X-Ray absorption spectroscopic studies on iron in soybean lipoxygenase: A model for mammalian lipoxygenases†

X-ray absorption spectroscopy studies are contributing both to the refinement of the geometry of metal sites already known from crystallographic studies and the elucidation of the coordination sphere in metal sites in metalloproteins where only limited information was available. Specific problems in the analysis of coordinating imidazoles can be overcome by applying multiple-scattering simulations. The model for the lipoxygenase iron site derived from the EXAFS study, a coordination sphere predominated by imidazoles, may require some adjustment in view of the possibility of coordination by water, and has received independent confirmation from studies of amino acid sequence homologies and the interaction with catechols. The spectroscopy of lipoxygenase is not consistent with the presence of the organic cofactor, PQQ, nor with its coordination to iron, but would be consistent with the presence, but not the coordination to iron, of the carboxy-hydroxy-indoloquinone-glutamic acid cofactor, CHIQG. From the observed amino acid sequence homologies it is concluded that plant lipoxygenases can be good models for the studies of mammalian lipoxygenases.