Nitrogenases types

The molybdenum-containing nitrogenase was the first to be identified and is the most thoroughly studied enzyme of the three known nitrogenase types. The solid-state crystal structure of the MoFe protein, obtained from Azetohacler vine-landii, has been solved in both a dithionite-reduced and an oxidized form. The... 

Iron Sulfur Compounds. Many molecular compounds (18—20) are known in which iron is tetrahedraHy coordinated by a combination of thiolate and sulfide donors. Of the 10 or more stmcturaHy characterized classes of Fe—S compounds, the four shown in Figure 1 are known to occur in proteins. The mononuclear iron site REPLACE occurs in the one-iron bacterial electron-transfer protein mbredoxin. The [2Fe—2S] (10) and [4Fe—4S] (12) cubane stmctures are found in the 2-, 4-, and 8-iron ferredoxins, which are also electron-transfer proteins. The [3Fe—4S] voided cubane stmcture (11) has been found in some ferredoxins and in the inactive form of aconitase, the enzyme which catalyzes the stereospecific hydration—rehydration of citrate to isocitrate in the Krebs cycle. In addition, enzymes are known that contain either other types of iron sulfur clusters or iron sulfur clusters that include other metals. Examples include nitrogenase, which reduces N2 to NH at a MoFe Sg homocitrate cluster carbon monoxide dehydrogenase, which assembles acetyl-coenzyme A (acetyl-CoA) at a FeNiS site and hydrogenases, which catalyze the reversible reduction of protons to hydrogen gas. 

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... 

NIS measurements have been performed on the rubredoxin (FeSa) type mutant Rm 2-A from Pyrococcus abyssi [103], on Pyrococcus furiosus rubredoxin [104], on Fe2S2 - and Fe4S4 - proteins and model compounds [105, 106], and on the P-cluster and FeMo-cofactor of nitrogenase [105, 107]. 

Fig. 6.11 The distribution in periplasmic space of the major molybdenum (and copper) enzymes except nitrogenase. Note the types of substrate.

A Time course of in vivo B H2-production activities and C nitrogenase of wild-type and mutants after transfer to BG1 lo medium. Vertical bars SD values for three replicates. Wild-type, WktshupL mutant, oAhoxH mutant,... 

Most proteins contain more than one polypeptide chain. The manner in which these chains associate determines quaternary structure. Binding involves the same types of noncovalent forces mentioned for tertiary structure van der Waals forces, hydrophobic and hydrophilic attractions, and hydrogen bonding. However, the interactions are now interchain rather than infrachain (tertiary structure determination). The quaternary structure of hemoglobin (four almost identical subunits) will be discussed in Chapter 4, that of superoxide dismutase (two identical subunits) will be discussed in Chapter 5, and that of nitrogenase (multiple dissimilar subunits) will be discussed in Chapter 6. 

In summary, reduction of dinitrogen by nitrogenase requires cooperativity among nitrogenase s subunits and involves three basic types of electron transfer steps ... 

To date, three types of nitrogenases have been identified, namely FeMo-, FeV- and FeFe- nitrogenases, depending upon the metal (in addition to iron) present in their cofactor. 

Despite the availability of the molecular structures of the different active sites of the FeMo-nitrogenases, the mechanism of nitrogen fixation remains obscure. The interest of our discussion, however, is centred on the various modes proposed to describe how molecular nitrogen might coordinate the FeMoco. Some of the reported schemes are inspired by the type of coordination found in model compounds. 

Relative hydrogen production of the wild type and the hypF defective (M539) T. roseopersicina strains in vivo under nitrogenase repressed and depressed conditions... 

Figure 10.9 Relative hydrogen production of the wild type and the hypF defective (MS39) I roseopersidna strains in vivo under nitrogenase repressed (white columns) and derepressed (black columns) conditions. Samples were measured after cultivation for three days.The amount of H2 evolved by the wild-type strain under non-nitrogenfixing condition was chosen as I.

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