Origin of the pH dependence of the midpoint reduction potential in Clostridium pasteurianum ferredoxin:oxidation state-dependent hydrogen ion association.

The origin of the dependence of E1/2 on pH exhibited by Clostridium pasteurianum 2(4Fe-4S) ferredoxin has been investigated. The results show that oxidation state-dependent pK values, which may arise from sites on the iron-sulfur centers, are responsible for the pH effect. Based on a model of two equivalent protonation sites/molecule, values of 7.4 for pKox and 8.9 for pKrd were obtained. The results of experiments which monitor changes in the hydrogen ion concentration with changes in protein oxidation state are reported. The magnitude of the changes in pH on reduction or reoxidation of the protein are in reasonable agreement with the proposed model. The conformation of C. pasteurianum ferredoxin was examined by nmr, epr, and CD spectroscopies to rule out a pH-dependent conformation equilibrium as the origin of the pH effect.