Respiratory chain and electron transport

NADH and FADHg are produced as a result of substrate level dehydrogenations. Oxidation of these reduced coenzymes by oxygen is accomplished by the intervention of a series of electron carriers between the primary reductant and the terminal oxidant (Fig. 2). The electron-transport components represent redox couples of increasing redox potential and are therefore favored thermodynamically. The respiratory chain can be separated into four multienzyme complexes NADH-Q reductase (complex I), succinate-Q reductase (complex II), QH2 Cytochrome c reductase (complex III), and cytochrome c oxidase (complex IV). At each of these successive oxidation-reduction steps, a certain amount of free energy is available, the amount being determined by the difference in the oxidation-reduction potential of the two sequential components. The difference in the redox potential between 

NADH and oxygen is approximately 1.13 V, representing an energy availability of 51 kcal/mol of NADH oxidized. At steps I, III, and IV of the respiratory chain, the free energy is coupled to the formation of ATP, a process which requires approximately 25 kcal to form 3 mol of ATP (Baltscheffsky and Baltscheffsky, 1974). 

The cytochromes, which have a prosthetic heme group at the active site, possess characteristic absorption maxima originating from the porphyrin rings as modified by the iron atom held in coordinate linkage. It is this iron that accounts for the electron acceptor and donor properties of the cytochromes. 

Some of the electron-transport carriers contain iron that is not found in a porphyrin prosthetic group. This is termed nonheme iron, the identification of iron depending on instrumental methods of electron paramagnetic resonance (EPR). Nonheme-iron carriers have an asymmetric EPR signal at g= 1.94, which is generated when they are reduced. In addition, they are usually found to be bound to sulfur as part of the active site. 

The ubiquinones (coenzyme Q) are a group of lipid-soluble benzo-quinones found in most aerobic organisms. For a long while their function was unclear, but studies with reconstituted electron-transport systems have shown that coenzyme Q is a component which occurs at a 

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