Principles of Biochemistry

The electron transport chain is a series of biochemical reactions in which electrons and hydrogen ions from NADH and FADH₂ are passed to intermediate carriers and then ultimately react with molecular oxygen to produce water. NADH and FADH₂ are oxidized in this process.

NADH + H⁺ → NAD⁺ + 2H⁺ +2e^-

FADH₂ → FAD + 2H⁺ +2e^-

The major components of the respiratory chain are as follows.

NAD → FAD→CoQ→ Cyt b → Cytc₁ →Cyt c → Cyt a→ Cyt a₃ →O₂ 

The enzymes and electron carriers needed for the ETC are located along the inner mitochondrial membrane. Within this membrane are four distinct protein complexes, each containing some of the molecules needed for the ETC process to occur. These four protein complexes, which are tightly bound to the membrane, are

Complex I : NADH-coenzyme Q reductase

Complex II: Succinate-coenzyme Q reductase

Complex III: Coenzyme Q-cytochrome c reductase

Complex iv: Cytochrome c oxidase

Hydrogen and electrons flow through the chain in steps from the more electronegative components to more electropositive oxygen through a redox span of 1.1V from NAD/NADH to O₂/2H₂O.

O₂ + 4e^- + 4H⁺   →  2H₂O

Energy is released at three places as the hydrogen ions and electrons flow through the respiratory chain. They are between NAD and FAD, between FAD and CoQ and between Cytochrome ‘a’ and ‘a₃’_. Thus 3ADP molecule captures this energy in the form of high energy phosphate and forms 3ATP. The resulting ATP passes on this free energy to drive those processes that require energy. Hence the ATP is called “the energy currency” of the cell. The production of ATP by oxidation is termed oxidative phosphorylation.