Electron Transport & Oxidative Phosphorylation

Electron Transport & Oxidative Phosphorylation is a metabolic pathway that uses the energy released from the Citric Acid Cycle and oxygen to produce ATP. It is the major ATP production mechanism in human carbohydrate metabolism. See also [1].

NADH-coenzyme Q oxidoreductase (complex I)

• NADH-quinone oxidoreductase
• NADH:ubiquinone oxidoreductase
• NADH quinone oxidoreductase (NQO1) with inhibitor dicoumarol

The reaction that is catalyzed by this enzyme is the two electron oxidation of NADH by coenzyme Q10 or ubiquinone (represented as Q in the equation below), a lipid-soluble quinone that is found in the mitochondrion membrane:

NADH + Q + 5H⁺(matrix) ⟶ NAD⁺ + QH₂ +4H⁺(intermembrane)

The start of the reaction, and indeed of the entire electron chain, is the binding of a NADH molecule to complex I and the donation of two electrons. The electrons enter complex I via a prosthetic group attached to the complex, flavin mononucleotide (FMN). The addition of electrons to FMN converts it to its reduced form, FMNH2. The electrons are then transferred through a series of iron–sulfur clusters: the second kind of prosthetic group present in the complex. There are both [2Fe–2S] and [4Fe–4S] iron–sulfur clusters in complex I.

Succinate-Q oxidoreductase (complex II)

• Succinate Dehydrogenase

Succinate-Q oxidoreductase, also known as complex II or succinate dehydrogenase, is a second entry point to the electron transport chain. It is unusual because it is the only enzyme that is part of both the citric acid cycle and the electron transport chain. Complex II consists of four protein subunits and contains a bound flavin adenine dinucleotide (FAD) cofactor, iron–sulfur clusters, and a heme group that does not participate in electron transfer to coenzyme Q, but is believed to be important in decreasing production of reactive oxygen species.

Succinate + Q -> Fumarate + QH2

Q-cytochrome c oxidoreductase (complex III)

• Complex III of Electron Transport Chain
• Cytochrome bc1 complex
• Cytochrome b

The reaction catalyzed by complex III is the oxidation of one molecule of ubiquinol and the reduction of two molecules of cytochrome c, a heme protein loosely associated with the mitochondrion. Unlike coenzyme Q, which carries two electrons, cytochrome c carries only one electron.

QH₂+ 2 Cyt c_ox + 2H⁺(matrix) -> Q + 2 Cyt c_red + 4H⁺(intermembrane)

Cytochrome c oxidase (complex IV)

• Cytochrome c oxidase

Cytochrome c oxidase, also known as complex IV, is the final protein complex in the electron transport chain. The mammalian enzyme has an extremely complicated structure and contains 13 subunits, two heme groups, as well as multiple metal ion cofactors – in all, three atoms of copper, one of magnesium and one of zinc. The fully oxidized form of CcO active site shows the heme, Cu+2 ion and an O2 molecule. Second heme binding site. ^[1] Bacterial CcO is composed of 2 subunits.

4Cyt c_red +O₂ + 8H⁺(matrix) -> 4Cyt c_ox + 2H₂O +4H⁺(intermembrane)