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.
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.
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.
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.