Iron nitrogenase

Substrate reduction by the iron nitrogenase is very similar to that observed with vanadium nitrogenases. Acetylene is a relatively poor substrate, and N2 reduction is accompanied by considerable H2 evolution. Acetylene reduction leads to the production of some ethane as well as ethylene. Beyond this, little has been investigated. Under optimal conditions for N2 reduction, the ratio of N2 reduced to H2 produced was 1 7.5 compared with 1 1 for molybdenum nitrogenase 192). 

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. 

Iron nitrogenase. FeFe proteins (e.g. from Azotobacter vinelandii and Rhodopseudomonas capsulatus) are hexameric (02 2 2) actually contain low... 

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. 

Molybdenum. Molybdenum is a component of the metaHoen2ymes xanthine oxidase, aldehyde oxidase, and sulfite oxidase in mammals (130). Two other molybdenum metaHoen2ymes present in nitrifying bacteria have been characteri2ed nitrogenase and nitrate reductase (131). The molybdenum in the oxidases, is involved in redox reactions. The heme iron in sulfite oxidase also is involved in electron transfer (132). 

Fig. 7. View of the FeMo-cofactor prosthetic group of the nitrogenase MoFe protein with some of the surrounding amino acid residues where ( ) represents the molybdenum coordinated to a-His-442 and homocitrate (at the top), ( ) represents the iron, interspersed with the sulfur (O) and carbon...

Molybdenum nitrogenase has been the subject of intensive study for more than 30 years, but much less work has been done on the vanadium and iron-only nitrogenases. Consequently, we first review the properties of Mo nitrogenase, and then in later sections outline what is known of the other two enzymes. 

As well as donating electrons to the MoFe protein, the Fe protein has at least two and possibly three other functions (see Section IV,C) It is involved in the biosynthesis of the iron molybdenum cofactor, FeMoco it is required for insertion of the FeMoco into the MoFe protein polypeptides and it has been implicated in the regulation of the biosynthesis of the alternative nitrogenases. 

NifM is required for maturation of VnfH, and NifS and U seem to be important for provision of sulfide and probably iron for the biosynthesis of the vanadium nitrogenase. The apo VFe protein has been isolated from an A. vinelandii strain deleted for nifKD and nifB (169). It was an hexamer that could be activated in vitro by the addi-... 

Although, as indicated in Fig. 12, there is clear genetic evidence for a third nitrogenase in Azotobacter vinelandii, the initial preparations of this enzyme had low activity and contained small quantities of molybdenum as well as iron, and thus the activity might have been... 

An Fe-only nitrogenase has also been isolated from a nifH mutant of Rhodospirillum rubrum and was characterized as an a2/82<% hex-amer containing only iron, no molybdenum or vanadium, with an o 2Fe4S4-containing Fe protein. A factor could be extracted from the FeFe protein into NMF that combined with apo-MoFe protein to form an active enzyme 193). 

Part 1 of the nitrogenase protein contains another interconnected group of Fe-S atoms, this one with eight iron atoms and seven sulfur atoms. This [8Fe-7S] group collects electrons and transmits them to the binding center. Part 2 of nitrogenase contains a third Fe-S group, this one made up of four iron atoms and four sulfur atoms. This part of the enzyme also binds two molecules of ATP. 

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... 

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