The relation of pH and oxidation-reduction potential to the association state of the ferredoxin . ferredoxin:NADP+ reductase complex.

The interaction between spinach ferredoxin and ferredoxin:NADP+ reductase was studied by varying pH and oxidation-reduction state. The Kd of the oxidized ferredoxin . ferredoxin:NADP+ reductase complex increases with increasing pH in the range from pH 6 to pH 8; the Kd is pH-independent above pH 9. These data are interpreted as showing 1 proton binding/complex at neutral pH. The extent of association of the complex was also varied by manipulation of salt concentration, was also varied by manipulation of salt concentration, and of concentration of the individual proteins. Increased association accompanied a negative shift in potential of ferredoxin relative to that of ferredoxin: NADP+ reductase. Oxidation-reduction titrations showed that the oxidation-reduction potential of ferredoxin is reduced to above -510 mV when in complex with ferredoxin:NADP+ reductase, a change of about -90 mV; the potential of ferredoxin:NADP+ reductase is changed little (no more than +20 mV). Conversely, these data also showed that the oxidation-reduction state of ferredoxin strongly affected its association with the flavoprotein, increasing the Kd at least 30-fold on reduction of ferredoxin.