The Respiratory Chain Complexes

The respiratory chain consists of a series of proteins containing oxidation-reduction groups. The chain is located in the inner membrane of the mitochondrion. Chemical treatment of the mitochondrial membrane results in the isolation of five complexes which have been designated as complexes I, II, III, IV and V. The enzymes of the inner membrane of the mitochondria appear to exist as components of these five complexes. Four of the complexes (I, II, III and IV) are parts of the electron transport system, while the fifth is concerned with the conservation and transfer of energy and synthesis of ATP. Two electron carries, coenzyme Q (CoQ) or ubiquinone (UQ) and cyytochrome c can be easily removed from the respiratory chain, and have been called mobile carriers. It has been suggested that coenzyme Q acts as a mobile electron carrier between complexes I and III and II and III, and cytochrome c between complexes III and IV. Some workers include both coenzyme Q and cytochrome c in complex. III. Alternative names for the five complex are given below.

Complex I – NADH/NADPH: CoQ reductase.
Complex II – Succinate: CoQ reductase.
Complex III – Reduced CoQ (CoQH2): cytochrome c reductase.
Complex IV – Cytochrome oxidase.
Complex V – ATPase (ATP synthesizing system)

Complexes I and II are dehydrogenase complexes (DH). Pairs of hydrogens released during aerobic glycolysis and three dehydrogenation reactions of the Kerbs cycle are accepted by NAD, which is reduced to NADH2. This compound serves as an energy donor for the respiratory chain and passes hydrogens to complex I. In the fourth dehydrogenation reaction of the Kerbs cycle (succinate to fumarate) the hydrogens released are accepted by complex II. The pathway of electrons during oxidation of NADH is complexes I, III and IV. In the oxidation of succinate the pathway is complexes II, III and IV. Thus complexes I and II are alternates. Hydrogens from complexes I and II accepted by the mobile carrier coenzyme Q and transferred to complex III. Electrons are transferred from complex III to complex IV through the mobile carrier cytochrome C.

Three molecules of ATP are generated per pair of hydrogen/electrons passing down the respiratory chain (one each through complexes I, III and IV, which are the energy coupling sites). Here energy released during electron transport is conserved and utilized for synthesis of ATP. ATP generation takes place in complex V. Complex II is a dummy as far as ATP generation is concerned.