Nitrogenase reduction of carbon-containing compounds

Abstract

Nitrogenase is an enzyme found in many bacteria and archaea that catalyzes biological dinitrogen fixation, the reduction of N2 to NH3, accounting for the major input of fixed nitrogen into the biogeochemical N cycle. In addition to reducing N2 and protons, nitrogenase can reduce a number of small, non-physiological substrates. Among these alternative substrates are included a wide array of carbon-containing compounds. These compounds have provided unique insights into aspects of the nitrogenase mechanism. Recently, it was shown that carbon monoxide (CO) and carbon dioxide (CO2) can also be reduced by nitrogenase to yield hydrocarbons, opening new insights into the mechanism of small molecule activation and reduction by this complex enzyme as well as providing clues for the design of novel molecular catalysts. This article is part of a Special Issue entitled: Metals in Bioenergetics and Biomimetics Systems.

Keywords: Carbon; Carbon dioxide; Carbon monoxide; Reduction; Substrate.

Figure 1

Mo-nitrogenase with cofactors. Shown is one functioning half of the Monitrogenase. The top shows the Fe protein (left) and an αβ-dimer half of the MoFe protein (right). Shown below are the metal clusters and ATP, with Fe in rust, S in yellow, C in gray, O in red, N in blue, and Mo in magenta. Taken from PDB 2AFK.

Figure 2

FeMo-cofactor and key residues. Shown is the FeMo-cofactor with key MoFe protein amino acid side chains. Colors are Fe in rust, S in yellow, C in gray, O in red, N in blue, and Mo in magenta. PDB 2AFK.

Figure 3

Metal hydrides and the FeMo-cofactor. Shown are provisional binding sites for bridging hydrides (H⁻) and protons (H⁺) on one 4Fe-4S face of the FeMo-cofactor. All hydrogen species are highlighted in red.

Figure 4

Position of α-69^Gly and α-70^Val. Shown are the locations of α-69^Gly and α-70^Val near the FeMo-cofactor, with the van der Waals surface of the surrounding protein shown as mesh.

Figure 5

Standard Gibb’s free energy change diagrams for N₂ and CO₂ reduction. Shown is the standard Gibb’s free energy change (ΔG°) between intermediates for the N₂ (panel A) and CO₂ (panel B) reduction pathways. Also shown are possible energy states for metal bound intermediates (lower dashed traces in both panels). All reaction standard Gibb’s free energy changes were calculated from known standard Gibb’s free energy of formation values [–100].