Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

772 Chapter 14: Energy Conversion: Mitochondria and Chloroplasts
on to oxygen at a special bimetallic center, where it remains clamped between a
heme-linked iron atom and a copper ion until it has picked up a total of four electrons.
Only then are the two oxygen atoms of the oxygen molecule safely released
as two molecules of water (Figure 14–25).
The cytochrome c oxidase reaction accounts for about 90% of the total oxygen
uptake in most cells. This protein complex is therefore crucial for all aerobic life.
Cyanide and azide are extremely toxic because they bind to the heme iron atoms
in cytochrome c oxidase much more tightly than does oxygen, thereby greatly
reducing ATP production.
The Respiratory Chain Forms a Supercomplex in the Crista
Membrane
By using cryoelectron microscopy to examine proteins that have been very gently
isolated, it can be shown that the three protein complexes that form the respiratory
chain assemble into an even larger supercomplex in the crista membrane.
As illustrated in Figure 14–26, this structure is thought to help the mobile electron
carriers ubiquinone (in the crista membrane) and cytochrome c (in the crista
space) transfer electrons with high efficiency. The formation of the supercomplex
depends on the presence of the mitochondrial lipid cardiolipin (see Figure
14–11), which presumably works like a hydrophobic glue that holds the components
together.
In addition to the three proton pumps in the supercomplex just discussed,
one of the enzymes in the citric acid cycle, succinate dehydrogenase, is embedded
in the mitochondrial crista membrane. In the course of oxidizing succinate
to fumarate in the matrix, this enzyme complex captures electrons in the form of
a tightly bound FADH 2 molecule (see Panel 2–9, pp. 106–107) and passes them to
4 electrons entering,
one at a time,
from cytochrome c
CRISTA
SPACE
C
4 H + (4 pumped protons)
e –
e –
electrons donated,
one at a time, from
cytochrome c
copper atoms
e –
protein
side chains
heme a 3
heme a
MATRIX
e –
4
copper
atom
iron atoms
H + 4 H +
O 2
inputs
2 H 2 O
outputs
4 electrons
collected and
O 2 bound here
active site
Figure 14–25 The reaction of O 2 with electrons in cytochrome c oxidase. Electrons from cytochrome c pass through the complex via bound
copper ions (blue spheres) and hemes (red) to an O 2 molecule bound between heme a 3 and a copper ion. Iron ions are shown as red spheres. The
iron atom in heme a serves as an electron queuing point where electrons are held so that they can be released to an O 2 molecule (not shown) that
is held at the bimetallic center active site, which is formed by the central iron of the other heme (heme a 3 ) and a closely apposed copper atom. The
four protons needed to reduce O 2 to water are removed from the matrix. For each O 2 molecule that undergoes the reaction 4e – + 4H + + O 2 → 2H 2 O,
another four protons are pumped out of the matrix by mechanisms that are driven by allosteric changes in protein conformation (see Figure 14–28).
MBoC6 m14.27/14.25