Iron-sulfur clusters FeMoco-cofactor

Fig. 1. Schematic illustration of the enzyme nitrogenase being composed of the molybdenum-iron (MoFe) protein, an oc2p2 tetramer with two unique iron-sulfur clusters (P-cluster) and two iron-molybdenum cofactors (FeMoco), and the iron protein with one [4Fe-4S]-cluster and two ATP binding sites.

The enzyme system responsible for N2 reduction, called the nitrogenase complex, consists of two separate proteins. As outlined in Figure 20.4, one protein-called component I, nitrogenase, or molybdenumiron protein-catalyzes the reduction of N2, and the other-called component II, nitrogenase reductase, or iron protein-transfers electrons from ferredoxin or flavodoxin to component I. Both component I and component II contain Fe4S4 iron-sulfur clusters, and component I also contains molybdenum, in the form of a tightly bound iron-molybdenum cofactor (FeMoCo). 

The MoFe protein is an aifli tetramer of Mr 220 kDa, and its a and subunits are encoded by the nifD and nifK gene, respectively (Figure 1(a) and Table 1). It contains, in preparations with the highest activity, 2 molybdenum (Mo), 30 to 34 iron (Fe), and an approximately equivalent number of acid-labile sulfur (S ) atoms (Table 1). This metal content is consistent with the presence of two different types of unique metal clusters in the protein, that is, the [8Fe-7S] cluster (P cluster), which is bridged between each a/3 subunit pair, and the [Mo-7Fe-9S-homocitrate] cluster (FeMo cofactor or FeMoco), which is located within each a subunit (Figure la). ... 

All of these beautiful model complexes for nitrogen fixation have one disadvantage in common they do not represent any of the structural features of the iron-vanadium cofactor in the nitrogenase. In contrast, cubane clusters constimting the vanadium-iron-sulfur moiety of the FeMoco are expedient structural models, with the disadvantage, however, that they do not catalyse the reduction of N2. 

In a typical nitrogenase C pasteurianum , both proteins, MoFe-protein and Fe-protein, share one [Fe-4S] cluster of the ferredoxin type. Later on, it was shown that all molybdenum, and partly sulfur, can be isolated as an insoluble substance of the relative molecular mass from 1000 to 1500. This substance was named the iron-molybdenum cofactor (FeMoco) it contains 2Mo, 6 to 8 Fe and 6 or more S atoms. 

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