Energetics of Electron-Transfer and Protonation Reactions of the Quinones in the
Photosynthetic Reaction Center of Rhodopseudomonas viridis
Received August 4, 1997 Revised Manuscript Received December 9, 1997
Abstract: The electron-transfer reactions involving the quinones
in the bacterial photosynthetic reaction
center (bRC) are coupled to a proton uptake by the bRC. In this
study, we calculated the energies of the
different states of the bRC occurring during these electron-transfer
and protonation reactions by an
electrostatic model. We considered the possibility that titratable
groups of the bRC can change their
protonation during these reactions. The protonation probabilities
of titratable groups were obtained by a
Monte Carlo calculation. In contrast to earlier studies by other
groups, we used atomic partial charges
derived from quantum-chemical calculations. Our calculated
reaction energies are in agreement with
experiments. We found that the proton uptake by the bRC is coupled
more strongly to changes of the
redox state of the quinones than to changes of their protonation state.
Thus, the proton uptake by the
bRC occurs predominantly before the protonation of QB.
According to our computations, the reduction
of QB. - to the doubly
negative state QB2- is
energetically even more unfavorable in the bRC than in
solution. Therefore, we suggest that the second electron transfer
from QA to QB occurs after QB
has
received its first proton. We found that the
QA. -QB. -
state is more populated at pH 7.5 than the
QA. -QB.H
state. The low population of the
QA. -QB.H
state may be the reason why the singly protonated QB
could
not be detected spectroscopically. Our calculations imply that the
first protonation of QB. - is
a prerequisite
for the second electron transfer between QA and
QB. Therefore, a pH dependence of the
equilibrium
between the states
QA. -QB. -
and
QA. -QB. H
can also explain the experimentally observed pH dependence
of the rate for the second electron-transfer step. On the basis of
our calculated reaction energies, we
propose the following sequence for the electron-transfer and
protonation reactions: (1) first electron transfer
from QA to QB, (2) first protonation of
QB (at the distal oxygen close to Ser L223), (3) second
electron
transfer from QA to QB, and (4) second
protonation of QB (at the proximal oxygen close to His
L190).
Web Release Date: February 3, 1998