title: Manganese lipoxygenase oxidizes bis-allylic hydroperoxides and octadecenoic acids by different mechanisms.
abstract: Manganese lipoxygenase (MnLOX) oxidizes (11R)-hydroperoxylinolenic acid (11R-HpOTrE) to a peroxyl radical. Our aim was to compare the enzymatic oxidation of 11R-HpOTrE and octadecenoic acids with LOO-H and allylic C-H bond dissociation enthalpies of ~88 and ~87kcal/mol. Mn(III)LOX oxidized (11Z)-, (12Z)-, and (13Z)-18:1 to hydroperoxides with R configuration, but this occurred at insignificant rates (<1%) compared to 11R-HpOTrE. We next examined whether transitional metals could mimic this oxidation. Ce(4+) and Mn(3+) transformed 11R-HpOTrE to hydroperoxides at C-9 and C-13 via oxidation to a peroxyl radical at C-11, whereas Fe(3+) was a poor catalyst. Our results suggest that MnLOX oxidizes bis-allylic hydroperoxides to peroxyl radicals in analogy with Ce(4+) and Mn(3+). The enzymatic oxidation likely occurs by proton-coupled electron transfer of the electron from the hydroperoxide anion to Mn(III) and H(+) to the catalytic base, Mn(III)OH(-). Hydroperoxides abolish the kinetic lag times of MnLOX and FeLOX by oxidation of their metal centers, but 11R-HpOTrE was isomerized by MnLOX to (13R)-hydroperoxy-(9Z,11E,15Z)-octadecatrienoic acid (13R-HpOTrE) with a kinetic lag time. This lag time could be explained by two competing transformations, dehydration of 11R-HpOTrE to 11-ketolinolenic acid and oxidation of 11R-HpOTrE to peroxyl radical; the reaction rate then increases as 13R-HpOTrE oxidizes MnLOX with subsequent formation of two epoxyalcohols. We conclude that oxidation of octadecenoic acids and bis-allylic hydroperoxides occurs by different mechanisms, which likely reflect the nature of the hydrogen bonds, steric factors, and the redox potential of the Mn(III) center.
authors.affiliation: Department of Pharmaceutical Biosciences, Uppsala University, SE751 24 Uppsala, Sweden. Ernst.Oliw@farmbio.uu.se
authors.forename: Ernst H
authors.initials: EH
authors.lastname: Oliw
authors.forename: Fredrik
authors.initials: F
authors.lastname: Jernerén
authors.forename: Inga
authors.initials: I
authors.lastname: Hoffmann
authors.forename: Margareta
authors.initials: M
authors.lastname: Sahlin
authors.forename: Ulrike
authors.initials: U
authors.lastname: Garscha
mesh_terms: D000818:Animals; D002384:Catalysis; D020134:Catalytic Domain; D008054:Lipid Peroxides / Q000737:chemistry*; D008084:Lipoxygenase / Q000737:chemistry*; D008297:Male; D008345:Manganese / Q000737:chemistry; D010084:Oxidation-Reduction; D012756:Sheep; D013229:Stearic Acids / Q000737:chemistry*
chemical_list: C114874:5-octadecenoic acid; D008054:Lipid Peroxides; D013229:Stearic Acids; D008345:Manganese; C112608:manganese lipoxygenase; D008084:Lipoxygenase