Hamberg M
Department of Medical Biochemistry and Biophysics, Karolinska Institutet, Stockholm, Sweden.
Acta Chem Scand (Cph). 1996 Mar;50(3):219-24. doi: 10.3891/acta.chem.scand.50-0219.
Lipoxygenases catalyze dioxygenation of polyunsaturated fatty 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 pentadienyl radical. 8(R)-Dioxygenase, recently discovered in the fungus Gaeumannomyces graminis, catalyzes formation of 8-hydroperoxy 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 omega 6-lipoxygenase and an epoxy alcohol synthase. The latter enzyme catalyzes isomerization of fatty acid hydroperoxides into alpha,beta- and gamma,delta-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 alpha,beta (E) and gamma,delta (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.
脂氧合酶催化多不饱和脂肪酸的双加氧反应，生成脂肪酸氢过氧化物。该反应首先涉及从双烯丙基亚甲基立体特异性地提取一个氢原子，随后双原子氧在戊二烯基自由基的一个末端碳原子上进行异面进攻。最近在禾顶囊壳菌中发现的8(R)-双加氧酶，通过从C-8提取一个氢原子并在同一碳原子上插入双原子氧，催化亚油酸和油酸的8-氢过氧化物衍生物的形成。同位素标记研究表明，在此过程中C-8的构型发生了反转。寄生水霉，一种鱼类寄生虫，含有ω-6脂氧合酶和环氧醇合酶。后一种酶催化脂肪酸氢过氧化物异构化为α,β-和γ,δ-环氧醇。用18O标记的氢过氧化物进行的实验表明，氢过氧化物向环氧醇的转化包括末端氢过氧化物氧向两个共轭双键之一的分子内转移。反应以几何构型保持进行，即母体脂肪酸氢过氧化物的α,β (E)和γ,δ (Z)双键的环氧化分别产生反式和顺式环氧化物。禾顶囊壳菌以及海洋红藻龙须菜含有邻二醇合酶，可催化脂肪酸氢过氧化物异构化为邻二羟基脂肪酸。使用18O标记的氢过氧化物的研究表明，氢过氧化物向二醇的转化是通过末端氢过氧化物氧向邻亚甲基的分子内转移发生的。用立体特异性氘代脂肪酸氢过氧化物进行的实验表明，两种邻二醇合酶催化的分子内羟基化反应在羟基化碳的绝对构型保持的情况下进行。
