Iron-sulfur clusters FeMo-cofactor

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). ... 

When compared to haem and iron—sulfur clusters, there are a number of much more complex organic cofactors which are found in metalloproteins (Rees, 2002). An albeit selective gallery is presented in Fig. 4.6. They include the Mo-cofactor found in a great number of Mo-dependent enzymes, the FeMo-cofactor and the... 

It is recalled that in Chapter 9, Section 2, the electrochemical behaviour of the FeMo cofactor from FeMo-nitrogenase, was reported. It possesses a heteronuclear iron-molybdenum-sulfur (MoFe7S9) cluster, which has similarities with the above discussed iron-sulfur proteins. 

More complex assemblies of iron and sulfur, sometimes extended to other metals like nickel, molybdenum, vanadium, or other iron centers are found in some enzymes, that catalyze the transformation of small molecules [1, 14]. Among these centers, we will focus next on the P cluster and the FeMo cofactor of nitrogenase and on the H cluster of the iron-only hydrogenase. 

The MoFe-protein is an a2 2 tetramer (with the subunits coded by the nifD and nifK genes, respectively), with a total molecular weight of 240,000. The two subunits are of similar size for example, the isolated a and /3 subunits of A. vinelandii MoFe-protein have 491 and 522 amino acids, respectively (46). In general, the amino acid sequences of MoFe-proteins are less well conserved than are Fe-protein sequences, so that the MoFe-protein sequences from A. vinelandii and C. pasteuria-num are only 36% identical (47). Associated with the MoFe-protein tetramer are approximately 2 molybdenum atoms, 30 iron atoms, and 30 sulfur atoms that are organized into two types of metal centers the FeMo-cofactor and the P-cluster pair. The structures and properties of these centers have been extensively probed by a wide variety of techniques. 

As we will see in subsequent chapters, many metalloproteins have their metal centres located in organic cofactors, like the tetrapyrrole porphyrins and corrins, in metal clusters like the Fe—S clusters in iron—sulfur proteins, or in even more complex cofactors, like the FeMo-cofactor of nitrogenase. We discuss their structures briefly here before moving on in a selected number of cases to the way in which metal insertion into the cofactor is engineered. 

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