Nitrogenase catalyzes biological dinitrogen fixation, the reduction of N2 to 2NH3. Recently, the binding site for a non-physiological alkyne substrate (propargyl alcohol, HC≡C-CH 2OH) was localized to a specific Fe-S face of the FeMo-cofactor approached by the MoFe protein amino acid α-70Val. Here we provide evidence to indicate that the smaller alkyne substrate acetylene (HC≡CH), the physiological substrate dinitrogen, and its semi-reduced form hydrazine (H2N-NH2) interact with the same Fe-S face of the FeMo-cofactor. Hydrazine is a relatively poor substrate for the wild-type (α-70Val) MoFe protein. Substitution of the α-70 Val residue by an amino acid having a smaller side chain (alanine) dramatically enhanced hydrazine reduction activity. Conversely, substitution of α-70Val by an amino acid having a larger side chain (isoleucine) significantly lowered the capacity of the MoFe protein to reduce dinitrogen, hydrazine, or acetylene.