Effect of Isotope Substitution and Controlled Dehydration on the Photoinduced Electron Transport Reactions of Quinone Acceptors and Multiheme Cytochrome c in Bacterial Photosynthetic Reaction Center

S. K. Chamorovsky, P. M. Krasil'nikov, and P. P. Knox^*

School of Biology, Lomonosov Moscow State University, Moscow, 119992 Russia; fax: (095) 939-1115; E-mail: serch@bioh.phys.msu.su; shaitan@biophys.msu.ru; knox@biophys.msu.ru

^* To whom correspondence should be addressed.

Received November 20, 2001

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Isotope substitution of H₂O by ²H₂O causes an increase in the rate of dark recombination between photooxidized bacteriochlorophyll (P⁺) and reduced primary quinone acceptor in Rhodobacter sphaeroides reaction centers (RC) at room temperature. The isotopic effect declines upon decreasing the temperature. Dehydration of RC complexes of Ectothiorhodospira shaposhnikovii chromatophores containing multiheme cytochrome c causes a decrease in the efficiency of transfer of a photomobilized electron between the primary and secondary quinone acceptors and from cytochrome to P⁺. In the case of H₂O medium these effects are observed at a lower hydration than in ²H₂O-containing medium. In the E. shaposhnikovii chromatophores subjected to dehydration in H₂O, the rate of electron transfer from the nearest high-potential cytochrome heme to P⁺ is virtually independent of hydration within the P/P₀ range from 0.1 to 0.5. In samples hydrated in ²H₂O this rate is approximately 1.5 times lower than in H₂O. However, the isotopic effect of this reaction disappears upon dehydration. The intramolecular electron transfer between two high-potential hemes of cytochrome c in samples with ²H₂O is inhibited within this range of P/P₀, whereas in RC samples with H₂O there is a trend toward gradual inhibition of the interheme electron transfer with dehydration. The experimental results are discussed in terms of the effects of isotope substitution and dehydration on relaxation processes and charge state of RC on implementation of the reactive states of RC providing electron transfer control.

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KEY WORDS: purple bacteria, photosynthetic reaction center, cytochromes, electron transport, deuteration, dehydration

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