Electron Transport and Cellular Respiration

View IUBMB-Nicholson Animation of Membrane Respiratory Chain

A major source of cellular energy production, in the form of ATP, is derived from the proton motive force supplied to mitochondrial ATP synthase. The main driver of the proton gradient across the inner mitochondrial membrane is the electron transport system. NADH and FADH2 produced by glycolysis, fatty acid oxidation and the TCA cycle serve as a electron donors and cofactors for the protein complexes involved in the electron transport system. The net result is an influx of protons into the intermembrane space of the mitochondria and the production of water as a byproduct in the inner mitochondrial membrane.

2 H⁺ + 2 e⁺ + 1/2 O₂ → H₂O + energy

There are four membrane-bound protein complexes that participate in the electron transport system.

Complex I is a NADH dehydrogenase (or NADH-Coenzyme Q Reductase) composed of FMN, Coenzyme Q and Fe-S clusters. Using NADH as the initial electron donor, complex I generates a net result of 4 protons transferred from the matrix to the intermembrane space of the mitochondria, and the transfer of two electrons and two protons to reduce coenzyme Q located outside the complex within the inner mitochondrial membrane.

Complex II is Succinate Dehydrogenase (or Succinate-Coenzyme Q Reductase), which is one of the enzymes of the TCA cycle. It is composed of a bound FAD and three Fe-S clusters. Complex II generates a net result of the production of fumarate from succinate and reduction of one coenzyme Q located outside the complex within the inner mitochondrial membrane.

Complex III is a Cytochrome C Reductase (or Coenzyme Q -Cytochrome C Reductase). It is composed of cytochrome c₁, cytochrome b_L, cytochrome b_H and a Fe-S Rieske protein. Complex III also contains two coenzyme Q binding sites, which utilize reduced coenzyme Q in two ways. The first step is to transfer electrons via the Fe-S protein to Cytochrome c₁, which reduces cytochrome c. Secondly, the reduced coenzyme Q circulates electrons thru cytochrome c₁, cytochrome b_L and intermediate oxidative states of coenzyme Q resulting in the regeneration of one reduced coenzyme Q, one more reduced cytochrome c, and the translocation of four protons.

Complex IV is a Cytochrome c Oxidase. It is composed of heme-containing cytochrome a and cytochrome a₃ plus two copper complexes. This complex generates a net result of the formation of two water molecules, the oxidation of four Cytochrome cs, and the translocation of two protons.

ATP Synthase is often termed Complex V of the electron transport system since it uses the proton motive force of the translocated protons to drive the nanomotor, which condenses ADP and phosphate to form ATP in the matrix.

Cytochrome c: Sigma's Cytochrome c products are supplied mainly in the oxidized form of the protein. The reduced form of cytochrome c can be prepared with either sodium dithionite or sodium ascorbate, followed by gel filtration. (See Dixon, H.B., and McIntosh, R., Nature, 213(74), 399-400 (1967))

Sigma employs two methods for purification of cytochrome c; either trichloroacetic acid (TCA) is used during preparation or acetic acid. The "TCA" procedure may reduce the amount of superoxide dismutase (SOD), but tends to cause dimerization or "acid-modified structures." In contrast the "acetic acid" method may have slightly higher amounts of SOD, but a lower percentage dimeric cytochrome c. Ion-exchange chromatography is used during purification. Any SOD present can readily be removed using size exclusion chromatography.

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