Catabolism of fatty acid

Figure 30-19. The analogous first three reactions in the catabolism of leucine, valine, and isoleucine. Note also the analogy of reactions and to reactions of the catabolism of fatty acids (see Figure 22-3). The analogy to fatty acid catabolism continues, as shown in subsequent figures.

Gerhard B (1993) Catabolism of fatty acid acids. In Moore TS (ed) Lipid metabolism in plants. CRS Press, Baton Rouge, p 527... 

Biotin (6.24) consists of an imidazole ring fused to a tetrahydrothiophene ring with a valeric acid side chain. Biotin acts as a co-enzyme for carboxylases involved in the synthesis and catabolism of fatty acids and for branched-chain amino acids and gluconeogenesis. 

FIGURE 17-16 The fatty acids in the endoplasmic reticulum. This alternative to /3 oxidation begins with oxidation of the carbon most distant from the a carbon—the oj (omega) carbon. The substrate is usually a medium-chain fatty acid shown here is lauric acid (laurate). This pathway is generally not the major route for oxidative catabolism of fatty acids... 

The pathways of amino acid catabolism are quite similar in most organisms. The focus of this chapter is on the pathways in vertebrates, because these have received the most research attention. As in carbohydrate and fatty acid catabolism, the processes of amino acid degradation converge on the central catabolic pathways, with the carbon skeletons of most amino acids finding their way to the citric acid cycle. In some cases the reaction pathways of amino acid breakdown closely parallel steps in the catabolism of fatty acids (Chapter 17). 

Pantothenic acid participates as part of coenzvme A in carbohydrate metabolism (2-carbon transfer-acetate, or pyruvate), lipid metabolism (biosynthesis and catabolism of fatty acids, sterols, +phospholipids), protein metabolism (acetylations of amines and amino acids), porphyrin metabolism, acetylcholine production, isoprene production. 

Fatty acids are utilized as fuels by most tissues, although the brain, red and white blood cells, the retina, and adrenal medulla are important exceptions. Catabolism of fatty acids requires extramitochondrial activation, transport into mitochondria, and then oxidation via the /3-oxidative pathway. The initial step is catalyzed by fatty acyl-CoA synthetase (also called thiokinase and fatty acyl-CoA ligase), as shown in Equation (19.5). The product, fatty acyl-CoA, then exchanges the CoA for carnitine, as shown in Equation (19.6) ... 

Figure 11.8. Pathways for the catabolism of free fatty acids in cheese during ripening (reprinted from Cheese Chemistry, Physics and Microbiology, 3rd edn P.F. Fox et al. (eds.), Collins, Y.F., McSweeney, P.L.H., Wilkinson, M.G., Lipolysis and Catabolism of fatty acids in cheese, pp. 373-379, 2004, with permission from Elsevier).

During the catabolism of fatty acids with an odd number of carbon atoms and the amino acids valine, isoleucine and threonine the resultant propionyl-CoA is converted to succinyl-CoA for oxidation in the TCA cycle. One of the enzymes in this pathway, methylmalonyl-CoA miitase, requires vitamin B12 as a cofactor in the conve sion of methylmalonyl-CoA to succinyl-CoA. The 5 -deoxyadenosine derivative of cobalamin is required for this reaction. 

Crockett, E.L. and B.C. Sidell. Peroxisomal /3-oxidation is a significant pathway for catabolism of fatty acids in a marine teleosts. Am. J. Physiol. 264 R1004-R1009, 1993. 

At rest, skeletal muscle utilizes the catabolism of fatty acids and branched-chain amino acids to maintain cellular... 

This reaction is part of the oxidation pathway for catabolism of fatty acids (Figure 18.16). 

FIG. 4. Catabolism of fatty acids by Penicillium roqueforti (modified from Kinsella and Hwang, 1976). 

In plants and in some bacteria, but not in animals, acetyl-GoA can serve as the starting material for the biosynthesis of carbohydrates. Animals can convert carbohydrates to fats, but not fats to carbohydrates. (Acetyl-GoA is produced in the catabolism of fatty acids.) Two enzymes are responsible for the ability of plants and bacteria to produce glucose from fatty acids. Isocitrate lyase cleaves isocitrate, producing glyoxylate and succinate. Malate synthase catalyzes the reaction of glyoxylate with acetyl-GoA to produce malate. 

Even though both the anabolism and the catabolism of fatty acids require successive reactions of two-carbon units, the two pathways are not the exact reversal of each other. The differences between the two pathways are summarized in Table 21.2. The sites in the cell in which various anabolic and catabolic reactions take place are shown in Figure 21.18. 

The pathway of catabolism of fatty acids includes reactions in which unsaturated, as well as saturated, fatty acids can be metabolized. Odd-numbered fatty acids can... 

Provide an overviecv of the synthesis and catabolism of fatty acids by fisting the types of chemical reactions used. 

What common feature is shared by the catabolism of fatty acids having an odd number of carbon atoms and the catabolism of the amino acids isoleucine, methionine, and valine ... 

Catabolism of fatty acids includes the reduction of NAD+ and the sequential splitting of the fatty-acyl chain into 2-carbon acetyl units. A fatty acid enters the oxidative pathway only after it has become linked to the large molecule, coenzyme A (CoA). The abstraction of a hydride ion (H = + 2e ) by NAD+ does not... 

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). 

The first phase in the catabolism of fatty acids involves their release from triglycerides, either those stored in adipose tissue or those ingested from the diet. The hydrolysis of triglycerides is catalyzed by a group of enzymes called lipases. 

Early studies demonstrated that oils and fatty acids stimulated the production of the polyene macrolide antibiotics fungichromin and filipin, but not amphotericin B or candicidin (89). The stimulation polyene antibiotic biosynthesis by oils might be a simple precursor effect. Catabolism of fatty acids results in a increased pool of acetyh QiA. which is subsequently used for polyene biosynthesis. 

The catabolism of fatty acids occurs in the mitochondria through a systematic process called beta-oxidation, whereby 2-carbon fragments are successively chopped from the fatty acid molecule to form acetyl CoA which then enters the Krebs cycle. The term lipolysis indicates mobilization of fats... 

Acetyl CoA is a common point—a crossroad— for many of the biochemical reactions of the body. It can be used for building other substances including (1) new fatty acids and cholesterol, (2) the formation of the ketone body, acetoacetic acid, and (3) the nerve transmitter substance, acetylcholine. It is formed during the catabolism of fatty acids, glucose, and amino acids hence, excesses of these nutrients can be stored as fat. As already pointed out, acetyl CoA can enter the KreErs cycle. However, entry into this cycle is dependent upon continued availability of carbohydrates. 

A variation on the Claisen condensation is an important biochemical reaction responsible for carbon-carbon bond formation in the biosynthesis of fatty acids. Also, a reverse Claisen condensation occurs in the catabolism of fatty acids. We have seen that the base-catalyzed condensation of two car-boxylate esters occurs because the proton a to the carbonyl group is slightly acidic. The a-hydrogen atom in P-keto esters has a pA of about 10.5. This p/f value is too high for P-keto esters to be of much use in biochemical reactions. At pH 7, the ratio of the conjugate base (the enolate anion) to the keto ester is less than 0.001. 

Acetyl CoA is produced by the catabolism of carbohydrates, fats, and certain amino acids. The catabolism of fatty acids predominates over the catabolism of carbohydrates in certain illnesses, such as diabetes. When there is not enough oxaloacetate to react with the available CoA, a Claisen condensation of two acetyl CoA molecules produces acetoacetyl CoA. 

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