Two components constitute Mo-dependent nitrogenase (EC 1.18.6.1)--the Fe protein (a homodimer encoded by nifH) and the MoFe protein (an alpha 2 beta 2 tetramer encoded by nifDK). The MoFe protein provides the substrate-binding site and probably contains six prosthetic groups of two types--four Fe-S centres and two Fe- and Mo-containing cofactors. To determine the distribution and catalytic function of these metalloclusters, we and others are attempting to change the catalytic and spectroscopic features of nitrogenase by substituting specific amino acids targeted as potential metallocluster ligands, particularly those to the FeMo-cofactor, which is responsible for the biologically unique electron paramagnetic resonance signal (S = 3/2) of nitrogenase, and is believed to be the N2-reducing site. Here we describe mutant strains of Azotobacter vinelandii that have single amino-acid substitutions within the MoFe protein alpha-subunit. These substitutions alter both substrate-reduction properties and the unique electron paramagnetic resonance signal, indicating that the FeMo-cofactor is associated with both the alpha-subunit and the substrate-reducing site.

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