Two related, integral membrane polypeptides sequester electron transfer cofactors into two quasi-symmetrical branches, each of which incorporates a quinone. In type II reaction centers [photosystem (PS) II and proteobacterial reaction centers], electron transfer proceeds down only one of the branches, and the mobile quinone on the other branch is used as a terminal acceptor. PS I uses iron-sulfur clusters as terminal acceptors, and the quinone serves only as an intermediary in electron transfer. Much effort has been devoted to understanding the unidirectionality of electron transport in type II reaction centers, and it was widely thought that PS I would share this feature. We have tested this idea by examining in vivo kinetics of electron transfer from the quinone in mutant PS I reaction centers. This transfer is associated with two kinetic components, and we show that mutation of a residue near the quinone in one branch specifically affects the faster component, while the corresponding mutation in the other branch specifically affects the slower component. We conclude that both electron transfer branches in PS I are active.

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Section: Discussionsupporting

confidence: 92%

Evidence for two active branches for electron transfer in photosystem I

Guergova-Kuras

Boudreaux

Joliot

et al. 2001

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

327

288

View full text Add to dashboard Cite

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“…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%

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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“…(2) Flash illumination with a xenon flash (4 Ixs/20 mJ). The transmission of a 2 l.ts probe beam was monitored immediately before and 45 ms after the actinic flash using a photodiode array detector as described [10,11]. The intensity of the actinic flash was varied by neutral density filters and the extent of the flash-induced photoconversion estimated from the series of light minus dark difference spectra as described previously [9].…”

Section: Piis0014-5793(96)01249-5mentioning

confidence: 99%

The light intensity dependence of protochlorophyllide photoconversion and its significance to the catalytic mechanism of protochlorophyllide reductase

1996

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The light-dependent step in chlorophyll synthesis by higher plants involves hydrogen transfer from NADPH ÷ to the porphyrin protochlorophyllide catalysed by the photoenzyme protochlorophylllde rednctase. The light intensity dependence of the process has been studied in vitro using wheat etioplast membranes. The results suggest that a single photochemical event is involved in the photoconversion. In support of this conclusion we also demonstrate that illumination of these membranes with a train of ultrashort (150 fs) flashes resulted in chlorophyll accumulation. The significance of the findings in terms of possible mechanisms for the reductase are discussed.Key words: Protochlorophyllide reductase; Light intensity dependence; Photoenzyme mechanism sion. The ultra-fast rate of the process [4-6] suggests chlorophyll formation is possible from a single photoreaction whereas Litvin and co-workers maintain two successive [7] or parallel [8] photoreactions in the process.In the current paper, we have studied the light dependence of protochlorophyllide photoconversion in isolated wheat etioplast membranes. The data on analysis suggest a single photoreaction in the process. It has also been demonstrated that such membranes are capable of accumulation of low, but finite levels of chlorophyllide when illuminated with a train of ultra-short femtosecond flashes. The significance of the data is discussed in terms of possible mechanisms of photoconversion.

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Observation of the Reduction and Reoxidation of the Primary Electron Acceptor in Photosystem I

Cited by 106 publications

References 42 publications

Evidence for two active branches for electron transfer in photosystem I

Evidence for two active branches for electron transfer in photosystem I

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

The light intensity dependence of protochlorophyllide photoconversion and its significance to the catalytic mechanism of protochlorophyllide reductase

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