Iron also nitrogenase

Also required for the vanadium and iron-only nitrogenases, which are encoded by vnf and anf genes, respectively. 

Figure 12 Assembly of the molybdenum nitrogenase. Pathways of FeMo cofactor biosynthesis, Fe protein maturation, and MoFe protein assembly are indicated in dotted, dashed, and solid arrows, respectively. The nif gene products that are also required for the vanadium and iron-only nitrogenase systems are indicated by ...

The an/H-encoded Fe protein of the iron-only nitrogenase is very homologous to the ni/H-encoded Fe protein of the molybdenum nitrogenase and the vn -encoded Fe protein of the vanadium nitrogenase. This Fe protein also has an ax homodimeric composition and contains a [4Fe-4S] cluster. 

The iron-only nitrogenase has been isolated from A. vinelandii, R. rubum, and R. capsulatus (Table 1). It is also similar to the molybdenum nitrogenase except that the molybdenum is replaced by iron in the protein. It has the molybdenum iron (MoFe) protein equivalent in the iron-iron (FeFe) protein and the iron (Fe) protein equivalent. 

The vanadium-nitrogenase and iron-only nitrogenases, also known as alternative nitrogenases, are related to the molybdenum nitrogenase but are all genetically distinct. A fourth and completely different nonconventional type of nitrogenase is known it also contains iron and molybdenum, but couples dinitrogen reduction to the oxidation of superoxide and carbon monoxide ]17]. 

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). 

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. 

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