Stereochemical aspects of fatty acid oxidation: Hydroperoxide isomerases

Abstract

Lipoxygenases catalyze dioxygenation of polyunsaturated fatts acids to produce fatty-acid hydroperoxides. The reaction involves initial stereospecific abstraction of a hydrogen atom from a bis-allylic methylene group followed by antarafacial attack by dioxygen at one of the terminal carbon atoms of the penudienyl radical. 8(R)-Dioxygenase, recently discovered in the fungus Goeumannomyces graminis, catalyzes formation of 8-trydroperoxy derivatives of linoleic and oleic acids by abstracting one hydrogen from C-8 and inserting dioxygen at the same carbon atom. Isotope-labeling studies show that the configuration at C-8 is inverted during this process. The fungus Saprolegnia parasitica, a fish parasite, contains an ω6-lipoxygenase and an epoxy alcohol synthase. The latter enzyme catalyzes isomerization of fatty acid hydroperoxides into α,β- and γ,δ-epoxy alcohols. Experiments with 18O-labelled hydroperoxides demonstrate that the hydroperoxide → epoxy alcohol conversion consists of intramolecular transfer of the terminal hydroperoxide oxygen to either of the two conjugated double bonds. The reactions proceed with retention of geometrical configuration, i.e. epoxidation of the α,β (E) and γ,δ (Z) double bonds of the parent fatty acid hydroperoxide gives rise to trans and cis epoxides, respectively G. graminis, as well as the marine red alga Gracilariopsis lemaneiformis, contain vicinal diol synthases that catalyze isomerization of fatty-acid hydroperoxides into vicinal dihydroxy fatty acids. Studies using 18O-labelled hydroperoxides show that the hydroperoxide → diol conversions occur by intramolecular transfer of the terminal hydroperoxide oxygen to the vicinal methylene group. Experiments with stereospecifically deuteriated fatty-acid hydroperoxides demonstrate that the intramolecular hydroxylations catalyzed by the two vicinal diol synthases proceed with retention of absolute configuration of the carbon hydroxylated. © Acta Chemica Scandinavica 1996.