The effect of pH on the formal reduction potential of adrenodoxin in the presence and absence of adrenodoxin reductase: the implication in the electron transfer mechanism

We have investigated the formal reduction potentials (E degrees') of adrenodoxin with and without adrenodoxin reductase in order to elucidate the mechanism of electron transfer from adrenodoxin reductase (a flavoprotein) to adrenodoxin (an iron-sulfur protein). It was found by our spectropotentiostatic method that adrenodoxin showed no variation of E degrees' at different pH's in the absence of adrenodoxin reductase. The average E degrees' was -252 +/- 2 mV in the pH range between 6.0 and 8.3. In the presence of adrenodoxin reductase, adrenodoxin exhibited, on the other hand, a pH dependence of E degrees' at pH higher than 7.2 with a slope of -59 mV per pH unit: Adrenodoxin molecule possesses one protonation site with a pKa of 7.2. Cyclic voltammograms of adrenodoxin additionally revealed that the reoxidation reaction of reduced adrenodoxin is very slow in the absence of adrenodoxin reductase, but that it is readily reoxidized in the presence of adrenodoxin reductase.