CRYSTALLOGRAPHIC DETERMINATION OF THE ACTIVE-SITE IRON AND ITS LIGANDS IN SOYBEAN LIPOXYGENASE L-1

Five ligands of the active site iron atom in soybean lipoxygenase L-1 have been identified from the electron density map of the crystallized enzyme. The position of the iron atom can be readily and independentl y located from an anomalous difference electron density map. The ligan ds identified are His-499, His-504, His-690, Asn-694, and Ile-839, the carboxy-terminal residue. Our previous view that these three histidin es are essential for activity and binding of iron, based on site-speci fic mutation studies, is confirmed. A sixth protein ligand is not pres ent, and the sixth coordination site opens into a wide cleft. The stru cture of the soybean lipoxygenase was solved by multiple anomalous iso morphous replacements.