Abstract
The protonation of the singly reduced quinones Q− A and Q− B and the doubly reduced quinone Q2− B is discussed. Q− A and Q− B are not protonated directly in contrast to Q2− B which forms QBH2. Experimental evidence is presented showing that QBH2 leaves the reaction center (RC). It is shown that this process is compatible with the three-dimensional structure of the RC. A model for the protonation involving a chain of protonatable residues connecting QB with the aqueous interface was tested by replacing Glu-212 of the L subunit with the non-protonatable residue Gin. The rate of protonation, deduced from a cytochrome turnover assay, was greatly reduced in the mutant, implicating Glu-212 as a proton donor. Replacing Glu-212 with the protonatable residue Asp did not significantly affect the protonation rate. From the pH dependence of the electron transfer from Q− A to QB the pK of Glu-212 was determined to be 9.5.
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Feher, G., McPherson, P.H., Paddock, M., Rongey, S.H., Schönfeld, M., Okamura, M.Y. (1990). Protonation of Quinones in Reaction Centers from Rb. sphaeroides . In: Baltscheffsky, M. (eds) Current Research in Photosynthesis. Springer, Dordrecht. https://doi.org/10.1007/978-94-009-0511-5_6
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DOI: https://doi.org/10.1007/978-94-009-0511-5_6
Publisher Name: Springer, Dordrecht
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