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

770 Chapter 14: Energy Conversion: Mitochondria and Chloroplasts
electrons out to
cytochrome c
heme c
Rieske protein
e – e –
CRISTA
SPACE
cytochrome
c 1
MATRIX
iron–sulfur
cluster
heme b L
(A)
heme b H
electrons in from
ubiquinol (QH 2 )
(C)
cytochrome b
(B)
Figure 14–22 The structure of cytochrome c reductase. Cytochrome c reductase (also known as the cytochrome b-c 1
complex) is a dimer of two identical 240,000-dalton halves, each composed of 11 different protein molecules in mammals.
(A) A structure graphic of the entire dimer, showing in color the three proteins that form the functional core of the enzyme
complex: cytochrome b (green) and cytochrome c 1 (blue) are colored in one half, and the Rieske protein (purple) containing an
Fe 2 S 2 iron–sulfur cluster (red and yellow) is colored in the other. These three protein subunits interact across the two halves.
(B) Transfer of electrons through cytochrome c reductase to the small, soluble carrier protein cytochrome c. Electrons entering
from ubiquinol near the matrix side of the membrane are captured by the iron–sulfur cluster of the Rieske protein, which moves
its iron–sulfur group back and forth to transfer these electrons to heme c (red). Heme c then transfers them to the carrier
molecule cytochrome c. MBoC6 n14.308/14.22
As detailed in Figure 14–23, only one of the two electrons from each ubiquinol is transferred through this path. To increase
proton pumping, the second ubiquinol electron is passed to a molecule of ubiquinone bound to cytochrome c reductase on the
opposite side of the membrane—near the matrix. (C) This shows the symbol for cytochrome c reductase used throughout this
chapter. (PDB code: 1EZV.)
redox loop mechanism used is called the Q cycle because while one of the electrons
received from each QH 2 molecule is transferred from ubiquinone through
the complex to the carrier protein cytochrome c, the other electron is recycled
back into the quinone pool. Through the mechanism illustrated in Figure 14–23,
the Q cycle increases the total amount of redox energy that can be stored in the
electrochemical proton gradient. As a result, two protons are pumped across the
crista membrane for every electron that is transferred from NADH dehydrogenase
to cytochrome c.
The Cytochrome c Oxidase Complex Pumps Protons and
Reduces O 2 Using a Catalytic Iron–Copper Center
The final link in the mitochondrial electron-transport chain is cytochrome c oxidase.
The cytochrome c oxidase complex accepts electrons from the soluble electron
carrier cytochrome c, and it uses yet a different, third mechanism to pump
protons across the inner mitochondrial membrane. The structure of the mammalian
complex is illustrated in Figure 14–24. The atomic-resolution structures,
combined with studies of the effect of mutations introduced into the enzyme by
genetic engineering of the yeast and bacterial proteins, have revealed the detailed
mechanisms of this electron-driven proton pump.