Crystal structure of FeMo-co. Fe is show in rust, Mo in magenta, S in yellow, carbide in dark-grey, carbon in gray, N in blue and O in red. The Fe atoms of catalytic 4Fe-4S face are labelled as 2, 3, 6, and 7. Two amino acids, α-70Val and α-195His, that approach the FeMo-co are also shown. The image was created using PDB coordinate 2AFI.