Ligand induced formation of transient dimers of mammalian 12/15-lipoxygenase: A key to allosteric behavior of this class of enzymes?

Mammalian lipoxygenases (LOXs) have been implicated in cellular defense response and are important for physiological homeostasis. Since their discovery lipoxygenases have been believed to function as monomeric enzymes which exhibit allosteric properties. In aqueous solutions the rabbit 12/15-LOX is mainly present as hydrated monomer but changes in the local physiochemical environment suggested a monomer-dimer equilibrium. Since the allosteric character of the enzyme can hardly be explained employing a single ligand binding site model we proposed that the binding of allosteric effectors may shift the monomer-dimer equilibrium towards dimer formation. To test this hypothesis we explored the impact of an allosteric effector [13(S)-HODE] on the structural properties of rabbit 12/15-LOX by small angle X-ray scattering (SAXS). Our data indicate that the enzyme undergoes ligand-induced dimerization in aqueous solution and molecular dynamics simulations suggested that lipoxygenase dimers may be stable in the presence of substrate fatty acids. These data provide direct structural evidence for the existence of lipoxygenase dimers, where two non-covalently linked enzyme molecules might work in unison and, therefore, such mode of association might be related to the allosteric character of 12/15-LOX. Introduction of negatively charged residues (W181E+H585E and L183E+L192E) at the inter-monomer interface disturbs the hydrophobic dimer interaction of the wild-type LOX and this structural alteration may lead to functional distortion of mutant enzymes.