Temporary Stabilization of Electron on Quinone Acceptor Side of Reaction Centers from the Bacterium Rhodobacter sphaeroides Wild Type and Mutant SA(L223) Depending on Duration of Light Activation

P. P. Knox^1*, N. I. Zakharova¹, N. H. Seifullina¹, I. Yu. Churbanova¹, M. D. Mamedov², and A. Yu. Semenov²

¹Department of Biophysics, Faculty of Biology, Lomonosov Moscow State University, Moscow 119899, Russia; fax: (7-095) 939-1115; E-mail: knox@biophys.msu.ru.

²Belozersky Institute of Physico-Chemical Biology, Lomonosov Moscow State University, Moscow 119992, Russia; fax: (7-095) 939-3188; E-mail: semenov@genebee.msu.su

^* To whom correspondence should be addressed.

Received October 20, 2003; Revision received November 19, 2003

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The dark reduction of photooxidized bacteriochlorophyll (P⁺) by photoreduced secondary quinone acceptor (Q_B^-) in isolated reaction centers (RC) from the bacterium Rhodobacter sphaeroides wild type and mutant strain SA(L223) depending on the duration of light activation of RC was studied. The kinetics of the dark reduction of P⁺ decreased with increasing light duration, which is probably due to conformational changes occurring under prolonged light activation in RC from the wild type bacterium. In RC from bacteria of the mutant strain in which protonatable amino acid Ser L223 near Q_B is substituted by Ala, the dependence of reduction kinetics of P⁺ on duration of light was not observed. Such dependence, however, became observable after addition of cryoprotectors, namely glycerol and dimethylsulfoxide, to the RC samples from the mutant strain. It was concluded that substitution of Ser L223 with Ala disturbs the native mechanism of electrostatic stabilization of the electron in the RC quinone acceptor site. At the same time, an additional modification of RC hydrogen bonds by glycerol and dimethylsulfoxide probably includes various possibilities for more effective time delay of the electron on Q_B.

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KEY WORDS: purple bacteria, photosynthetic reaction center, quinone acceptors, electron transport

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