Iron-sulfur centers nitrogenase

More complex iron-sulfur centers in the MoFe-protein of Mo-nitrogenase are discussed next. 

Paramagnetic centers are found in many specific enzymes and proteins that function in animal tissues and bacteria (oxygenases, sulfite or nitrite reductases, xanthine oxidase, nitrogenase, etc.) besides mitochondria and microsomes. The catalytic site can include Mo, Cu, Co, Ni, Mn, and other metal ions. Elowever heme- and iron-sulfur centers constitute the majority of the paramagnetic centers found outside the mitochondria. 

Molybdoenzymns At present, 6 oligomeric oxi-doreductases are known, which contain Mo as an essential constituent 1. Nitrogenase (see) 2. Nitrate reductase, EC 1.6.6.3 (see) 3. Xanthine oxidase, EC 1.2.3.2 (see), from animals and bacteria 4. Aldehyde oxidase, EC 1.2.3.1 from animal liver, which catalyses the reaction R-CHO + HjO R-COOH + 2H + 2e 5. Sulfite oxidase, EC 1.18.3.1 from mammalian and bird liver (Af, 114,000 2 subunits), which catalyses the reaction S03 + HjO-> SO/ + 2H + 2e this enzyme also contains a b5-like cytochrome and passes electrons directly to cytochrome c in the respiratory chain and 6. Formate dehydrogenase, EC 1.2.1.2, a membrane-bound protein from E. coli, containing one atom each of molybdenum and selenium, one heme group and nonheme iron-sulfur centers. It is NAD -dependent and catalyses the reaction HCOO + NAD CO2 -I- NADH. 

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. 

A large number of studies devoted to metal-sulfur centers are motivated by the occurrence of such arrangements at the active site of various metalloenzymes [1-13]. Mononuclear complexes with Mo=0 func-tion(s) and possessing sulfur ligands in their coordination sphere have been extensively investigated since they can be seen as models of the active site of enzymes such as nitrate- and DM SO reductases or sulfite- and xanthine oxidases [1-4]. On the other hand, a large variety of mono-, di-, and polynuclear Mo—S centers have been synthesized in order to produce functional models of the Mo-nitrogenase since the exact nature (mono-, di- or polynuclear) of the metal center, where N2 interacts within the iron-molybdenum cofactor (FeMo—co) of the enzyme is still unknown [4-8]. 

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