In photosynthetic complexes, this process involves
the sequential transfer of one electron to a quinone
acceptor, QB, forming a semiquinone. Subsequently,
another electron is transferred in a process that leads
to the transfer of two protons from the solution to
the quinone. In both cases, the electron donor to
QBis another quinone, QA. If the proton transfer pre-
cedes electron transfer, a protonated semiquinone
intermediate state is involved (Figure 5.11). If electron
transfer precedes proton transfer, the intermediate
state is a doubly reduced quinone. If the transfer
is concerted, then only one step takes place and no
intermediate states are formed.
In both of the two-step processes, the overall reac-
tion rate will be limited by either proton or electron
transfer. If proton transfer is first and rate-limiting,
then the overall rate is given by the rate of proton
transfer. However, if proton is rate-limiting but
second, then the overall rate is given by the product
of the proton-transfer rate and the fraction of the
intermediate, namely the doubly reduced quinone.
If electron transfer is first and rate-limiting, then the
overall rate reflects the electron-transfer rate. However, if the electron
transfer is second and rate-limiting, then the rate is proportional to the
product of the electron-transfer rate and the amount of the intermediate,
the protonated quinone. For the concerted transfer, the modeling involves
a proton-dependent electron transfer with no intermediate.
Each of these possible combinations of rate-limiting steps and inter-
mediates predicts a different rate dependence on the pH and free-energy
dependence for the reaction. For the bacterial reaction center, data were
consistent with a rate-limiting electron-transfer step followed by a rapid
proton transfer, although the concerted model was also possible (Graige
et al. 1996). The coupling of proton and electron transfer occurs for many
other biological electron-transfer cofactors. For example, the oxidation of
tyrosine is always coupled with release of the phenolic proton, resulting
in a neutral tyrosyl radical. Even in complex multi-electron enzymatic
reactions, reductive or oxidative steps are usually found to have associ-
ated proton-transfer steps. For example, proton and electron transport are
coupled in cytochrome coxidase, a protein complex involved in respira-
tion (see Chapter 6), and energetic calculations are most consistent with
a proton-transport process that acts in concert with electron transport
and is controlled by a protonatable amino acid residue (Glu-286; Olsson
et al. 2005).
The coupling of electron and proton transfer requires that proteins
have a means to store and donate, or accept, protons as a reaction occurs.
CHAPTER 5 EQUILIBRIA AND REACTIONS INVOLVING PROTONS 109
kH(1)kH(1)kekeQAQBH
QAQB^2
QA keH QA(QBH)
QB
H(1)H(1)Figure 5.11Possible reaction paths for
the coupling of protons to the transfer of
an electron to the quinone QB, which has
previously been reduced to the state QB−.
The electron donor is a reduced quinone,
QA−, and the transfer of the electron may
occur either along the upper path, in which
case proton transfer precedes electron
transfer, or along the lower path, in which
case electron transfer precedes proton
transfer. For a concerted mechanism,
no intermediate is formed and the final
state is formed directly. Modified from
Graige et al. (1996).