Enzyme nitrogenase

Free-living bacteria are, however, used as the source of the enzyme nitrogenase, responsible for N2 fixation (1,4,26,80), for research purposes because these ate easier to culture. The enzyme is virtually identical to that from the agriculturally important thizobia. These free-living N2-fixets can be simply classified into aerobes, anaerobes, facultative anaerobes, photosynthetic bacteria, and cyanobacteria. 

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.

In Nature, nitrogen fixation is mediated by the enzyme nitrogenase according to Eq. (1) (6)... 

X-ray crystallographic structures of the enzyme nitrogenase first became available in 1992 with refinements of the structures continuing to the present time. As of this... 

Table 3.4 lists values for A Eq and for some important oxidation and spin states found in bioinorganic molecules. Data are taken from reference 24 and from Table 1 of reference 25 for hemoglobin, myoglobin, and the picket-fence porphyrin model compound, FeTpivPP(l-Melm).25 The myoglobin and hemoglobin model compounds are discussed in Section 4.8.2. Reference 26 provides the Table 3.4 data on iron sulfur clusters found in many bioinorganic species.26 The unusual iron-sulfur and iron-molybdenum-sulfur clusters found in the enzyme nitrogenase are discussed more fully below and in Chapter 6. 

Figure 6.1 Cartoon illustrating the structure of the enzyme nitrogenase.

Chemists and biochemists have studied the complex enzyme nitrogenase for all of modem scientific times. Many models for the enzyme s efficient reduction and protonation of dinitrogen to the useful product ammonia have been put forward. Many different research groups have based these models on analytical and instmmental observations. Crystallization of the enzyme s subunits and subsequent X-ray crystallographic structures in the 1990s yielded an intimate portrait informing all aspects of research on nitrogenase. In spite of the many structural and analytical successes, aspects of the enzyme s structure and function remain controversial or unclear up to the present time. 

Molybdate, although present in small amounts in soil, is an essential nutrient for nitrogen fixation, specifically in the enzyme nitrogenase. It does not move readily through soil and is therefore considered to be of limited mobility. 

In this text, iron-sulfur clusters are discussed because they appear in proteins and enzymes (1) cytochrome b(6)f, Rieske [2Fe-2S] cluster (Section 7.5 and Figure 7.26) (2) cytochrome bci, Rieske [2Fe-2S] cluster (Section 7.6 and Figure 7.30) and (3) aconitase, [4Fe-4S] cluster (Section 7.9.2.1, and Figure 7.50). The iron-sulfur protein (ISP) component of the cytochrome b(6)f and cytochrome bci complexes, now called the Rieske ISP, was first discovered and isolated by John S. Rieske and co-workers in 1964 (in the cytochrome bci complex). More information about the RISP is found in Section 7.5.1. Section 7.9.2 briefly discusses other proteins with iron-sulfur clusters—rubredoxins, ferrodoxins, and the enzyme nitrogenase. The nitrogenase enzyme was the subject of Chapter 6 in the hrst edition of this text— see especially the first edition s Section 6.3 for a discussion of iron-sulfur clusters. In this second edition, information on iron-sulfur clusters in nitrogenase is found in Section 3.6.4. See Table 3.2 and the descriptive examples discussed in Section 3.6.4. 

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