Recruitment of a foreign qiunone into the a(1) site of photosystem I: Altered kinetics of electron transfer in phylloquinone biosynthetic pathway mutants studied by time-resolved optical, EPR, and electrometric techniques

Alexey Yu Semenov, Ilya R. Vassiliev, Art Van Der Est, Mahir D. Mamedov, Boris Zybailov, Gaozhong Shen, Dietmar Stehlik, Bruce A. Diner, Parag R. Chitnis, John H. Golbeck

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Interruption of the menA or menB gene in Synechocystis sp. PCC 6803 results in the incorporation of a foreign quinone, termed Q, into the A1 site of photosystem I with a number of experimental indicators identifying Q as plastoquinone-9. A global multiexponential analysis of time-resolved optical spectra in the blue region shows the following three kinetic components: 1) a 3-ms lifetime in the absence of methyl viologen that represents charge recombination between P700+ and an FeS- cluster; 2) a 750-μs lifetime that represents electron donation from an FeS- cluster to methyl viologen; and 3) an ~15-μs lifetime that represents an electrochromic shift of a carotenoid pigment. Room temperature direct detection transient EPR studies of forward electron transfer show a spectrum of P700+ Q- during the lifetime of the spin polarization and give no evidence of a significant population of P700+ FeS- for t ≤ 2-3 μs. The UV difference spectrum measured 5 μs after a flash shows a maximum at 315 nm, a crossover at 286 nm, and a minimum at 255 nm as well as a shoulder at 290-295 nm, all of which are characteristic of the plastoquinone-9 anion radical. Kinetic measurements that monitor Q at 315 nm show a major phase of forward electron transfer to the FeS clusters with a lifetime of ~15 μs, which matches the electrochromic shift at 485 nm of the carotenoid, as well as an minor phase with a lifetime of ~250 μs. Electrometric measurements show similar biphasic kinetics. The slower kinetic please can be detected using time-resolved EPR spectroscopy and has a spectrum characteristic of a semiquinone anion radical. We estimate the redox potential of plastoquinone-9 in the A1 site to be more oxidizing than phylloquinone so that electron transfer from Q- to F(x) is thermodynamically unfavorable in the menA and menB mutants.

Original languageEnglish (US)
Pages (from-to)23429-23438
Number of pages10
JournalJournal of Biological Chemistry
Issue number31
Publication statusPublished - Aug 4 2000


All Science Journal Classification (ASJC) codes

  • Biochemistry
  • Molecular Biology
  • Cell Biology

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