Student Reader

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+.
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