Fe-proteins

AH three nitrogenases comprise two separately puriftable component proteins. Each has a specific homodimeric Fe protein. The Fe... 

Fig. 4. Requirements, substrates, and products of Mo-nitrogenase catalysis, where I is the MoFe protein II the Fe protein and Pi is inorganic phosphate. The generating system is composed of creatine phosphate and creatine phosphokinase to recycle the inhibitory MgADP produced during catalysis to...

The VFe protein also has the equivalent of P-cluster pairs which have similar properties to those found in the MoFe protein (159). No information is available on whether P-cluster pairs exist in the FeFe protein, but because of the relatively high sequence identity and the similar genetic basis of its biosynthesis, the occurrence seems highly likely. The catalytic role assigned to the P-cluster pair involves accepting electrons from the Fe protein for storage and future deUvery to the substrate via the FeMo-cofactor centers. As of this writing (ca early 1995), this role has yet to be proved. 

Other non-haem proteins, distinct from the above iron-sulfur proteins are involved in the roles of iron transport and storage. Iron is absorbed as Fe" in the human duodenum and passes into the blood as the Fe protein, transferrin, The Fe is in a distorted octahedral environment consisting of 1 x N, 3x0 and a chelating carbonate ion which... 

Mr 220-250 kDa. Figure 1 shows an overall electron transfer pathway for the nitrogenases where the Fe proteins act as very specific, essential electron donors to the larger proteins. This is not the only role for the Fe proteins (see Section IV,C) and their role in the mechanism is almost certainly more complex than that of a simple electron transfer agent (see below. Section V). Electron transfer from the Fe protein to... 

The Fe proteins are homodimers containing a single Fe4S4 cluster. Site-directed mutagenesis experiments showed that the cluster was probably held between the two subunits by ligation to two of the invariant cysteine residues from each subunit (16). This observation was confirmed later by X-ray crystallography (1) of the Fe protein... 

Fig. 2. The structure of the Fe protein (Av2) from Azotobacter vinelandii, after Geor-giadis et al. (1). The dimeric polypeptide is depicted by a ribbon diagram and the Fe4S4 cluster and ADP by space-filling models (MOLSCRIPT (196)). The Fe4S4 cluster is at the top of the molecule, bound equally to the two identical subunits, Emd the ADP molecule spans the interface between the subunits with MoO apparently binding in place of the terminal phosphate of ATP.

The nitrogenase proteins are generally characterized by two letters indicating the species and strains of bacteria and the numerals 1 for the MoFe protein and 2 for the Fe protein. Thus, the Fe protein from Azotobacter vinelandii is Av2 and the MoFe protein from Klebsiella pneumoniae is Kpl. 

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. 

Fe protein polypeptide NifH is also involved in FeMoco and MoFe protein biosynthesis MoFe protein polypeptides... 

Modifies and activates NifH to form active Fe protein Involved in molybdenum processing... 

The purified preparations of the apo-MoFe proteins from both organisms included a small additional polypeptide of around 20 kDa. This was shown to be the nifY product in K. pneumoniae (94) and a non-nif protein denoted y in A. vinelandii (95). These proteins are apparently essential for effective reaction with FeMoco and are associated with the MoFe protein polypeptide through its interaction with the Fe protein and MgATP (96, 97) (see Section IV,C,3). These observations demonstrate a third role for the Fe protein in generating a form of apo-MoFe protein that is capable of accepting FeMoco. 

A comprehensive description of the mechanism of molybdenum nitrogenase has been provided by the Lowe-Thorneley scheme 102) (Figs. 8 and 9). In this scheme the Fe protein (with MgATP) functions as a single electron donor to the MoFe protein in the Fe protein cycle (Fig. 8), which is broken down into four discrete steps, each of which may be a composite of several reactions ... 

The reduced Fe protein MgATP complex forms a complex with the MoFe protein... 

An electron is transferred from the Fe protein to the MoFe protein with concomitant hydrolysis of MgATP to MgADP and Pi... 

The oxidized Fe protein MgADP complex dissociates from the reduced MoFe protein... 

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