Evidence for interstitial carbon in nitrogenase FeMo cofactor

Abstract

The identity of the interstitial light atom in the center of the FeMo cofactor of nitrogenase has been enigmatic since its discovery. Atomic-resolution x-ray diffraction data and an electron spin echo envelope modulation (ESEEM) analysis now provide direct evidence that the ligand is a carbon species.

Figure 1

Carbon in the center of the FeMo cofactor. A) The [Mo:7Fe:9S:C]:homocitrate FeMo cofactor. B) Average electron density in a sphere of given radius for all carbon (black), nitrogen (blue) and oxygen (red) atoms in the structure. The two central atoms in the asymmetric unit (green) closely follow the trace for carbon. C) Electron density at the atomic positions (ρ₀) vs. residual B-factor shows C, N, and O to occupy distinct areas. The central atoms (green) fall within the carbon area. D) X-band 3-pulse ESEEM frequency domain spectra of WT (green), U-¹⁵N (blue) and U-¹³C (black) labeled nitrogenase. The inset depicts ¹³C resonances recorded with two different τ values.