We investigated the geometry of the iron ligand sphere of the native rabbit 15-lipoxygenase (15-LOX) by X-ray absorption spectroscopy using synchrotron radiation. The soybean LOX-1 was used as a reference compound because its iron ligand sphere is well characterized. For structural information the X-ray absorption spectra were evaluated using the Excurve Program (CCLRC Daresbury Laboratory, Warrington, U.K.). From the positions of the absorption edges and from the intensities of the 1s–3d pre-edge transition peaks a six-coordinate ferrous iron was concluded for the rabbit 15-LOX. Evaluation of the extended region of the absorption spectra suggested six nitrogen and/or oxygen atoms as direct iron ligands, and the following binding distances were determined (means±S.D.; estimated accuracy is ±0.001 nm for bond distances, on the basis of more than 22 X-ray absorption spectra): 0.213±0.001 nm, 0.213±0.001 nm, 0.236±0.001 nm, 0.293±0.001 nm, 0.189±0.001 nm and 0.242±0.001. Lyophilization of the LOX altered the binding distances but did not destroy the octahedral iron ligand sphere. For construction of a structural model of the iron ligand sphere the binding distances extracted from the X-ray spectra were assigned to specific amino acids (His-360, -365, -540, -544 and the C-terminal Ile-662) by molecular modelling using the crystal coordinates of the soybean LOX-1 and of a rabbit 15-LOX–inhibitor complex.

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