Iron-sulfur containing proteins are found widely in nature as electron transfer reagents. The most important of these enzymes are ferredoxins and nitrogenases. The nitrogenase enzyme system contains two proteins called the Fe protein and a protein containing both molybdenum and iron, the MoFe protein. Both of these proteins contain Fe-S clusters, although the structures of the clusters in the Fe protein and MoFe protein are different. The MoFe protein contains the active site responsible for reducing N2 to NH3. The function of the Fe protein is to reduce the MoFe protein. (For information on the structures, relationships, and reactions of nitrogenases see references 3 and 4.)

The research reported here focuses on the function of the Fe protein. The Fe protein is a dimer containing a single Fe4S4 cluster. This cluster has a very simple structure, which is shown in the accompanying diagram. The Fe and S atoms occupy the alternate corners of a cube, so that the four Fe atoms form a tetrahedron, which interlocks with a tetrahedron defined by the four S atoms.

When nitrogen is reduced to ammonia by the nitrogenase, the MoFe protein of the nitrogenase binds a molecule of N2, which is reduced to NH3 in the enzyme’s active site. In order to reduce nitrogen, the MoFe protein must first be reduced itself. The only agent known to do this is the Fe protein of the nitrogenase. Under laboratory conditions, when, for example, dithionite [S2O62-] is used as the reducing agent to supply electrons to the cluster, the Fe4S4 cluster of the Fe protein shuttles between the +2 and +1 oxidation states via a one-electron transfer. (Formally, since S is assigned an oxidation state of -2, it appears that, in the oxidized state, the cluster has 2 Fe atoms with +3 oxidation state and 2 Fe atoms with +2 oxidation state; while in the reduced form, the cluster has 1 Fe atom with +3 oxidation state and 3 Fe atoms with +2 oxidation state.) The behavior of Fe protein in such experiments implies that a one-electron transfer occurs in vivo, but recent research suggests that this conclusion may be all wrong.

New evidence has recently come from chemistry professor Eckard Munck at Carnegie Mellon University and biochemist Barbara Burgess at the University of California, Irvine, that the conventional picture of the mechanism for the reduction of Fe protein may be incorrect (1,2). By employing Mossbauer and electron paramagnetic resonance spectroscopies, these researchers showed that, in the reduced form of Fe protein, each iron atom in the Fe4S4 cluster is actually in the ferrous (+2) state, which means that, because each sulfur is present as sulfide, S2-, the cluster is neutral. Since many previous studies had shown that the oxidized form of the Fe4S4 cluster in Fe protein had a charge of +2, the finding that the reduced Fe4S4 cluster was neutral suggests that a two-electron reduction had taken place. The current work confirms the earlier work of Watt and Reddy (3), who concluded that the Fe protein can be reduced to an all-ferrous state. A schematic summary of the interlocking oxidations and reductions in the nitrogenase system is shown below.

It may be that the reason the Fe protein is a dimer containing a single Fe4S4 cluster is that the dimeric structure is required for the transfer of two electrons. The effect of the polypeptide portion of these proteins on the properties of the Fe4S4 cluster may also explain why each MoFe protein contains two cube-shaped electron-acceptor clusters, which are similar to the Fe protein clusters, but modified by their own protein component (Mo-modified). Each Mo-modifed Fe4S4 cluster functions as a one-electron acceptor, so that each MoFe protein accepts two electrons. (For structures of the various iron species in the nitrogenase system see references 4 and 5.) Muenck and coworkers suggest that the “current understanding of how nitrogenase functions is incorrect and all aspects of the reaction mechanism need to be re-examined.” Stay tuned for further developments

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