The electron transport system is a chain of reactions that converts energy from nutrients into a proton gradient across membranes. It consists of four complexes: Complex I transfers electrons from NADH to ubiquinone and pumps protons; Complex II transfers electrons from succinate to ubiquinone without pumping protons; Complex III transfers electrons from ubiquinone to cytochrome c and pumps protons; Complex IV transfers electrons from cytochrome c to oxygen, pumping protons and producing water. The proton gradient powers ATP synthase to produce ATP through oxidative phosphorylation.

1. ELECTRON TRANSPORT SYSTEM:
The Electron Transport System also called the Electron Transport Chain, is a
chain of reactions that converts redox energy available from oxidation of
NADH and FADH2, into proton-motive force which is used to synthesize ATP
through conformational changes in the ATP synthase complex through a
process called oxidative phosphorylation.
NADH + 1/2O2 + H+
+ ADP + Pi → NAD+
+ ATP + H2O
Components of the Electron Transport Chain:
When electrons flow down the energy gradient from NADH to O2 and FADH2
to O2, the four protein complexes that catalyze the redox reaction are:
Complex I = NADH-Q reductase complex (NADH to O2)
Complex II = succinate dehydrogenase (FADH2 to O2)
Complex III= Cytochrome c reductase complex
Complex IV = Cytochrome c oxidase complex
1. Complex I (NADH-Q reductase complex)
 Complex I in the electron transport chain is formed of NADH
dehydrogenases and the Fe-S centers that catalyzes the transfer of two
electrons from NADH to ubiquinone (UQ).
 At the same time, the complex translocate four H+
ions through the
membrane, creating a proton gradient.
 NADH + H+
+ CoQ → NAD+
+ CoQH2
 NADH is first oxidized to nAD+ by reducing FMN to FMNH2 in a two-
step electron transfer.
 FMNH2 is then oxidized to FMN where the two electrons are first
transferred to Fe-S centers and then to ubiquinone.
2. Complex II (Succinate dehydrogenase)

2.  Complex II consists of succinic dehydrogenase, FAD, and Fe-S centers.
 The enzyme complex catalyzes the transfer of electrons from other
donors like fatty acids and glycerol-3 phosphate to ubiquinone through
FAD and Fe-S centers.
 This complex runs parallel to the Complex II, but Complex II doesn’t
translocate H+ across the membrane, as in Complex I.
 Succinate + FADH2 + CoQ → Fumarate + FAD+
+ CoQH2
3. Complex III (Cytochrome c reductase complex)
 Complex III consists of cytochrome b, c, and a specific Fe-S center.
 The enzyme complex, cytochrome reductase, catalyzes the transfer of two
electrons from reduced CoQH2 to two molecules of cytochrome c.
 Meanwhile, the protons (H+
) from the ubiquinone are release across the
membrane aiding to the proton gradient.
 The CoQH2 is oxidized back to CoQ while the iron center (Fe3+
) in the
cytochrome c is reduced to Fe2+
.
 CoQH2 + 2 Cyt c (Fe3+
) → CoQ + 2 cytc c (Fe2+
) + 4H+
4. Complex IV (Cytochrome c oxidase complex)
 Complex IV consists of cytochrome a and a3, which is also termed
cytochrome oxidase.
 This is the last complex of the chain and is involved in the transfer of two
electrons from cytochrome c to molecular oxygen (O2) forming water.
 In the meantime, four protons are translocated across the membrane
aiding the proton gradient.
 4 cytc c (Fe 2+
) + O2 → 4cytc c (Fe3+
) + H2O