Electron Transfers in Oxidative Phosphorylation
The numbered steps below correspond to the numbered steps in the electron-transport chain animation in Figure 9, in the
main page of the tutorial. (These are the same as the numbers on the electron carriers (purple) in Figure 9). We
recommend that you view the movie first, and refer to the text below for clarification of the steps in the movie.
An electron from NADH is first accepted by the protein complex NADH-Q reductase, also known as
the NADH dehydrogenase complex. This is the largest of the electron carriers, consisting of more than 22 protein
chains. The NADH-Q reductase complex accepts an electron from NADH and passes the electron to the next electron
carrier, ubiquinone, which has a higher reduction potential.
Ubiquinone, abbreviated as Q, is an organic molecule (not a protein)
dissolved in the hydrophobic region of the inner membrane of the mitochondrion. It can move freely within the
hydrophobic region of the membrane, by diffusion. [Note that ubiquinone diffuses
from one region of the membrane to another (i.e.,within the walls of the membrane),
whereas polar molecules can diffuse from one side of the membrane to the other side (i.e., across the
membrane) through channels.]
Ubiquinone has a higher reduction potential than the NADH-Q reductase. Hence, when ubiquinone in the oxidized form
comes in contact with the NADH-Q reductase complex (by a random collision), this mobile electron carrier accepts an
electron from NADH-Q reductase (i.e., gets reduced).
(Note: Because the electron-transport chain has mobile electron carriers, the
electron-carriers need not be located next to each other, as they are shown in Figure 8. It is the difference in
reduction potential, not spatial arrangement, that causes the electron to flow sequentially from one carrier to
The free energy released by the spontaneous transfer of electrons from the NADH-Q reductase complex to ubiquinone is
used for a very important purpose. As seen by the blue arrow in Figure 8, this free energy is used
to pump protons (H+ ions) out of the matrix, through the
NADH-Q reductase (which spans the membrane), and into the intermembrane space, building up a significant
proton-concentration gradient. As we will see later, this proton gradient ultimately
provides the energy needed to generate ATP! Hence NADH-Q reductase acts as both an electron
carrier and a proton pump.Ubiquinone is an electron carrier only; it is not a proton
pump. Therefore, ubiquinone does not increase the H
+ concentration in the intermembrane space.
The reduced form of ubiquinone then continues to move through the hydrophobic region of the membrane by diffusion.
When the ubiquinone comes in contact with the next carrier in the electron-transport chain, the electron is transferred
to this protein complex, known as cytochrome reductase, or the cytochrome b-c1
complex. This complex is actually a dimer, i.e., it consists of two membrane-spanning protein
subunits. Electrons from ubiquinone are first accepted by the subunit called cytochrome b, which then passes the
electron to the other protein subunit of cytochrome reductase, which is called cytochrome c1. As shown in
Table 2, the cytochrome c1 subunit has a higher reduction potential than the cytochrome b subunit.
From cytochrome reductase, the electron is picked up by another mobile electron carrier,
cytochrome c (not to be confused with the cytochrome c1 subunit of cytochrome
reductase). Cytochrome c is a small protein containing one heme group. When the oxidized form of cytochrome c contacts
the cytochrome reductase complex by a random collision, its heme group can accept an electron from the heme group of
the cytochrome c1 subunit (in cytochrome reductase). Cytochrome c then carries this electron until the
carrier collides with the final protein carrier in the electron-transport chain, cytochrome oxidase.
Like NADH-Q reductase, cytochrome reductase acts as both an electron carrier and a proton pump.
As the electron is spontaneously transferred from one group to another in the protein complex, free energy is released.
This free energy is used to pump protons from the matrix, across the inner mitochondrial membrane (through cytochrome
reductase), and into the intermembrane space. Hence, the proton gradient is increased further.
Cytochrome oxidase (Figure 6a and 6b) is the best understood of all the electron-carrier
proteins involved in oxidative phosphorylation. In many ways this protein is similar to NADH-Q reductase and cytochrome
reductase, which are discussed above. Cytochrome oxidase accepts an electron from cytochrome c, and passes it to
O2, the final electron acceptor in this chain. The mechanism for this final electron transfer is described
in the yellow box, below. (It is interesting to note that azide, which is used in
airbags, is toxic to us because it binds to cytochrome oxidase and
blocks this important electron transfer.) As with the other proteins, the free energy from the
spontaneous oxidation-reduction reaction is used to pump more protons into the intermembrane space, increasing the
proton gradient even further.
Mechanism for Safely Transferring Electrons to Molecular Oxygen
Cytochrome oxidase also has an important, unique feature that is necessary because it transfers its electrons to
O2. O2 has a difficult time picking up one extra electron to form the free-radical species
O2-; however, once O2 has accepted one electron it becomes very reactive, and can
easily accept more electrons, or participate in other chemical reactions. (A free radical is a group that contains
an unpaired electron. Free radicals are extremely reactive.) Many of the chemical reactions that the free radical
O2- could participate in, such as the destruction of fatty acids that make up membranes, would
be very harmful to the body. Cytochrome oxidase acts as an enzyme to help add the first electron to O2.
However, recall that cytochrome oxidase can only transfer one electron at a time to oxygen, and that adding only
one electron would result in a dangerous free radical. Hence, the protein must have a mechanism to hold the oxygen
in place until all four electrons have been transferred to O2 (i.e., until the
oxygen has been reduced completely to H2O), so that the free radical generated after the first electron
transfer does not escape and do great harm to the cells. Cytochrome oxidase contains a special
bimetallic center consisting of a heme (iron-containing) group in close proximity to a
copper atom. Oxygen is trapped between these two metal atoms until it has been completely reduced to H2O.
The water molecules that are generated can then exit the protein complex.