Chemistry Reference
In-Depth Information
FIGURE 5.18
The mitochondrial electron-transport chain. (From
Voet & Voet, 2004
:
pp. 1591.)
D
E
0
o
(more of these haem iron proteins in Chapter 13) to coenzyme Q. However, here the
is only 0.085 V,
corresponding to a
D
G
0
of
e
16.4 kJ/mol, which is not sufficient to allow proton pumping.
Complex III (Coenzyme Q: Cytochrome c Oxidoreductase) transfers electrons from coenzyme Q to cyto-
chrome c, through a sequence of cytochromes and iron
D
E
0
o
e
sulfur cofactors. Here, the
for the couple CoQ/
D
G
0
of
cytochrome c is 0.19 V, corresponding to a
36.7 kJ/mol, again enough to power the synthesis of an ATP
molecule and to ensure that protons are pumped across the inner mitochondrial membrane.
Finally, Complex IV, Cytochrome c Oxidase, takes the electrons coming from four molecules of cytochrome c,
a small, water-soluble haem protein, moving outside of the membrane in the intermembrane space and carries out
the four-electron reduction of a molecule of dioxygen to two molecules of water. The
e
D
E
0
o
for the couple cyto-
D
G
0
of
chrome c/O
2
is by far the highest of the four complexes, 0.58 V, corresponding to a
112 kJ/mol, and
there is certainly proton pumping, which must involve conformational changes since, in this Complex, unlike the
three others, there are only one-electron cytochromes and copper atoms, with no obvious proton exchanges
possible (more information about cytochrome c oxidase in Chapters 13 and 14).
We now turn our attention to how the gradient of protons pumped by Complexes I, III, and IV across the
inner mitochondrial membrane into the intermembrane space, together with the associated membrane potential,
is used to turn the molecular rotor which ensures ATP synthesis. Without entering into the detail, we can
calculate that the
D
G for pumping a proton from the mitochondrial matrix to the intermembrane space is
21.5 kJ/mol. Since the estimated
D
G (the real
in vivo free energy) for synthesis of an ATP molecule is between
þ
50 kJ/mol, we can estimate that at least two protons (most likely three) need to be pumped per ATP
generated. From experimental data we know that two electrons descending the respiratory chain from NADH
(i.e., via Complexes I, III, and IV) to oxygen will produce 3 ATP molecules. By comparison two electrons
entering via FADH
2
and passing through Complexes II, III, and IV to oxygen will result in formation of only 2
ATP molecules.
If, during electron transfer along the respiratory chain, protons are translocated from the matrix to the inter-
membrane space, how, we may ask, is this proton gradient used to synthesise ATP? Where better to start than with
the enzyme itself, the proton-translocating ATP synthase (
Figure 5.19
)
(
Capaldi & Aggeler, 2002
)
. It is composed
of two parts: one called F
0
which is inserted into the inner mitochondrial membrane and contains the proton
translocation channel; the second, F
1
, consists of a stalk, which connects with the F
0
component, to which a roughly
40 and
þ
a
3
,
b
3
,
g
,
d
,
e
), two of which, the
a
and
b
subunits make up the bulk of F
1
. Both bind nucleotides, although only the
b
subunits are directly involved in
12. I much prefer the description of F
1
as a ball on a stick, rather than, as sometimes found in textbooks, a lollipop.
