X-ray structure of ferredoxin from Peptoniphilus asaccharolyticus.This monomeric 55-residue protein is drawn in ribbon form colored cyan. Its two [4Fe–4S] clusters are drawn in ball-and-stick form with S yellow and Fe orange. The Cys side chains that ligand each Fe atom are drawn in ball-and-stick form with C_β green and S yellow. [Based on an X-ray structure by Elinor Adman, Larry Sieker, and Lyle Jensen, University of Washington. PDBid 1DUR.]