Unsaturated fatty acids degradation

In the area of moleculady designed hot-melt adhesives, the most widely used resins are the polyamides (qv), formed upon reaction of a diamine and a dimer acid. Dimer acids (qv) are obtained from the Diels-Alder reaction of unsaturated fatty acids. Linoleic acid is an example. Judicious selection of diamine and diacid leads to a wide range of adhesive properties. Typical shear characteristics are in the range of thousands of kilopascals and are dependent upon temperature. Although hot-melt adhesives normally become quite brittle below the glass-transition temperature, these materials can often attain physical properties that approach those of a stmctural adhesive. These properties severely degrade as the material becomes Hquid above the melt temperature. 

The main pathways of the breakdown of fatty acids in biological systems involve oxidation at various points along the chain or oxidation at certain double bonds of specific unsaturated fatty acids (Coultate, 2001 90-98). The main forms of oxidation are termed a-, (3- and to-. They are named depending on which carbon of the chain is attacked. Of these (3-oxidation is the most general and prevalent. Degradation proceeds by the liberation of two-carbon (acetyl-CoA) fragments from the chain. The enzymes responsible for oxidation are widely found in plants, animals and micro-organisms. 

Flavor is one of the major characteristics that restricts the use of legume flours and proteins in foods. Processing of soybeans, peas and other legumes often results in a wide variety of volatile compounds that contribute flavor notes, such as grassy, beany and rancid flavors. Many of the objectionable flavors come from oxidative deterioration of the unsaturated lipids. The lipoxygenase-catalyzed conversion of unsaturated fatty acids to hydroperoxides, followed by their degradation to volatile and non-volatile compounds, has been identified as one of the important sources of flavor and aroma components of fruits and vegetables. An enzyme-active system, such as raw pea flour, may have most of the necessary enzymes to produce short chain carbonyl compounds. 

Vitamin E in the diet is known to provide valuable antioxidant properties for humans, preventing the destruction of cellular materials, e.g. unsaturated fatty acids in biological membranes, and also helping to prevent heart disease. Other materials are similarly known to have beneficial antioxidant properties, and we are encouraged to incorporate sufficient levels of antioxidant-rich foods into our diets to minimize the risks of cardiovascular disease, cell degradation, and cancer. 

Most fatty acids are saturated and even-numbered. They are broken down via p-oxidation (see p.l64). In addition, there are special pathways involving degradation of unsaturated fatty acids (A), degradation of fatty acids with an odd number of C atoms (B), a and ro oxidation of fatty acids, and degradation in peroxisomes. 

Unsaturated fatty acids usually contain a cis double bond at position 9 or 12—e.g., linoleic acid (18 2 9,12). As with saturated fatty acids, degradation in this case occurs via p-oxida-tion until the C-9-ds double bond is reached. Since enoyl-CoA hydratase only accepts substrates with trans double bonds, the corresponding enoyl-CoA is converted by an iso-merase from the ds-A, cis- A isomer into the trans-A, cis-A isomer [1]. Degradation by p-oxidation can now continue until a shortened trans-A, ds-A derivative occurs in the next cycle. This cannot be isomerized in the same way as before, and instead is reduced in an NADPH-dependent way to the trans-A compound [2]. After rearrangement by enoyl-CoA isomerase [1 ], degradation can finally be completed via normal p-oxidation. 

Raw potato possesses little aroma. Approximately 50 compounds have been reported to contribute to raw potato aroma. Raw potatoes have a high content of LOX, which catalyses the oxidation of unsaturated fatty acids into volatile degradation products (Scheme 7.2) [187]. These reactions occur as the cells are disrupted, e.g. during peeling or cutting. Freshly cut, raw potatoes contain ( ,Z)-2,4-decadienal, ( ,Z)-2,6-nonadienal, ( )-2-octenal and hexanal, which are all products of LOX-initiated reactions of unsaturated fatty acids [188,189]. It is reported that two compounds represent typical potato aroma in raw potato methional and ( ,Z)-2,6-nonadienal [189]. Other important volatiles in raw potatoes produced via the LOX pathway are l-penten-3-one, heptanal, 2-pen-tyl furan, 1-pentanol and ( , )-2,4-heptadienal [189]. Pyrazines such as 3-iso-propyl-2-methoxypyrazine could be responsible for the earthy aroma of potato [35]. Some of the most important character-impact compounds of raw potatoes are summarised in Table 7.8. Aroma compounds from cooked, fried and baked potatoes have previously been reviewed [35]. 

Dehydrogenation Oxidation of the products formed in the above reaction yields a-p-unsaturated acyl CoA derivatives. This reac tion is analagous to the dehydrogenation described in the p-oxidation scheme of fatty acid degradation (see p. 190). 

Fatty Acid Oxidation Yields Large Amounts of ATP Additional Enzymes Are Required for Oxidation of Unsaturated Fatty Acids in Mitochondria Ketone Bodies Formed in the Liver Are Used for Energy in Other Tissues Summary of Fatty Acid Degradation Biosynthesis of Saturated Fatty Acids... 

Unsaturated fatty acids such as oleoyl-CoA can also be degraded to acetyl-CoA in mitochondria. Three cycles of /3 oxidation result in an acyl-CoA (A3-cis-dodecenoyl-CoA) that is not a substrate for acyl-CoA dehydrogenase. This problem is circumvented by isomerization of the double bond to the t -tmns position by enoyl-CoA isomerase. Complete oxidation of the remainder of the molecule by the enzymes of f3 oxidation is now possible. 

Organic components of polishes are widespread. Thus most products emit complex VOC mixtures that may consist of alkanes, various alcohols, acetates, C2-C4-benzenes, terpenes and derivatives of naphthalene. This is illustrated by the range of compounds emitted by a furniture polish (Figure 15.2b) and a shoe polish (Figure 15.2c). Many modern floor waxes are based on natural ingredients like alkyd resins. On oxidative degradation of unsaturated fatty acids, volatile aliphatic aldehydes (C5-C11) with unpleasant smell (Ruth, 1986) are formed and the emission rates may remain at high levels over months and even years (Salthammer, 1999). 

Unsaturated fatty acids require the action of additional enzymes in order to be completely degraded by (3-oxidation. 

The yield of ATP is reduced slightly for unsaturated fatty acids, since the additional metabolic reactions which enable them to be degraded by the (3-oxidation pathway either involve using NADPH or bypass an FADH2-producing reaction (see Fig. 4). 

The short chain fatty acids (C to. ) detected in plain cashews could be detected in roasted samples also, in higher amounts. Dodecenoic acid was the only unsaturated fatty acid among them. These fatty acids are formed by the degradation of glycerides by mild enzymatic action or hydrolysis. 

The volatiles from cooked meat contain large numbers of aliphatic compounds including aldehydes, alcohols, ketones, hydrocarbons and acids. These are derived from lipids by thermal degradation and oxidation (J7) and many may contribute to desirable flavor. In addition, the aldehydes, unsaturated alcohols and ketones produced in these reactions, as well as the parent unsaturated fatty acids, are reactive species and under cooking conditions could be expected to interact with intermediates of the Maillard reaction to produce other flavor compounds. 

Ecostar (St. Lawrence Starch Company). This product associates PE with a mixture of starch and auto-oxidant unsaturated fatty acids. The global content of starch is between 6 and 15%. The degradation process then follows two mechanisms in the first, the starch is fragmented, then assimilated by microorganisms, whereas in the second, the interaction between the auto-oxidants and the metallic complexes from soil or water gives peroxides that attack the synthetic polymer chains. 

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