The Respiratory Chain of Plant Mitochondria

The oxidation-reduction potentials of the cytochromes of the respiratory chain of mung bean (Phaseolus aureus) mitochondria have been measured under strictly anaerobic conditions with a combined spectrophotometric/potentiometric method. The midpoint potentials at pH 7.2 are as follows: cytochrome a: +190 millivolts; a(3): +380 millivolts; b(553): +75 millivolts; b(557): +42 millivolts; b(562): -77 millivolts; c(547) and c(549): +235 millivolts. (The subscripts refer to the difference absorbance maxima observed for these cytochromes in reduced-minus-oxidized difference spectra recorded at 77 K.) The same values of midpoint potentials at pH 7.2 are obtained with mitochondria depleted of energy by aerobic incubation with ADP and uncoupler in the presence of inorganic phosphate, or with coupled mitochondria energized with ATP in the absence of inorganic phosphate. Coupling site II is placed between b(553)/b(557) and c(549)/c(547) in these mitochondria, and coupling site III is placed between a and a(3).