FeFe nitrogenase

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. 

Biological N2 fixation (1), i.e., the reduction of N2 to NH3 catalyzed by FeMo, FeV, or FeFe nitrogenases, is one of the fundamental synthetic processes of nature (2-4). In spite of intense efforts over the last decades, its molecular mechanism is poorly understood, in particular because the pivotal chemical question has remained unanswered how do nitrogenases achieve to activate and convert the inert N2 molecule to ammonia under ambient conditions and mild redox potentials. 

The reduction of N2 catalyzed by nitrogenases is always accompanied by proton reduction leading to H2. This obligatory H2 evolution is different for the three types of nitrogenases. Equations 56-58 give the NH3/H2 ratios observed with FeMo, FeV, and FeFe nitrogenases, respectively (150-152). 

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). 

The uncertain role of the Mo atom is emphasized by the existence of the alternative nitrogenases, which contain a FeV or a FeFe cofactor. 

Biological N2 fixation is catalyzed by Fe/Mo, Fe/V, or FeFe (the Fe-only) nitrogenases (150, 151). The extremely different reaction conditions of the biological and the Haber-Bosch processes of N2 reduction, that is, standard temperature and pressure and biological redox potentials on the one hand, red-hot temperatures and high pressures on the other hand, make the quest for low molecular weight competitive catalysts particularly challenging. 

Gollan, U., Schneider, K., Muller, A., Schuddekopf, K., and Klipp, W. 1993. Detection of the in vivo incorporation of a metal cluster into a protein. The FeMo cofactor is inserted into the FeFe protein of the alternative nitrogenase in Rhodobacter capsulatus. Eur. J. Biochem. 215, 25-35. 

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. 

S-adenosylmethionine (SAM or AdoMet) superfamily, aconitase, and others), enzymes that contain Fe-S heteroatomic clusters (nitrogenase iron-molybdenum cofactor (FeMoco), carbon monoxide dehydrogenase (CODH), and acetyl CoA synthase (ACS)), and enzymes that contain unique ligation sets around specialized iron centers ([NiFe] and [FeFe] hydrogenases) (Figure 1). ... 

For the most common types of biological clusters ([2Fe-2S], [3Fe-4S], and [4Fe-4S]), the assembly machinery includes iron chaperones, cysteine desulfurases, electron transfer proteins, molecular chaperones, and scaffold proteins on which the nascent cluster is assembled prior to insertion into a target protein. For the more complex and unusual clusters, such as FeMoco of nitrogenase, the [NiFe] center of hydrogenase, or the Fl-cluster of [FeFe] hydrogenase, significantly less is known about the cluster assembly process. As we shall discover below, one common theme that these systems share in the synthesis of their respective metallocofactors is the involvement of radical chemistry provided by radical SAM enzymes. 

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

A second iron metalloprotein that is larger (about 220.000 Daltons) and also eventually contains another transition metal that is different for the three kinds of nitrogenase enzymes Mo (MoFe protein), V (VFe protein) or Fe (FeFe protein). It is the MoFe protein that is tiie best known. It contains ... 

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