What are the components of the electron transport chain?

The electron transport chain is made up of four multi-subunit protein complexes found in the inner mitochondrial membrane. These complexes sequentially oxidize NADH and FADH2, transferring the released electrons through a series of acceptors resulting in the production of adenosine triphosphate (ATP). 

1. Complex I: NADH-Coenzyme Q Oxidoreductase

Complex I of the electron transport chain consists of flavin mononucleotide (FMN), NADH dehydrogenase, 8 iron-sulfur (Fe-S) clusters, and coenzyme Q (CoQ). This complex is also known as ubiquinone oxidoreductase. FMN receives two electrons from NADH generated during earlier stages of cellular respiration. These electrons are transferred through Fe-S clusters to CoQ. CoQ, in its oxidized form (ubiquinone, UQ), can accept electrons to become ubisemiquinone (UQH) and then ubiquinol (UQH2). The reduced UQH2 then transfers electrons to Complex III of the electron transport chain. During this electron transport process, Complex I pumps four protons from the mitochondrial matrix to the intermembrane space, contributing to the electrochemical gradient that drives ATP synthesis in oxidative phosphorylation.

2. Complex II: Succinate-Coenzyme Q Oxidoreductase

Complex II contains enzymes similar to those in Complex I, but this complex does not couple electron transport with proton pumping. In Complex II, FADH2 is reduced to FAD by the enzyme succinate dehydrogenase present in the inner mitochondrial membrane. Succinate, an intermediate in the Krebs cycle, is simultaneously oxidized to fumarate, which returns to the mitochondrial matrix, re-enters the Krebs cycle, and goes through a series of enzymatic reactions. The electrons from the oxidation of FADH2 are then transferred through Fe-S clusters to CoQ, reducing it to UQH2. The electrons carried by UQH2 are eventually sent to Complex III. 

3. Complex III: Cytochrome bc1 Oxidoreductase

Complex III is composed of cytochrome b and cytochrome c1 complexes that contain a heme prosthetic group and a 2Fe-2S cluster known as the Rieske center. UQH2 transfers electrons from Complex I or Complex II to Complex III in a series of redox reactions known as the Q cycle, which involves the transfer of electrons from UQH2 to cytochrome c1 and cytochrome b. This eventually results in the regeneration of one UQH2 and the transfer of electrons to Complex IV, which is the final complex in the electron transport chain. During each Q cycle, four protons are pumped into the intermembrane, adding to the proton gradient across the inner mitochondrial membrane.

4. Complex IV: Cytochrome c Oxidase

Complex IV, the last complex in the electron transport chain, receives electrons from Complex III and transfers them to oxygen, which is the last electron acceptor in cellular respiration. Complex IV contains cytochrome a, cytochrome a3, a copper atom (Cu3) and a copper atom pair (CuA4) center that can hold four electrons and acts as a redox center. During electron transport, two protons from the matrix are transported across the inner mitochondrial membrane into the intermembrane space. Complex IV also pumps protons into the intermembrane space, adding to the proton gradient established by Complexes I and III. The cumulative proton gradient across the inner mitochondrial membrane creates an electrochemical potential that drives ATP synthesis through oxidative phosphorylation. The complete oxidation of one NADH molecule is estimated to result in the synthesis of three ATP molecules, while the complete oxidation of one FADH2 molecule yields about two ATP molecules.