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Electron transport chainComments
parentCellular respiration
siblingsGlycolysisKreb’s CyclePyruvate decarboxylation

Electron transport chain

How are reduced energy currencies like NADH used to make ATP? It harnesses the exergonicity of electron transfers. The reaction ½ O2 + 2 H+ → H2O is very exergonic; the reactants have a very high reduction potential, a measure of electron affinity. Rather than allowing e- transfer to make HO in one hugely exergonic step, the electron transport inserts lots of little harnessed steps.

The e- move from cofactors of lower to higher reduction potentials and from one complex or carrier to the next. The exergonic e- transfers are coupled to H+ translocation from the mitochondrial matrix to the inner membrane space. This establishes a proton gradient. The flow of H+ from the inner membrane space back into the matrix is harnessed by ATP synthase to drive endergonic ATP formation.

Complex IComplex I (NADH-CoQ oxidoreductase) pumps four H+ for every two e- tranferred. Two e- (H·) from NADH and free H+ travel one-at-a-time along FMN and then a a series of Fe-S (iron sulfide) centers until finally being onto Q to form QH2 (ubiquinol). Ubiquinol will transfer its e- to Complex III. These exergonic electron transfers drive the translocation of 4 H+ to the inner membrane space.
Complex IIComplex II (succinate:CoQ oxidoreductase) is used in the citric acid cycle. It takes e- one-at-a-time from the succinate → fumarate oxidation. The e- travels from FAD to Fe-S (iron sulfide) centers before being deposited on Q to form QH2. No H+ is pumped.
CoQUnlike the e- carriers Complex I, II, III and IV which are embedded in the membrane, CoQ (ubiquinone) is a mobile e- carrier. It is written CoQ or simply Q -- QH· is ubisemiquinone; QH2 is uniquinol. It is ubiquinol which is formed by Complex I and II, and it transfers its two e- to Complex III.
Complex IIIComplex III (CoQ-cytochrome c oxidoreductae) pumps four H+ for every two e- transferred. The reduced QH2 from Complex I or Complex II gives up its e- to Complex III. The e- travel through Complex III to an attached cytochrome c, then the reduced cytochrome c travels to Complex IV. These exergonic electron transfers drive the translocation of 4 H+ to the inner membrane space.
Cyt CLike Q, cytochrome c (abbreviated cyt c) is a mobile electron carrier which is laden with two e- by Complex IV and then transfers Complex IV, where it unloads its two e-.
Complex IVComplex IV (cytochrome c oxidase) pumps 2 H+ for every 2 e- transferred. The e- from reduced cytochrome c are transferred into Complex IV, which uses them and four H+ (from the matrix) to reduce O2 to H2O and also drive the translocation of 2 H+ to the inner membrane space.
Proton gradientAll in all, four e- are transferred: two from NADH and H+ via Complex I; two from succinate & fumarate via Complex II. Other reactions sometimes generate QH2 which then proceeds along to Complexes III and IV normally. Four H+ (two by Complex III and two by Complex IV) are translocated for each of the two QH2 produced. Additionally, Complex I translocates four H+.