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

766 Chapter 14: Energy Conversion: Mitochondria and Chloroplasts
Figure 14–15 The structure of the heme group attached covalently to
cytochrome c. The porphyrin ring of the heme is shown in red. There are six
different cytochromes in the respiratory chain. Because the hemes in different
cytochromes have slightly different structures and are kept in different local
environments by their respective proteins, each has a different affinity for an
electron, and a slightly different spectroscopic signature.
exploited by the membrane protein complexes in the respiratory chain to move
electrons both within and between complexes.
Unlike the colorless atoms H, C, N, and O that constitute the bulk of biological
molecules, transition metal ions are often brightly colored, which makes the proteins
that contain them easy to study by spectroscopic methods using visible light.
One family of such colored proteins, the cytochromes, contains a bound heme
group, in which an iron atom is tightly held by four nitrogen atoms at the corners
of a square in a porphyrin ring (Figure 14–15). Similar porphyrin rings are responsible
both for the red color of blood and for the green color of leaves, binding an
iron in hemoglobin or a magnesium in chlorophyll, respectively.
Iron–sulfur proteins contain a second major family of electron-transfer cofactors.
In this case, either two or four iron atoms are bound to an equal number of
sulfur atoms and to cysteine side chains, forming iron–sulfur clusters in the protein
(Figure 14–16). Like the cytochrome hemes, these clusters carry one electron
at a time.
The simplest of the electron-transfer cofactors in the respiratory chain—and
the only one that is not always bound to a protein—is a quinone (called ubiquinone,
or coenzyme Q). A quinone (Q) is a small hydrophobic molecule that is
freely mobile in the lipid bilayer. This electron carrier can accept or donate either
one or two electrons. Upon reduction (note that reduced quinones are called quinols),
it picks up a proton from water along with each electron (Figure 14–17).
In the mitochondrial electron-transport chain, six different cytochrome
hemes, eight iron–sulfur clusters, three copper atoms, a flavin mononucleotide
(another electron-transfer cofactor), and ubiquinone work in a defined sequence
to carry electrons from NADH to O 2 . In total, this pathway involves more than
60 different polypeptides arranged in three large membrane protein complexes,
each of which binds several of the above electron-carrying cofactors.
As we would expect, the electron-transfer cofactors have increasing affinities
for electrons (higher redox potentials) as the electrons move along the respiratory
chain. The redox potentials have been fine-tuned during evolution by the protein
environment of each cofactor, which alters the cofactor’s normal affinity for electrons.
Because iron–sulfur clusters have a relatively low affinity for electrons, they
predominate in the first half of the respiratory chain; in contrast, the heme cytochromes
predominate further down the chain, where a higher electron affinity is
required.
COOH COOH
CH 2 CH 2
CH 2 CH 2
H 3 C
N + N
CH 3
Fe
H 3 C
N + N
CH 3
H
C
S
CH 3 HC S
CH 3
protein
MBoC6 m14.22/14.15
NADH Transfers Its Electrons to Oxygen Through Three Large
Enzyme Complexes Embedded in the Inner Membrane
Membrane proteins are difficult to purify because they are insoluble in aqueous
solutions, and they are easily disrupted by the detergents that are required to solubilize
them. But by using mild nonionic detergents, such as octylglucoside or
dodecyl maltoside (see Figure 10–28), they can be solubilized and purified in their
native form, and even crystallized for structure determination. Each of the three
different detergent-solubilized respiratory-chain complexes can be re-inserted
Figure 14–16 The structure of an iron–sulfur cluster. These dark brown
clusters consist either of four iron and four sulfur atoms, as shown here, or
of two irons and two sulfurs linked to cysteines in the polypeptide chain via
covalent sulfur bridges, or to histidines. Although they contain several iron atoms,
each iron–sulfur cluster can carry only one electron at a time. Nine different iron–
sulfur clusters participate in electron transport in the respiratory chain.
Cys
S
Fe
S
S
S
Fe
Fe
Fe
S
S
S
Cys
Cys
S
Cys