Electron Transfer Is Coupled to ATP Formation at Three Sites

Electron Transfer Is Coupled to ATP Formation at Three Sites 

The number of molecules of ATP formed per pair of electrons transferred down the respiratory chain to 02 is termed the P-to-O ratio. When NADH is used as the reducing substrate, the measured P-to-O ratio is about 2.5 with succinate it is about 1.5. Reductants that feed electrons directly to cytochrome c give a P to O of about 1.0. This last observation indicates that complex IV, which conducts electrons 

The rate of respiration by a suspension of mitochondria can be increased dramatically by the addition of a small amount of ADP. An oxidizable substrate (succinate) and Pi first are added at the times indicated by the vertical arrows, but the respiratory rate remains low until ADP is added. A second period of rapid respiration is obtained when more ADP is added, indicating that essentially all of the added ADP gets used up. The P-to-0 ratio can be determined by dividing the amount of ADP added by the amount of oxygen taken up during the period of rapid respiration. 

Structures of two uncouplers of oxidative phosphorylation, 2,4-dinitrophenol (DNP) and carbonylcyanide-p-trifluoromethoxyphenylhydrazone (FCCP). DNP has a weakly dissociable proton on the oxygen atom FCCP has a similar proton on one of the nitrogens. 

Finally, the intermediate P-to-O ratio obtained with succinate indicates that a coupling site occurs in complex III, but probably not in complex II. Thus, the respiratory chain appears to have three distinct coupling sites for ATP synthesis. 

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