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.

  1. 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. 
  1. 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 another.)

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.

  1. 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.
  1. 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.

  1. 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.