Essential Cell Biology 5th edition

474 CHAPTER 14 Energy Generation in Mitochondria and Chloroplasts
Figure 14–23 The iron in a heme group
can serve as an electron acceptor.
(A) Ribbon structure showing the position
of the heme group (red ) associated with
cytochrome c (green). (B) The porphyrin ring
of the heme group (light red ) is attached
covalently to side chains in the protein.
The heme groups of different cytochromes
have different electron affinities because
they differ slightly in structure and are held
in different local environments within each
protein.
H 3 C
H
H 3 C C
S
COOH COOH
CH 2 CH 2
CH 2 CH 2
CH 3
N + N
Fe
N + N
CH 3
CH 3 HC S
CH 3
(A)
(B)
cytochrome c
in the NADH dehydrogenase complex, for example, passes electrons to
ubiquinone. Later in the pathway, iron atoms that are held in the heme
groups bound to cytochrome proteins are commonly used as electron
carriers (Figure 14–23). These heme groups give cytochromes, such as
the cytochrome c reductase and cytochrome c oxidase complexes, their
color (“cytochrome” ECB5 e14.25/14.25
from the Greek chroma, “color”). Like other electron
carriers, the cytochrome proteins increase in redox potential the further
down the mitochondrial electron-transport chain they are located. For
example, cytochrome c, a small protein that accepts electrons from the
cytochrome c reductase complex and transfers them to the cytochrome c
oxidase complex, has a redox potential of +230 mV—a value about midway
between those of the cytochromes with which it interacts (see Figure
14–22).
QUESTION 14–7
Two different diffusible electron
carriers, ubiquinone and cytochrome
c, shuttle electrons between the
three protein complexes of the
electron-transport chain. Could the
same diffusible carrier, in principle,
be used for both steps? Explain your
answer.
Cytochrome c Oxidase Catalyzes the Reduction of
Molecular Oxygen
Cytochrome c oxidase, the final electron carrier in the respiratory chain,
has the highest redox potential of all. This protein complex removes
electrons from cytochrome c, thereby oxidizing it—hence the name
“cytochrome c oxidase.” The exceptionally high electron affinity stems in
part from a special oxygen-binding site within cytochrome c oxidase that
contains a heme group plus a copper atom (Figure 14–24). It is here that
nearly all the oxygen we breathe is consumed, when the electrons that
had been donated by NADH at the start of the electron-transport chain
are handed off to O 2 to produce H 2 O.
In total, four electrons donated by cytochrome c and four protons
extracted from the aqueous environment are added to each O 2 molecule
in the reaction 4e – + 4H + + O 2 → 2H 2 O. In addition to the protons
that combine with O 2 , four other protons are pumped across the membrane
during the transfer of the four electrons from cytochrome c to O 2 .
This pumping occurs because the transfer of electrons drives allosteric
changes in the conformation of cytochrome c oxidase that cause protons
to be ejected from the mitochondrial matrix (Figure 14−25).
Oxygen is useful as an electron sink because of its very high affinity for
electrons. However, once O 2 picks up one electron, it forms the superoxide
radical O 2 – ; this radical is dangerously reactive and will avidly take up
another three electrons wherever it can find them, a tendency that can
cause serious damage to nearby DNA, proteins, and lipid membranes.