Nitrogenase is composed of two separate proteins designated the Fe-protein and the MoFe-protein. The Fe-protein of nitrogenase has been reported to have four distinct functions. In order for N2 reduction to occur, the Fe-protein must transfer electrons to the MoFe-protein (Howard, Rees 2000; Burgess, Lowe 1996). The Fe-protein is a dimer that couples the hydrolysis of two MgATPs to the transfer of a single electron; and in addition, it is also associated with the precursor of the MoFe-protein, apodinitrogenase, to facilitate its activation with the FeMo-cofactor. Our comparison of the polypeptide sequences of different Fe-proteins has revealed the existence of a remarkable degree of conservation of the amino acid sequence. A consensus sequence for the Fe-protein derived from this analysis is shown in Figure 1.
dd1dyvsyDvLGDWCGGFAmP i renkAqe i YiVmsg emMAmYAANNIskGi1k DOMAIN#IV DOMAIN#V
Figure 1. NifH consensus sequence. Upper case letters indicate residues that are invariant. Star above the cysteins (C) indicates cluster ligands. The conserved prolines are shown in bold. Underlined regions marked with domain numbers are arbitrary.
Apart from the structural genes nifHDK that encode the Fe- and MoFe-proteins, a number of nif accessory genes are involved in biological nitrogen fixation (Jacobson et al. 1989). The ability of the Fe-protein to accomplish these different tasks in the N2-fixation process, depends upon certain specific post-translational events that convert the newly synthesized inactive protein into its matured, functional form (Rangaraj et al. 2000 and references therein). It was reported that the products of three nif genes - nifM, nifS and nifU - have been implicated in playing a role in the maturation and assembly of the Fe-protein. The role of nif S and nifU in the activity of nitrogenase is discussed in detail by D.R. Dean in this volume.
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