Five ligands of the active site iron atom in soybean lipoxygenase L-l have been identified from the electron density map of the crystallized enzyme. The position of the iron atom can be readily and independently located from an anomalous difference electron density map. The ligands identified are His-499, His-504, His-690, Asn-694, and Ile-839, the carboxy-terminal residue. Our previous view that these three histidines are essential for activity and binding of iron, based on site-specific mutation studies, is confirmed. A sixth protein ligand is not present, and the sixth coordination site opens into a wide cleft. The structure of the soybean lipoxygenase was solved by multiple anomalous isomorphous replacements.
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