Gene/Protein Disease Symptom Drug Enzyme Compound
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Query: UMLS:C1832526 (PCC)
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Reaction centers of photosystem I contain three different [4Fe-4S] clusters named FX, FA, and FB. The terminal photosystem I acceptors (FA, FB) are distributed asymmetrically along the membrane normal, with one of them (FA or FB) being reduced from FX and the other one (FB or FA) reducing soluble ferredoxin. In the present work, kinetics of electron transfer has been measured in PSI from the cyanobacterium Synechocystis sp. PCC 6803 after inactivation of FB by treatment with HgCl2. Photovoltage measurements indicate that, in the absence of FB, reduction of FA by FX is still faster than the rate of FX reduction [(210 ns)-1]. Flash-absorption measurements show that the affinity of ferredoxin for HgCl2-treated PSI is only decreased by a factor of 3-4 compared to untreated photosystem I. The first-order rate of ferredoxin reduction by FA-, within the photosystem I/ferredoxin complex, has been calculated from measurements of P700+ decay. Compared to control PSI, this rate is several orders of magnitude smaller (6 s-1 versus 10(4)-10(6) s-1). Moreover, it is smaller than the rate of recombination from FA-, resulting in inefficient ferredoxin reduction (yield of 25%). After reconstitution of FB, about half of the reconstituted photosystem I reaction centers recover fast reduction of ferredoxin with kinetics similar to that of untreated photosystem I. These results support FB as the direct partner of ferredoxin and as the more distal cluster of photosystem I with respect to the thylakoid membrane, in accordance with a linear electron-transfer pathway FX-->FA-->FB-->ferredoxin.
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PMID:Electron transfer in photosystem I reaction centers follows a linear pathway in which iron-sulfur cluster FB is the immediate electron donor to soluble ferredoxin. 952 64

The PsaC subunit of photosystem I (PS I) binds two [4Fe-4S] clusters, F(A) and F(B), functioning as electron carriers between F(X) and soluble ferredoxin. To resolve the issue whether F(A) or F(B) is proximal to F(X), we used single-turnover flashes to promote step-by-step electron transfer between electron carriers in control (both F(A) and F(B) present) and HgCl2-treated (F(B)-less) PS I complexes from Synechococcus sp. PCC 6301 and analyzed the kinetics of P700+ reduction by monitoring the absorbance changes at 832 nm in the presence of a fast electron donor (phenazine methosulfate (PMS)). In control PS I complexes exogenously added ferredoxin, or flavodoxin could be photoreduced on each flash, thus allowing P700+ to be reduced from PMS. In F(B)-less complexes, both in the presence and in the absence of ferredoxin or flavodoxin, P700+ was reduced from PMS only on the first flash and was reduced from F(X)- on the following flashes, indicating lack of electron transfer to ferredoxin or flavodoxin. In the F(B)-less complexes, a normal level of P700 photooxidation was detected accompanied by a high yield of charge recombination between P700+ and F(A)- in the presence of a slow donor, 2,6-dichlorophenol-indophenol. This recombination remained the only pathway of F(A)- reoxidation in the presence of added ferredoxin, consistent with the lack of forward electron transfer. F(A)- could be reoxidized by methyl viologen in F(B)-less PS I complexes, although at a concentration two orders of magnitude higher than is required in wild-type PS I complexes, thus implying the presence of a diffusion barrier. The inhibition of electron transfer to ferredoxin and flavodoxin was completely reversed after reconstituting the F(B) cluster. Using rate versus distance estimates for electron transfer rates from F(X) to ferredoxin for two possible orientations of PsaC, we conclude that the kinetic data are best compatible with PsaC being oriented with F(A) as the cluster proximal to F(X) and F(B) as the distal cluster that donates electrons to ferredoxin.
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PMID:PsaC subunit of photosystem I is oriented with iron-sulfur cluster F(B) as the immediate electron donor to ferredoxin and flavodoxin. 954 61

Photovoltage responses accompanying electron transfer on the acceptor side of photosystem I (PS I) were investigated in proteoliposomes containing PS I complexes from the cyanobacterium Synechococcus sp. PCC 6301 using a direct electrometrical technique. The relative contributions of the F(X) --> F(B) and the F(X) --> F(A) electron transfer reactions to the overall electrogenicity were elucidated by comparing the sodium dithionite-induced decrease in the magnitude of the total photoelectric responses in control and in F(B)-less (HgCl2-treated) PS I complexes. The results obtained suggest that the electrogenesis on the acceptor side of PS I is related to electron transfers between both F(X) and F(A) and F(A) and F(B). Based on the electrogenic nature of the latter reaction in PS I complexes, we conclude that F(A) rather than F(B) is the acceptor proximal to F(X).
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PMID:Electrogenicity accompanies photoreduction of the iron-sulfur clusters F(A) and F(B) in photosystem I. 970 6

An electrometrical technique was used to investigate electron transfer between the terminal iron-sulfur centers F(A)/F(B) and external electron acceptors in photosystem I (PS I) complexes from the cyanobacterium Synechococcus sp. PCC 6301 and from spinach. The increase of the relative contribution of the slow components of the membrane potential decay kinetics in the presence of both native (ferredoxin, flavodoxin) and artificial (methyl viologen) electron acceptors indicate the effective interaction between the terminal 14Fe-4S] cluster and acceptors. The finding that FA fails to donate electrons to flavodoxin in F(B)-less (HgCl2-treated) PS I complexes suggests that F(B) is the direct electron donor to flavodoxin. The lack of additional electrogenicity under conditions of effective electron transfer from the F(B) redox center to soluble acceptors indicates that this reaction is electrically silent.
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PMID:Electrometrical study of electron transfer from the terminal FA/FB iron-sulfur clusters to external acceptors in photosystem I. 1062 38