Structural properties of plant and mammalian lipoxygenases. Temperature-dependent conformational alterations and membrane binding ability.

Lipoxygenases form a heterogeneous family of lipid peroxidizing enzymes, which have been implicated in the synthesis of inflammatory mediators, in cell development and in the pathogenesis of various diseases with major health and political relevance (atherosclerosis, osteoporosis). The crystal structures of various lipoxygenase-isoforms have been reported, and X-ray coordinates for enzyme-ligand complexes are also available. Although the 3D-structures of plant and animal lipoxygenase-isoforms are very similar, recent small-angle X-ray scattering data suggested a higher degree of motional flexibility of mammalian isozymes in aqueous solutions. To explore the molecular basis for these differences we performed dynamic fluorescence measurements that allowed us to study temperature-induced conformational changes arising from three-dimensional fluctuations of the protein matrix. For this purpose, we first investigated the impact of elevated temperature on activity, secondary structure, tertiary structure dynamics and conformational alterations. Applying fluorescence resonance energy transfer we also tested the membrane binding properties of the two lipoxygenase-isoforms, and compared their binding parameters. Taken together, our results indicate that the rabbit 12/15-lipoxygenase is more susceptible to temperature-induced structural alterations than the soybean enzyme. Moreover, the rabbit enzyme exhibits a higher degree of conformational flexibility of the entire protein molecule (global flexibility) and offers the possibility of augmented substrate movement at the catalytic center (local flexibility).