Geometric comparsion of 379th position of E. coli HpaB with different amino acid residues. a The geometry of Arg379 determined from the published crystal structure of E. coli HpaB (PDB ID: 6 EB0). b Geometry of Cys379 predicted from homology modeling. The brown circle indicates the site where 4HPA and FAD bind, and the purple circle denotes the extra binding region of 4HPA [15, 24]. 4HPA (colored yellow) bindings were identified using the crystal structure of the HpaB–FAD–4HPA complex from T. thermophilus HB8 (PDB ID: 2YYJ). Distance between Ser462 in the C-terminal helical arm to Arg379 or Cys379 were predicted to be 2.70 Å and 4.75 Å, respectively.