Electron Transport in Mitochondria Is Coupled to Proton Translocation

The coupling between electron transport from NADH (or FADH2) to O2 and proton transport across the inner mito-chondrial membrane, which generates the proton-motive force, also can be demonstrated experimentally with isolated mitochondria (Figure 8-14). As soon as O2 is added to a suspension of mitochondria, the medium outside the mitochondria becomes acidic. During electron transport from NADH to O2, protons translocate from the matrix to the intermembrane space; since the outer membrane is freely permeable to protons, the pH of the outside medium is lowered briefly. The measured change in pH indicates that about 10 protons are transported out of the matrix for every electron pair transferred from NADH to O2.

When this experiment is repeated with succinate rather than NADH as the reduced substrate, the medium outside the mitochondria again becomes acidic, but less so. Recall that oxidation of succinate to fumarate in the citric acid cycle generates FADH2 (see Figure 8-9). Because electrons in FADH2 have less potential energy (43.4 kcal/mol) than electrons in NADH (52.6 kcal/mole), FADH2 transfers electrons to the respiratory chain at a later point than NADH does. As a result, electron transport from FADH2 (or succinate) results in translocation of fewer protons from the matrix, and thus a smaller change in pH (see Figure 8-13).

pH electrode

O2 solution ^

No O

pH electrode

O2 solution ^

No O

Mitochondrion

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