8.6: Electron Transport Chains
Embedded in multiple folds of the inner membrane of mitochondria are numerous copies of the electron transport chain—a series of four protein complexes and associated organic molecules—that are critical for extracting energy. Electrons enter the chain using carrier molecules nicotinamide adenine dinucleotide, or NADH, and flavin adenine dinucleotide, or FAD, which are produced previously, such as during the citric acid cycle.
To begin, NADH carries two electrons into complex I––reducing NADH back to NAD+––transferring them first to the cofactor flavin mononucleotide, or FMN, which is then oxidized as it passes the electrons to an iron-sulfur protein.
The cluster then passes the electrons to a carrier, ubiquinone, or Q, which uptakes two protons as it carries the electrons to complex III. As a result of the energy release, four protons are actively pumped through complex I into the intermembrane space, producing a proton gradient.
FADH2 directly carries two electrons to complex II––reducing FADH2 to FAD+––where, after a redox reaction, they are transferred to another iron-sulfur protein, and then to the carrier, Q, which also uptakes two protons from the mitochondrial matrix as it carries the electrons into complex III.
Here, in the third complex, there are two cycles. First, an electron is transferred to an iron-sulfur protein, pumping the two hydrogen ions into the intermembrane space. After passing through an intermediate cytochrome molecule––called cytochrome c1––the electron passes to and reduces a cytochrome c electron carrier.
Second, another electron passes to a cytochrome b complex, bringing two more protons into complex III, then is transferred to an iron-sulfur protein, cytochrome c1, and to the carrier cytochrome c, pumping the two protons into the intermembrane space.
Finally, the cytochrome c electron carriers attach to complex IV, and two electrons reduce a cytochrome a3 and a copper molecule, allowing an O2 molecule to bind.
Once both oxygen molecules are completely reduced, they pick up two hydrogen ions to form water. During this process, four more protons are pumped into the intermembrane space.
Thus, at the end of the electron transport chain, ten protons are pumped into the mitochondrial intermembrane space for transport into an ATP synthase complex for chemiosmosis and energy production. The oxidized electron carriers can return to the citric acid cycle to pick up more electrons.