Metalloporphyrin Catalysts of Oxygen Reduction - Fuel Cell Catalysis: A Surface Science Approach - page 637

in which Cu B and Fe are bridged by a peroxo (O 22 ) moiety, although this nuclear
configuration lies on the reaction path to the hydroperoxo intermediate. Decay of
the hydroperoxo intermediate to compound P M is calculated to require oxidation of
Tyr244 to the neutral phenoxyl radical and transfer of the terminal O of the hydroper-
oxyl group onto Cu B as hydroxide. The A ! P M conversion is calculated to be close to
thermoneutral (potential energy change between 21 and þ 6 kcal/mol).
This and related computational work suggests the importance of an interaction
between the terminal O atom of the heme-bound O 2 ligand and the distal Cu B and
phenol of Tyr244 (Fig. 18.4c), which is reminiscent of the mechanism of the O - O
bond activation in peroxidases. Just a single H þ is needed to affect the four-electron
reduction, and its primary role appears to be stabilizing the oxygen ligands at the
peroxo level. The P M intermediate is slowly (milliseconds) reduced to the resting
state of the enzyme as electrons are delivered by ferrocytochrome c. This reduction
is coupled to proton pumping.
The major conclusion from the studies of the mechanism of O 2 reduction by CcO is
that formation of a peroxo-level intermediate bridging two metal ions is not a pre-
requisite for four-electron reduction, at least in molecular complexes.
As mentioned earlier, all terminal oxidases perform two bioenergetic roles. One is
to clear the respiratory chain of low potential electrons by oxidizing the least-reducing
respiratory electron carrier and enabling continuous flow of electrons from food to
NAD þ to quinones. The other is to convert part of the free energy of the reduction
of the terminal oxidant into the proton-motive force. The redox potential of cyto-
chrome c (about 250 mV at pH 7; Fig. 18.2) determines the overall potential drop
(about 550 mV) available for the NADH dehydrogenase and cytochrome bc com-
ponents of the respiratory chain (Fig. 18.3). Approximately 450 mV of the 550 mV
difference between the standard redox potentials of the ferri-/ferrocytochrome c
couple and the O 2 /H 2 O couple is captured by CcO as the electrochemical proton
gradient. Two mechanisms are involved (Fig. 18.6). First, the enzyme draws four
protons required for the reduction of O 2 from the basic, negatively charged site of the
membrane (matrix; Fig. 18.3a). The four electrons, on the other hand, come from the
opposite site of the membrane (the intermembrane space, Fig. 18.3a). The annihilation
of these opposite charges at the O 2 reduction site—Reactions (18.3) and (18.4) in
Fig. 18.6—is equivalent to the translocation of four charges across the membrane
against the electrostatic potential. This consumes about 200 mV (the value of the trans-
membrane gradient) of the 550 mV difference between the standard potentials of the
ORR and the oxidation half-reaction—Reaction (18.2) in Fig. 18.6 [Wikstrom, 2004].
CcO expends about another 220 mV by physically moving four protons from the
matrix to the intermembrane space (IMS) of the mitochondrion (Fig. 18.3a), which com-
prises the proton pump. The mechanism of proton pumping is unknown [Brzezinski,
2006; Mills, 2000; Michel, 1999]. As a result, out of about 550 mV of the potential
difference between the electron donor (ferrocytochrome c) and the electron acceptor
(O 2 ), CcO captures about 450 mV (.80%) in a form that can be directly utilized by
the cell to satisfy its energy-dissipating requirements. The energy-transducing efficiency
of quinol oxidases is about 45%. These enzymes utilize a stronger reductant (e.g.,
ubiquinol, about 50 mV) [Ferguson-Miller, 1996] and operate against a lower electro-
chemical gradient (about180 mV) [Alberts et al., 2002].