Ferredoxins (from Latin ferrum: iron + redox, often abbreviated "fd") are iron–sulfur proteins that mediate electron transfer in a range of metabolic reactions. They contain iron and sulfur atoms organized as iron–sulfur clusters. These biomolecules accept or discharge electrons, with the effect of a change in the oxidation state of the iron atoms between +2 and +3, letting them act as electron transfer agents in biological redox reactions.
The term "ferredoxin" was coined by D.C. Wharton of the DuPont Co. and applied to the "iron protein" first purified in 1962 by Mortenson, Valentine, and Carnahan from the anaerobic bacterium Clostridium pasteurianum.[1] [2]
Other bioinorganic electron transport systems include rubredoxins, cytochromes, blue copper proteins, and the structurally related Rieske proteins.
Another redox protein, isolated from spinach chloroplasts, was termed "chloroplast ferredoxin".[3] The chloroplast ferredoxin is involved in both cyclic and non-cyclic photophosphorylation reactions of photosynthesis. In non-cyclic photophosphorylation, ferredoxin is the last electron acceptor thus reducing the enzyme NADP+ reductase. It accepts electrons produced from sunlight-excited chlorophyll and transfers them to the enzyme ferredoxin: NADP+ oxidoreductase .
Ferredoxins can be classified according to the nature of their iron–sulfur clusters and by sequence similarity.
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 differ significantly.[4] [5]
Ferredoxins are one of the most reducing biological electron carriers. They typically have a mid point potential of -420 mV.[6] 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.[7] 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 -500 mV[6] [8] to -340 mV.[9] Organisms typically host multiple types of ferredoxins.
Some ferrodoxins are called "highly reducing".[10]
Reactions of Fd can be coupled to the oxidation of aldehydes to acids. Examples are the glyceraldehyde to glycerate reaction (-580 mV), the carbon monoxide dehydrogenase reaction (-520 mV), and the 2-oxoacid:Fd Oxidoreductase reactions (-500 mV)[11] [8] like the reaction carried out by pyruvate synthase.[7]
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.[6] The Rnf complex is a widespread membrane protein in bacteria that reversibly transfers electrons between NADH and ferredoxin while pumping or ions across the cell 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.[12] 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.[13]
