Iron protein in nitrogenase

Nitrogenase.—A review of the recent chemistry of nitrogen fixation has appeared. The iron protein of nitrogenase from Clostridium pasteurianum acts as a one-electron redox system in the sequence ... 

In Earth history 2 billion years ago and human history 100 years ago, the center of nitrogen catalysis was a metal, even possibly the same metal, iron. The multibiUion-year-old enzyme that makes ammonia from nitrogen and hydrogen is an iron protein called nitrogenase, which contains iron, sulfur, and a brand-new element. 

Iron-sulfur proteins. In an iroinsulfiir protein, the metal center is surrounded by a group of sulfur donor atoms in a tetrahedral environment. Box 14-2 describes the roles that iron-sulfur proteins play in nitrogenase, and Figure 20-30 shows the structures about the metal in three different types of iron-sulfur redox centers. One type (Figure 20-30a l contains a single iron atom bound to four cysteine ligands. The electron transfer reactions at these centers... 

Iron-sulfur (Fe-S) proteins function as electron-transfer proteins in many living cells. They are involved in photosynthesis, cell respiration, as well as in nitrogen fixation. Most Fe-S proteins have single-iron (rubredoxins), or two-, three-, or four-iron (ferredoxins), or even seven/eight-iron (nitrogenases) centers. 

To successfully describe the structure and function of nitrogenase, it is important to understand the behavior of the metal-sulfur clusters that are a vital part of this complex enzyme. Metal-sulfur clusters are many, varied, and usually involved in redox processes carried out by the protein in which they constitute prosthetic centers. They may be characterized by the number of iron ions in the prosthetic center that is, rubredoxin (Rd) contains one Fe ion, ferredoxins (Fd) contain two or four Fe ions, and aconitase contains three Fe ions.7 In reference 18, Lippard and Berg present a more detailed description of iron-sulfur clusters only the [Fe4S4] cluster typical of that found in nitrogenase s Fe-protein is discussed in some detail here. The P-cluster and M center of MoFe-protein, which are more complex metal-sulfur complexes, are discussed in Sections 6.5.2. and 6.5.3. 

Kim, J. and D.C. Rees. Structural models for the metal centers in the nitrogenase molybdenum-iron protein. Science 257,1677-1682 (1992). 

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. 

Extended X-ray absorption fine structure (EXAFS) studies on the Fe/Mo/S aggregate in nitrogenase have made available structural data that are essential in the design of synthetic analog clusters. Analyses of the Mo K-edge EXAFS of both the Fe-Mo protein and the FeMoco (9) have shown as major features 3-4 sulfur atoms in the first coordination sphere at 2.35 A and 2-3 iron atoms further out from the Mo atom at 2.7 A. The Fe EXAFS of the FeMoco (10,11) shows the average iron environment to consist of 3.4 1.6 S(C1) atoms at 2.25(2) A, 2.3 +0.9 Fe atoms at 2.66(3) A, 0.4 0.1 Mo atoms at 2.76(3) A and 1.2 1.0 0(N) atoms at 1.81(7) A. In the most recent Fe EXAFS study of the FeMoco (11) a second shell of Fe atoms was observed at a distance of 3.75 A. 

Bacterial ferredoxins. Bacterial ferredoxin was first described in 1962 by Mortenson et al. (p who found a low-molecular iron protein involved in electron transfer of pyruvate hydrogenase and nitrogenase in C. pasteurianum. Subsequently, a number of ferredoxins have been found lii widely different types of bacteria such as photosynthetic bacteria and N2-fixing bacteria. These bacterial type ferredoxins have molecular... 

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