Oxygenated fatty acids enzymatic production

Many of the compounds derived from enzyme-catalysed oxidative breakdown of unsaturated fatty acids may also be produced by autoxidation [23]. While the enzymatically produced hydroperoxides in most cases yield one hydroperoxide as the dominant product, non-enzymatic oxidation of unsaturated fatty acids yields a mixture of hydroperoxides which differ in the position of the peroxide group and in the geometrical isomerism of the double bonds [24]. As the number of double bonds increases, the number of oxidation and oxygen-addition sites increases proportionally and thus the number of possible volatile... 

JAs are derived from linolenic acid via an octadecanoid pathway consisting of several enzymatic steps (Figure 36). Multiple compartments in plant cells participate in JA synthesis. The early steps of this pathway occur in chloroplasts, where linolenic acid is converted to OPDA by means of the three enzymes lipoxygenase (LOX), allene oxide synthase (AOS), and allene oxide cyclase (AOC).867-869 Linolenic acid is oxygenated by 13-LOX producing a peroxidized fatty acid 13-hydroperoxylinolenic acid. The product is subsequently metabolized by AOS to an unstable compound allene oxide. Allene oxide is sequentially converted by AOC to produce OPDA. An alternative pathway from another trienoic fatty acid, hexadecatrienoic acid (16 3), is present in chloroplasts.870 In this pathway, dinor OPDA is produced instead of OPDA. OPDA and dinor OPDA are transported into the peroxisome. An ABC transporter involved in this transport was identified in... 

An obstacle in the study of the enzymatic oxygenation of polyunsaturated fatty acids is the fact that these acids can be non-enzymatically transformed to hydroperoxides and many other products [51], Hematin and hematin-containing compounds, e.g. hemoglobin, myoglobin or cytochromes, catalyse the non-enzymatic formation of lipid hydroperoxides. Hydroperoxides may also be formed when unsaturated fatty acids are exposed to air. This autooxidation of polyunsaturated fatty acids is stimulated by light and heat and may eventually lead to complete eonsumption of the fatty acids. 

A variety of oxygenated derivatives is formed during the reaction of lipoxygenases with fatty acid substrates. The biosynthesis of the monohydroxy and dihydroxy acids proceeds via the corresponding hydroperoxy acids, as is the case in the plant systems. The immediate product of the lipoxygenase thus is a hydroperoxy acid. This product is reduced to the corresponding hydroxy acid, presumably both enzymatically by peroxidases and through non-enzymatic decomposition. However, the hydroperoxy acid can also be converted to a ketone or to an epoxy-hydroxy acid that can hydrolyze into a trihydroxy acid (see below). 

Lipid oxidation in foods is a complex chain of reactions that first consist of the introduction of a functional group containing two concatenated oxygen atoms (peroxides) into unsaturated fatty acids, in a free-radical chain reaction, that afterward gives rise to secondary oxidation products. Different pathways for lipid oxidation have been described radical mechanism or autoxidation, singlet oxygen-mediated mechanism or photooxidation, and enzymatic oxidation. 

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