Another Role for CO with Nitrogenase? CO Stimulates Hydrogen Evolution Catalyzed by Variant Azotobacter vinelandii Mo-Nitrogenases
journal contributionposted on 07.10.2014, 00:00 by Karl Fisher, Nathan D. Hare, William E. Newton
A likely entry/exit path for nitrogenase substrates, products, and/or protons involves residues α277Arg, α192Ser, and α356Gly, all of which are highly conserved among MoFe proteins from different organisms. The α192Ser and α277Arg residues form part of a hydrogen-bonded network that also involves α195His, which interacts with a FeMo cofactor-based sulfide. The terminal amino groups of α277Arg are also hydrogen-bonded directly to α281Tyr, which resides at the surface of the MoFe protein. Individual amino acid substitutions placed at position α277 or α192 resulted in a variety of effects on the catalytic and/or spectroscopic properties of the resulting variant MoFe protein. Of particular interest was the effect of CO on H2 evolution catalyzed by three MoFe protein variants, α277Cys, α192Asp, and α192Glu. All three variants exhibited CO stimulation of H2 evolution under high-electron flux conditions but not under low-electron flux conditions. This observation is best explained by these variants being redox-compromised but only at the most reduced redox states of the MoFe protein. Normally, these states are accessed and operational only under high-electron flux conditions, and the effect of added CO is to prevent access to these most reduced redox states, resulting in a normal rate of catalysis. Furthermore, via correlation of the effect of pH changes on H2 evolution activity for both the wild type and the α277Cys MoFe protein variant under argon, with or without 10% CO present, likely pathways for the delivery of a proton to the FeMo cofactor were identified.