Bacterial Nitrogenase

Nitrogenase Bacterial enzyme that catalyzes nitrogen fixation... 

Many key protein ET processes have become accessible to theoretical analysis recently because of high-resolution x-ray stmctural data. These proteins include the bacterial photosynthetic reaction centre [18], nitrogenase (responsible for nitrogen fixation), and cytochrome c oxidase (the tenninal ET protein in mammals) [19, 20]. Although much is understood about ET in these molecular machines, considerable debate persists about details of the molecular transfonnations. 

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. 

Oda Y, SK Samanta, FE Rey, L Wu, X Liu, T Yan, J Zhou, CS Harwood (2005) Functional genomic analysis of three nitrogenase isoenzymes in the photosynthetic bacterium Rhodopseudomonas palustris. J Bacterial 187 7784-7794. 

The prosthetic group associated with the molybdenum atom of the molybdenum cofactor found in most molybdenum-containing enzymes except nitrogenase (See Molybdenum Cofactor). Many of these enzymes catalyze two-electron redox reactions involving the net exchange of an oxygen atom between the substrate and water. In bacterial enzymes a nucleotide is linked to the phosphoryl group. 

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... 

Fig. 19.25 Schematic diagram of nitrogenase activity in a bacterial cell. Carbohydrate provides reducing capacity (ferredoxin), energy (MgATP), and organic precursors for the manufacture of amino acids. [From Skinner. K. J. Chem. Eng. News 1976.54(41). 22-35. Reproduced with permission.]...

Not only are two molecules of ATP hydrolyzed to pump each electron, but the Fe-protein must receive electrons from a powerful (low E°) reductant such as reduced ferredoxin, reduced flavodoxin, or dithionite. Klebsiella pneumoniae contains a pyruvate flavodoxin oxidoreductase (Eq. 15-35) that reduces either flavodoxin or ferredoxin to provide the low potential electron donor.29 30 In some bacteria, e.g., the strictly aerobic Azotobacter, NADPH is the electron donor for reduction of N2. The Fe-protein is thought to accept electrons from a chain that includes at least the ordinary bacterial ferredoxin (Fd) and a special one-electron-accepting azotoflavin, a flavoprotein that is somewhat larger than the flavodoxins (Chapter 15) and appears to play a specific role in N2 fixation.31 In Clostridium and Rhizobium reduced ferredoxins generated by cleavage of pyruvate reduce nitrogenase directly.32... 

The current phase of interest in dinitrogen fixation started in the early 1960 s when bacterial nitrogenase was first extracted in active form (Carnahan et al., 1960) and the chemistry of dinitrogen fixation received a further boost in 1965 when the first stable and well-characterized dinitrogen complexes (Ru(NH3)5(N2)2+) were isolated (Allen et al., 1965). 

Nitrogenase has been successfully isolated from several bacterial species (8) and seems to be remarkably similar in all cases. To date, no pure preparations of nitrogenase have been obtained from blue-green algae (9). 

As already noted, the nitrogenases have a more complex active center FeMoco. The structure of this center is now known in fair detail (see Fig. 17-E-8) as a result of an X-ray diffraction study of one of the bacterial nitrogenases. In all prior efforts to devise models for this site it was taken for granted that an Mo atom served to bind an N2 molecule. It was an enormous surprise therefore that the molybdenum atom is almost certainly not the bonding site for the N2 molecule. Its presence is probably not mandatory since nitrogenases having no Mo have been discovered. 

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