Alkyne substrate interaction within the nitrogenase MoFe protein

Nitrogenase catalyzes the biological reduction of N₂ to ammonia (nitrogen fixation), as well as the two-electron reduction of the non-physiological alkyne substrate acetylene (HC≡CH). A complex metallo-organic species called FeMo-cofactor provides the site of substrate reduction within the MoFe protein, but exactly where and how substrates interact with FeMo-cofactor remains unknown. Recent results have shown that the MoFe protein α-70^Val residue, whose side chain approaches one Fe-S face of FeMo-cofactor, plays a significant role in defining substrate access to the active site. For example, substitution of α-70^Val by alanine results in an increased capacity for the reduction of the larger alkyne propyne (HC≡C-CH₃), whereas, substitution by isoleucine at this position nearly eliminates the capacity for the reduction of acetylene. These and complementary spectroscopic studies led us to propose that binding of short chain alkynes occurs with side-on binding to Fe atom 6 within FeMo-cofactor. In the present work, the α-70^Val residue was substituted by glycine and this MoFe protein variant shows an increased capacity for reduction of the terminal alkyne, 1-butyne (HC≡C-CH₂-CH₃). This protein shows no detectable reduction of the internal alkyne 2-butyne (H₃C-C≡C-CH₃). In contrast, substitution of the nearby α-191^Gln residue by alanine, in combination with the α-70^Ala substitution, does result in significant reduction of 2-butyne, with the exclusive product being 2-cis-butene. These results indicate that the reduction of alkynes by nitrogenases involves side-on binding of the alkyne to Fe6 within FeMo-cofactor, and that a terminal acidic proton is not required for reduction. The successful design of amino acid substitutions that permit the targeted accommodation of an alkyne that otherwise is not a nitrogenase substrate provides evidence to support the current model for alkyne interaction within the nitrogenase MoFe protein.