text: Ferredoxins typically carry out a single electron transfer.
  However a few bacterial ferredoxins (of the 2[4Fe4S] type) have two iron sulfur clusters and can carry out two electron transfer reactions. Depending on the sequence of the protein, the two transfers can have nearly identical reduction potentials or they may be significantly different.
  Ferredoxins are one of the most reducing biological electron carriers. They typically have a mid point potential of -420 mV. The reduction potential of a substance in the cell will differ from its midpoint potential depending on the concentrations of its reduced and oxidized forms. For a one electron reaction, the potential changes by around 60 mV for each power of ten change in the ratio of the concentration. For example, if the ferredoxin pool is around 95% reduced, the reduction potential will be around -500 mV. In comparison, other biological reactions mostly have less reducing potentials: for example the primary biosynthetic reductant of the cell, NADPH has a cellular redox potential of -370 mV ( = -320 mV).
  Depending on the sequence of the supporting protein ferredoxins have  reduction potential from around -500mv to -340 mV. A single cell can have multiple types of ferredoxins where each type is tuned to optimally carry out different reactions.