Critical evaluation of a crystal structure of nitrogenase with bound N2 ligands

Recently, a 1.83 angstrom crystallographic structure of nitrogenase was suggested to show N-2-derived ligands at three sites in the catalytic FeMo cluster, replacing the three mu(2) bridging sulfide ligands (two in one subunit and the third in the other subunit) (Kang et al. in Science 368: 1381-1385, 2020). Naturally, such a structure is sensational, having strong bearings on the reaction mechanism of the enzyme. Therefore, it is highly important to ensure that the interpretation of the structure is correct. Here, we use standard crystallographic refinement and quantum refinement to evaluate the structure. We show that the original crystallographic raw data are strongly anisotropic, with a much lower resolution in certain directions than others. This, together with the questionable use of anisotropic B factors, give atoms an elongated shape, which may look like diatomic atoms. In terms of standard electron-density maps and real-space Z scores, a resting-state structure with no dissociated sulfide ligands fits the raw data better than the interpretation suggested by the crystallographers. The anomalous electron density at 7100 eV is weaker for the putative N-2 ligands, but not lower than for several of the mu(3) bridging sulfide ions and not lower than what can be expected from a statistical analysis of the densities. Therefore, we find no convincing evidence for any N-2 binding to the FeMo cluster. Instead, a standard resting state without any dissociated ligands seems to be the most likely interpretation of the structure. Likewise, we find no support that the homocitrate ligand should show monodentate binding.

Automated summary

Using standard crystallographic refinement and quantum refinement, the study evaluated the structure of nitrogenase and found that the original crystallographic raw data are strongly anisotropic. The researchers did not find convincing evidence for N-2 binding to the FeMo cluster or that the homocitrate ligand should show monodentate binding.