Oxidative degradation of fatty acids

The reverse of the above reaction is a key step in the oxidative degradation of fatty acids. This reverse Claisen condensation (catalyzed by thiolase) involves the cleavage of a carbon-carbon bond of a /3-keto ester of coenzyme A by another molecule of coenzyme A to give a new acyl derivative (RCO—SCoA) and ethanoyl (acetyl) derivative (CH3CO—SCoA) ... 

Peroxide value (PV, also referred to as initial peroxide) The presence of fatty acid hydroperoxides, formed by the oxidative degradation of fatty acids, is a measure of oxidative abuse and degradation of the lipid. Products generated by hydroperoxide degradation will confer a rancid note in edible applications. 

Beta-oxidation (P-oxidation). Oxidative degradation of fatty acids that occurs by the successive oxidation of the Tj-carbon atom. 

Fortifying foods with minerals and vitamins is becoming more and more common. Mineral deficiency is one of the most important nutritional problems in the world. The best method to overcome this problem is to make use of an external supply, which may be nutritional or supplementary, like the fortification of foods with highly bioavailable mineral sources. Major interests of mineral encapsulation are linked to the fact that this technique enables to reduce mineral reactions with other ingredients, when they are added to dry mixes to fortify a variety of foods, and it can also incorporate time-release mechanisms of the minerals into the formulations. For example, iron is the most difficult mineral to add to foods and ensure adequate absorption, and iron bioavailability is severely affected by interactions with food ingredients (e.g., tannins, phytates, and polyphenols). Additionally, iron catalyses the oxidative degradation of fatty acids and vitamins (Schrooyen et al., 2001). 

The leafy or grass-like smell of most green leaves is due to the presence of a series of aldehydes and alcohols derived by oxidative degradation of fatty acids, (.see Fig. 2.23.) The relative proportion of these compounds varies among plant species and often seasonally within the same species. The composition of mixtures of these compounds also is affected by aging and injury. 

The oxidation/reduction reactions that require one of the nicotinamide coenzymes are everywhere in metabolism in the glycolytic pathway, the citric acid cycle, the synthesis and degradation of fatty acids, the synthesis of steroids, and so on. Certain of... 

The most important process in the degradation of fatty acids is p-oxidation—a metabolic pathway in the mitochondrial matrix (see p. 164). initially, the fatty acids in the cytoplasm are activated by binding to coenzyme A into acyl CoA [3]. Then, with the help of a transport system (the carnitine shuttle [4] see p. 164), the activated fatty acids enter the mitochondrial matrix, where they are broken down into acetyl CoA. The resulting acetyl residues can be oxidized to CO2 in the tricarboxylic acid cycle, producing reduced... 

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. 

Scheme 7.1 Enzymatic degradation of fatty acids by the j -oxidation cycle and formation of various types of aroma compounds in fruits and vegetables...

The flavour of distillates from apple and pear is characterised by typical aroma compounds from these fruits formed by enzymatic degradation of fatty acids to C6-fragments like hexanol, trans-2-hexenol, as well as ethyl esters and acetates of hexanoic acid. In distillates of pears, especially of the variety Bartlett pear, the characteristic pear flavour is mainly dominated by the ethyl and methyl esters of frans-2-czs-4-decadienoic acid and trans-2-trans-A-decadienoic acid [27-29], The biogenesis of these monounsaturated, diunsaturated, and triunsaturated esters may be explained by -oxidation of unsaturated linoleic and linolenic acid in the fruits. The sesquiterpene compound a-farnesene, which is formed during postharvest ripening and storage of Bartlett pears [28], shows that quality and intensity of distilled pear spirits is mainly influenced by the quality and degree of ripeness of the fruits. 

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). 

The degradation of fatty acids occurs by an oxidation process in the mitochondria. The breakdown of the 16-carbon saturated fatty acid, palmitate, occurs in blocks of two carbon atoms by a cyclical process. The active substrate is the acyl-CoA derivative of the fatty acid. Each cycle involves four discrete enzymatic steps. In the process of oxidation the energy is sequestered in the form of reduced coenzymes of FAD and NAD+. These reduced coenzymes lead to ATP production through the respiratory chain. The oxidation of fatty acids yields more energy per carbon than the oxidation of glucose because saturated fatty acids are in the fully reduced state. 

The oxidative phosphorylation system contains over 80 polypeptides. Only 13 of them are encoded by mtDNA, which is contained within mitochondria, and all the other proteins that reside in the mitochondrion are nuclear gene products. Mitochondria depend on nuclear genes for the synthesis and assembly of the enzymes for mtDNA replication, transcription, translation, and repair (Tl). The proteins involved in heme synthesis, substrate oxidation by TCA cycle, degradation of fatty acids by /i-oxidalion, part of the urea cycle, and regulation of apoptosis that occurs in mitochondria are all made by the genes in nuclear DNA. 

Oxidation The individual reactions involved in the degradation of fatty acids by (3-... 

Figure 22.9 Reaction sequence for the degradation of fatty acids. Fatty acids are degraded by the repetition of a four-reaction sequence consisting of oxidation hydration, oxidation, and thiolysis.

Their discovery can be traced to the classical investigations of Knoop (1904). By feeding phenoxyalkanoic acids to dogs, he established that the degradation of fatty acids in the animal organism begins with oxidation at the carbon atom in the -position with respect to the carboxy radical, and terminates with decomposition... 

Degradation of fatty acids proceeds via an inducible set of enzymes that catalyze the pathway of P-oxidation [18]. P-Oxidation occurs via repeated cycles of reactions that are essentially the reverse of the reactions of fatty acid synthesis (Fig. 8). However, three major differences distinguish the two pathways. First, P-oxidation utilizes acyl-CoA thioesters and not acyl-ACPs. Second, the P-hydroxy intermediates have the opposite stereochemistry (L in P-oxidation and d in synthesis). Finally, the enzymes of P-oxidation share no homology with those of synthesis. 

Peroxisomes oxidize fatty acids that have more than 18 carbons and reduce their lengths to C18. The shorter chains are better substrates for (3 oxidation in the mitochondria. Therefore, clofibrate probably aids the degradation of fatty acids generally and thereby will lower the level of triglycerides. 

The catabolism of fatty acids begins in the cytoplasm, where they are activated by combining with CoA—SH. After being transported into mitochondria, the degradation of fatty acids occurs by the 3-oxidation pathway. The P-oxidation pathway produces reduced coenzymes (FADH2 and NADH) and cleaves the fatty acid chain into two-carbon fragments bound to coenzyme A (acetyl CoA). 

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