can have multiple types of ferredoxins where each type is tuned to optimally carry out different reactions. Reduction of ferredoxin The highly reducing ferredoxins are reduced either by using another strong reducing agent, or by using some source of energy to "boost" electrons from less reducing sources to the ferredoxin. Direct reduction Reactions that reduce Fd include the oxidation of aldehydes to acids like the glyceraldehyde to glycerate reaction (-580 mV), the carbon monoxide dehydrogenase reaction (-520 mV), and the 2-oxoacid:Fd Oxidoreductase reactions (-500 mV) like the reaction carried out by pyruvate synthase. Membrane potential coupled reduction Ferredoxin can also be reduced by using NADH (-320 mV) or (-414 mV), but these processes are coupled to the consumption of the membrane potential to power the "boosting" of electrons to the higher energy state. The Rnf complex is a widespread membrane protein in bacteria that reversibly transfers electrons between NADH and ferredoxin while pumping or ions across the membrane. The chemiosmotic potential of the membrane is consumed to power the unfavorable reduction of by NADH. This reaction is an essential source of in many autotrophic organisms. If the cell is growing on substrates that provide excess , the Rnf complex can transfer these electrons to and store the resultant energy in the membrane potential. The energy converting hydrogenases (Ech) are a family of enzymes that reversibly couple the transfer of electrons between and while pumping ions across the membrane to balance the energy difference. + + <=> + + + + <=> + + Electron bifurcation The unfavourable reduction of Fd from a less reducing electron donor can be coupled simultaneously with the favourable reduction of an oxidising agent through an electron bifurcation reaction. An example of the electron bifurcation reaction is the generation of for nitrogen fixation in certain aerobic diazotrophs. Typically in oxidative phosphorylation the transfer of electrons from NADH to Ubiquinone(Q) is coupled to charging the proton motive force. In Azotobacter the energy released by transferring one electron from NADH to Q is used to simultaneously boost the transfer of one electron from NADH to Fd. Direct reduction of high potential ferredoxins Some ferredoxins have a sufficiently high redox potential that they can be directly reduced by NADPH. One such ferredoxin is adrenoxin (-274mV) which takes part in the biosynthesis of many mammalian steroids. The ferredoxin Fd3 in the roots of plants that reduces nitrate and sulfite has