Proton-coupled electron transfer of unsaturated fatty acids and mechanistic insight into lipoxygenase

A proton-coupled electron transfer (PCET) process plays an important role in the initial step of lipoxygenases to produce lipid radicals which can be oxygenated by reaction with O₂ to yield the hydroperoxides stereoselectively. The EPR spectroscopic detection of free lipid radicals and the oxygenated radicals (peroxyl radicals) together with the analysis of the EPR spectra has revealed the origin of the stereo- and regiochemistry of the reaction between O₂ and linoleyl (= (2Z)-10-carboxy-1-[(1Z)-hept-1- enyl]dec-2-enyl) radical in lipoxygenases. The direct determination of the absolute rates of H-atom-transfer reactions from a series of unsaturated fatty acids to the cumylperoxyl (= (1-methyl-1-phenylethyl)dioxy) radical by use of time-resolved EPR at low temperatures together with detailed kinetic investigations on both photoinduced and thermal electron-transfer oxidation of unsaturated fatty acids provides the solid energetic basis for the postulated PCET process in lipoxygenases. A strong interaction between linoleic acid (=(9Z,12Z)-octadeca-9,12-dienoic acid) and the reactive center of the lipoxygenases (Fe^III-OH) is suggested to be involved to make a PCET process to occur efficiently, when an innersphere electron transfer from linoleic acid to the Fe^III state is strongly coupled with the proton transfer to the OH group.