Electron Transport Chain: Complex III and IV

Complex III, or Q-cytochrome c oxidoreductase, is a dimeric protein with eleven subunits per monomer. Its three catalytic components are cytochrome b with two b-type heme groups, cytochrome c1 with one c-type heme, and Rieske iron-sulfur protein containing Fe2-S2 clusters. Cytochrome b is encoded by the mitochondrial genome, enabling electron transfer from ubiquinol to cytochrome c.

Cytochrome c oxidase, or Complex IV, contains heme and copper ion cofactors that sequester oxygen atoms. These cofactors enable electron transfer from cytochrome c to oxygen, the terminal electron acceptor, reducing it to water. With thirteen subunits, three of its largest subunits are encoded by the mitochondrial genome, making it essential for completing the electron transport chain.

As electrons pass through complexes I, III, and IV, the released free energy pumps protons into the intermembrane space, creating a proton motive force. This proton gradient drives the rotation of ATP synthase, which catalyzes ATP synthesis from ADP and inorganic phosphate. The resulting ATP fulfills the cell's energy requirements through oxidative phosphorylation.

Superoxide generation in Complex III originates from the Q cycle, where an unstable radical called ubisemiquinone is formed. This radical can donate its unpaired electron to oxygen, generating superoxide anions. Complex III produces superoxides either inside the mitochondrial matrix or the intermembrane space, making it a major site of cellular oxidative damage.

Stigmatellin obstructs electron flux from ubiquinone to iron-sulfur proteins, preventing ubisemiquinone oxidation and reducing superoxide generation. Conversely, Antimycin A increases superoxide production by raising the steady-state concentration of ubisemiquinone. These drugs demonstrate how Q cycle intermediates directly regulate reactive oxygen species formation in mitochondria.

Cytochrome c oxidase acts as the regulatory center of oxidative phosphorylation through allosteric ATP inhibition. When the ATP/ADP ratio is high, phosphorylated Complex IV undergoes feedback inhibition by ATP. This mechanism allows cells to sense energy levels and adjust ATP synthesis in the mitochondria according to energy demand.

Electrons arrive at Complex III from ubiquinol and are transferred to cytochrome c, which carries them to Complex IV. Complex IV then accepts these electrons and donates them to oxygen, reducing it to water. This sequential electron transfer through the electron transport chain complex I and II, Complex III, and Complex IV releases energy for proton pumping.