Affinity and activity of non-native quinones at the QB site of bacterial photosynthetic reaction centers |
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Authors: | Xinyu Zhang M. R. Gunner |
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Affiliation: | 1. Physics Department MR-419, City College of New York, 160 Convent Ave., New York, NY, 10031, USA
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Abstract: | Purple, photosynthetic reaction centers from Rhodobacter sphaeroides bacteria use ubiquinone (UQ10) as both primary (QA) and secondary (QB) electron acceptors. Many quinones reconstitute QA function, while a few will act as QB. Nine quinones were tested for their ability to bind and reconstitute QA and QB functions. Only ubiquinone (UQ) reconstitutes both functions in the same protein. The affinities of the non-native quinones for the QB site were determined by a competitive inhibition assay. The affinities of benzoquinones, naphthoquinone (NQ), and 2-methyl-NQ for the QB site are 7 ± 3 times weaker than that at QA site. However, di-ortho-substituted NQs and anthraquinone bind tightly to the QA site (K d ≤ 200 nM), and ≥1,000 times more weakly to the QB site, perhaps setting a limit on the size of the site. With a low-potential electron donor, 2-methyl, 3-dimethylamino-1,4-NQ, (Me-diMeAm-NQ) at QA, QB reduction is 260 meV, more favorable than with UQ as QA. Electron transfer from Me-diMeAm-NQ at the QA site to NQ at the QB site can be detected. In the QB site, the NQ semiquinone is estimated to be ≈60–100 meV higher in energy than the UQ semiquinone, while in the QA site, the semiquinone energy level is similar or lower with NQ than with UQ. Thus, the NQ semiquinone is more stable in the QA than in the QB site. In contrast, the native UQ semiquinone is ≈60 meV lower in energy in the QB than in the QA site, stabilizing forward electron transfer from QA to QB. |
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