CHANGES IN THE IRON COORDINATION SPHERE OF FE(II) LIPOXYGENASE-1 FROMSOYBEANS UPON BINDING OF LINOLEATE OR OLEATE

Fe K-edge X-ray absorption spectra of the non-heme iron constituent of lipoxygenase-1 from soybeans were obtained. The spectrum of 2.5 mM Fe (II) lipoxygenase, mixed with 1.2 M linoleate in the absence of O-2, w as compared to the spectrum of the native (i.e. untreated) enzyme. In the lipoxygenase-linole ate complex, an edge shift to lower energy was observed. This indicated that the iron-ligand distances in this compl ex are slightly longer than those in the untreated enzyme species. The extended X-ray absorption fine strucutre spectrum of Fe(II) lipoxygen ase, prepared by anaerobic reduction of 2.5 mM Fe(III) lipoxygenase wi th 1.2 M linoleate, was very similar to the spectrum of the anaerobic lipoxygenase-linoleate complex. We conclude that the conformational di fferences between the iron coordination spheres of native and cycled F e(II) lipoxygenase must be ascribed to the presence of linoleate, and wt to changes in the enzyme that occur only after one cycle of oxidati on and reduction. Furthermore, spectra of 2.5 mM Fe(II) lipoxygenase m ixed with 1.2 M oleate, either in the absence or in the presence of O- 2, were also identical to the spectrum of the Fe(II) lipoxygenase-lino leate complex. This finding is in agreement with our observation that oleate is a competitive inhibitor of the lipoxygenase reaction. Moreov er, the similarity of the lipoxygenase-oleate complexes in the presenc e and absence of O-2 excludes the possibility that O-2 binding to the iron cofactor is induced upon binding of a fatty acid to lipoxygenase.