Peroxygenase pathway

Blee, E. Phytooxylipins The Peroxygenase Pathway. In Lipoxygenase and Lipoxygenase Pathway Enzymes. Piazza, G.J. Ed. AOCS Press Champaign, IL, 1996 pp. 138—161. 

Since AA is only a minor fatty acid in higher plants, eicosanoids are not of major importance for plant physiology. However, the oxygenation metabolites of linoleic acid and a-linolenic acid, called oxylipins [5,6], do play a role in plant defence reactions, in the formation of phytohormones and in the synthesis of cutin monomers [6,40-43]. Oxylipins constitute a family of lipids that are formed from free fatty acids by a cascade of reactions involving at least one step of dioxygen-dependent oxidation. The biosynthesis of oxylipins proceeds via a large number of metabolic pathways, most of which involve an unsaturated hydroperoxy fatty acid as intermediate (Scheme 10). Conversion of the hydroperoxide via the peroxide lyase pathway, the allene oxide pathway and the recently discovered peroxygenase pathway, leads to a complex pattern of oxidized lipid mediators. 

The epoxy compounds formed via the peroxygenase reaction may then be hydrolysed by epoxide hydrolases that are an integral part of the peroxygenase pathway, forming di- or trihydroxy fatty acids. This pathway is of importance for plant defence as it is hkely to be involved in the biosynthesis of cutin monomers, in the production of antifungal compounds and in detoxification mechanisms [6]. 

Blee, E. (1996) Phytooxylipins the peroxygenase pathway. In Lipoxygenase and Lipoxygenase Pathways (Piazza, G., ed.), pp. 138-161, AOCS Press, Champaign, IL... 

Blee, E. and Schuber, F. Biosynthesis of cutin monomers involvement of a lipoxygenase/ peroxygenase pathway, The Plant Journal 4(1) (1993), 113-123... 

Oxylipins are commonly found metabolites in higher plants and most of them originate from polyunsaturated fatty acid hydroperoxides by enzymatic transformations which have been extensively studied (for recent reviews, see 1,2). Two well characterized enzymes a lyase and an allene oxide synthase were shown to degrade hydroperoxides into compounds of physiological importance since they can ultimately yield fragrances or plant hormones such as jasmonic acid. We have recently reported a new fate for fatty acid hydroperoxides the peroxygenase pathway. It involves two enzyme activities i.e. a peroxygenase and an epoxide hydrolase which lead to the formation of epoxidized fatty acids and their derived dihydrodiols which are relevant to plant defense mechanisms. 

Figure 1 Stereochemical features of compounds formed from oleic acid by the peroxygenase pathway...

The P450 shunt pathway and peroxygenase activity of peroxidases share identical overall reaction equations. P450s generally have high Km values for H 202 values of 15 mM... 

Figure 1. Monooxygenase (peroxygenase) versus peroxidase pathways for reaction of the ferryl species ([Fe=0]" "3) with a substrate RH.

An obvious way to find out about the physiological function of an enzyme is to study the fate of the products that result from the reaction catalysed by the enzyme. The primary products of the lipoxygenase reaction, fatty-acid hydroperoxides, are potentially dangerous and should be quickly metabolized. Two major routes for metabolizing lipoxygenase products have been identified, collectively known as the lipoxygenase pathway [176]. This was recently extended by the discovery of the peroxygenase cascade [177]. 

Hydroperoxide-dependent peroxygenase (epoxygenase). This enzyme, detected in soybeans [7] and broad beans [8], controls the fatty acid hydroperoxide-dependent epoxidation of unsaturated fatty acids. Hydroperoxide molecule plays a role of oxygen donor for epoxidation. This pathway was proposed as the source of natural epoxides of linoleate, vemolic and coronaric acids, occurring in different amounts in many seed oils. 

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