Structural and thermochemical characterization of lipoxygenase-catechol complexes

A complex between native, iron(II) soybean lipoxygenase 3 and 4-nitrocatech ol, a known inhibitor of the enzyme, has been detected by isothermal titrat ion calorimetry and characterized by X-ray crystallography. The compound mo ors in the central cavity of the protein close to the essential iron atom, but not in a bonding arrangement with it. The iron ligands experience a sig nificant rearrangement upon formation of the complex relative to their posi tions in the native enzyme; a water molecule becomes bound to iron in the c omplex, and one histidine ligand moves away from the iron to become involve d in a hydrogen bonding interaction with the catechol. These changes in pos ition result in a trigonal pyramid coordination geometry for iron in the co mplex. Molecular modeling and force field calculations predict more than on e stable complex between 4-nitrocatechol and the central cavity of lipoxyge nase 3, but the interaction having the small molecule in the same orientati on as the one found in the crystal structure was the most favorable. These observations reveal specific details of the interaction between lipoxygenas e and a small molecule and raise the possibility that changes in the ligand environment of the iron atom could be a feature of the product activation reaction or the catalytic mechanism.