Comparison of the active site (left) and the electron transfer pathways (right) between: (A) E. coli NarGH (blue, PDB ID: 1R27) and NarGHI (orange, PDB ID 1Q16). (B) C. necator NapAB (blue, PDB ID: 3ML1), R. sphaeroides NapAB (orange, PDB ID: 1OGY), E. coli NapA (pink, PDB ID: 2NYA), and D. desulfuricans NapA (red, PDB ID: 2NAP), showing the respectively Mo ligands and the nearest [4Fe–4S] center. The electron transfer pathway also includes conserved residues (Arg94 in Nar and Lys56 in Nap, respectively) which possible role is to mediate the electron transfer between the pterin (via its exocyclic NH2) and the FeS center.