Kinetic properties of the acceptor quinone complex in Rhodopseudomonas viridis

Abstract

The kinetics of electron transfer from the primary (QA) to the secondary (QB) quinone acceptor in whole cells and chromatophores of Rhodopseudomonas viridis was studied as a function of the redox state of QB and of pH by using a photovoltage technique. Under conditions where QB was oxidized, the reoxidation of QA- was found to be essentially monophasic and independent of pH with a half-time of about 20 microseconds. When QB was reduced to the semiquinone form by a preflash, the reoxidation of QA- was slowed down showing a half-time between 40 and 80 microseconds at pH less than or equal to 9. Above pH 9, the rate of the second electron transfer decreased nearly one order of magnitude per pH unit. After a further preflash, the fast and pH-independent kinetics of QA- reoxidation was essentially restored. The concentration of QA still reduced 100 microseconds after its complete reduction by a flash showed distinct binary oscillations as a function of the number of preflashes, confirming the interpretation that the electron-transfer rate depends on the redox state of QB. After addition of o-phenanthroline, the reoxidation of QA- is slowed down to the time range of seconds as expected for a back-reaction with oxidized cytochrome. Under conditions where inhibitors of the electron transfer between the quinones fail to block this reaction in a fraction of the reaction centers due to the presence of the extremely stable and strongly bound semiquinone, QB-, these reaction centers show a slow electron transfer on the first flash and a fast one on the second, i.e., an out-of-phase oscillation.(ABSTRACT TRUNCATED AT 250 WORDS)

Proton transfer in the photosynthetic reaction center of Blastochloris viridis

“…Taking into account that the transfer of the first electron to Q B takes ∼20 μs in Bl. viridis [7], i.e. is somewhat faster than the components of voltage generation revealed here (cf.…”

“…viridis . Another striking finding is that the 5 μs component of voltage generation is apparently faster than the reported time constant of ET from to after the second flash (∼50 μs [7,8]). One possibility is that the 5 μs component reflects a terbutryn‐sensitive charge displacement at the Q B ‐site in response to formation.…”

“…In Bl. viridis, however, the voltage generation with τ of 400 μs [9] has seemed to be slower than the corresponding ET with τ of 50 μs [7,8]. Here we revisited the problem of voltage generation in Bl.…”

“…The reduction of by the next electron – e.g. after the second flash of light – yields a ubiquinol Q B H 2 [6–8]. By using capacitive potentiometry, Dracheva and co‐workers followed the electrogenic proton transfer (PT) that accompanied the reduction of Q B to Q B H 2 in the RCs of Bl.…”

Calculated coupling of electron and proton transfer in the photosynthetic reaction center of Rhodopseudomonas viridis

“…See text for details. Parson, 1975;Leibl and Breton, 1991;Mathis et al, 1992), resulting in the state DQAQB-. After a second photochemical cytochrome oxidation in the third step, the diradical state DQA-QB-is formed.…”