Fatty acid α-dioxygenases
Introduction
Lipoxygenases catalyze primary oxygenation of polyunsaturated fatty acids in plant tissue and the resulting reactive hydroperoxides are further metabolized into an array of oxygenated fatty acids (oxylipins). Attack by pathogens on leaves results in the activation of several enzymes needed for oxylipin biosynthesis including phospholipase, lipoxygenase, allene oxide synthase and divinyl ether synthase, and a number of the oxylipins formed by action of these enzymes play important roles in plants’ defense reactions against pathogens [1], [2], [3]. It was reported in 1998 that tobacco leaves infected with the bacterium Pseudomonas syringae accumulate a 75 kDa oxygenase (“PIOX”, for pathogen-induced oxygenase) showing homology to mammalian prostaglandin endoperoxide synthases-1 and -2 [4]. Studies of the catalytic properties of the tobacco oxygenase (herewith referred to as tobacco α-DOX1) and a homologous oxygenase from Arabidopsis (Arabidopsis α-DOX1) revealed that these enzymes catalyzed a non-lipoxygenase type of fatty acid dioxygenation, i.e. stereospecific oxygenation at the α-carbon of the fatty acid chain to provide unstable 2-hydroperoxy fatty acids, a new class of oxylipins [5]. The present paper will briefly review the biochemistry, molecular biology and functionality of α-dioxygenases from tobacco, Arabidopsis and other plants.
Section snippets
Tobacco and Arabidopsis α-dioxygenases
Incubation of linolenic acid with preparations containing tobacco α-DOX1 or Arabidopsis α-DOX1 led to the formation of a major reaction product which was identified as the long-chain aldehyde 8(Z),11(Z),14(Z)-heptadecatrienal [5]. In addition, small amounts of 2(R)-hydroxylinolenic acid and 8(Z),11(Z),14(Z)-heptadecatrienoic acid were produced. Inspection of the structures of the three compounds suggested an α-hydroperoxide as a common precursor. In agreement with this theory, when tobacco or
α-Dioxygenases belong to a family of heme-containing fatty acid dioxygenases
Database searches have identified several plant proteins showing homology to α-dioxygenases. Thus, the amino acid sequence of tobacco α-DOX1 shares 95% identity with the protein encoded by the Nicotiana attenuata PIOX cDNA [21], 85% with a pepper protein sequence (GenBank accession number AY040869), 84% with the tomato protein encoded by the EST278024, 75% with Arabidopsis α-DOX1 [4], and 63% with a rice α-oxygenase protein [19]. A pea fatty acid oxygenase with homology to tobacco α-DOX1 has
α-Dioxygenases and the hypersensitivity reaction (HR) in plants
Studies of different plant-bacteria interactions showed that the expression of tobacco α-DOX1 was induced in response to the infection with incompatible (Pseudomonas syringae pv syringae) and compatible (P. syringae pv tabaci) bacteria, but that the accumulation of transcripts and α-DOX1 protein appears earlier and reaches higher values when infection promotes a hypersensitive reaction or HR [4]. In a similar manner, the expression of the Arabidopsis α-DOX1 gene was induced earlier in plants
Conclusion
Lipoxygenases and endoperoxide synthases catalyze dioxygenation of polyunsaturated fatty acids to provide unsaturated fatty acid hydroperoxides and prostaglandin endoperoxides, respectively. The α-dioxygenases treated in this review emerge as a third group of enzymes catalyzing primary oxygenation of fatty acids. The products, unstable 2-hydroperoxide derivatives of fatty acids, may either decompose into chain-shortened fatty aldehydes, intermediates in the α-oxidation pathway, or undergo
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