Fat catabolism

Cascara, pentosan content, V, 271 Catabolism, fat, II, 150, 151 Catalase, theory of action on ethyl hydroperoxide, V, 53... 

A surgical operation or any form of physical injury in a previously healthy person initiates a series of metabolic changes in which protein tissue is catabolized, fat is oxidized, and water and salt are retained (BIO, C13). The endocrine system responds at the same time with an increased secretion rate and altered tissue utilization of many hor-... 

In the first stage of catabolism, fats, carbohydrates, and proteins are hydrolyzed to fatty acids, monosaccharides, and amino acids. 

We have seen that in the first two stages of catabolism, fats, carbohydrates, and proteins are converted to compounds that can enter the citric acid cycle (Section 25.5). Here we will look at the reactions that allow fats to enter the citric acid cycle. 

FIGURE 18.9 Comparison of the state of rednction of carbon atoms in biomolecnles —CH9— (fats) > —CHOH— (carbohydrates) > —C=0 (carbonyls) > —COOH (carboxyls) >C09 (carbon dioxide, the final product of catabolism). 

The initial stages of catabolism result in the conversion of both fats and carbohydrates into acetyl groups that are bonded through a thioester link to coenzyme A. Acetyl CoA then enters the next stage of catabolism—the citric acid cycle, also called the tricarboxylic acid (TCA) cycle, or Krebs tycle, after Hans Krebs, who unraveled its complexities in 1937. The overall result of the cycle is the conversion of an acetyl group into two molecules of C02 plus reduced coenzymes by the eight-step sequence of reactions shown in Figure 29.12. 

The catabolism of proteins is much more complex than that of fats and carbohydrates because each of the 20 amino acids is degraded through its own unique pathway. The general idea, however, is that the amino nitrogen atom is removed and the substance that remains is converted into a compound that enters the citric acid cycle. 

Diels-Alder reaction, 493 El reaction, 391-392 ElcB reaction, 393 E2 reaction, 386 Edman degradation, 1032 electrophilic addition reaction, 147-148. 188-189 electrophilic aromatic substitution, 548-549 enamine formation, 713 enol formation, 843-844 ester hydrolysis, 809-811 ester reduction, 812 FAD reactions. 1134-1135 fat catabolism, 1133-1136 fat hydrolysis, 1130-1132 Fischer esterification reaction, 796 Friedel-Crafts acylation reaction, 557-558... 

Figure 15-1. Outline of the pathways for the catabolism of dietary carbohydrate, protein, and fat. All the pathways lead to the production of acetyl-CoA, which is oxidized in the citric acid cycle, ultimately yielding ATP in the process of oxidative phosphorylation.

These data on fasting ketonuria in man as well as the earlier described tests on the exogenous and endogenous ketonuria of rats indicate that marked changes in ketonuria can occur after the administration of carbohydrate in quantities too small to decrease significantly the fat catabolism. It is difficult to harmonize such results with the antiketo-genesis theory. 

One suggestion to explain this discrepancy is that two pathways of fat catabolism are available and that ketone body formation is the resultant of only one type of breakdown.177 This latter type, also called the indirect fat utilization, 182 occurs in the liver the catabolism of fat in the muscle, called the -direct method, either involves no ketogenesis or the ketone bodies are immediately utilized and no accumulation occurs. 

We should note at this point that the TCA cycle is more than just a means of producing NADH for oxidative phosphorylation. The pathway also provides a number of useful intermediates for other, often synthetic, pathways. For example, citrate is the starting substance for fat synthesis (Chapter 9) succinyl-CoA is required for haem production and 2-oxoglutarate and oxaloacetate in particular are involved with amino acid and pyrimidine metabolism. Pathways which have dual catabolic/anabolic functions are referred to as amphibolic . 

Fat is the most important energy reserve in the human body. The energy is realized by conversion of the fats to fatty acids and glycerol and their oxidation to carbon dioxide and water. Let s see the consequences of these catabolic conversions. 

Fat synthesis The acetyl-CoA prodnced from amino acid catabolism is also a precursor for fatty acid and triacyl-glycerol synthesis, both in adipose tissne and liver (details of pathways are given in Chapter 11). Unfortnnately, the quantitative significance of this pathway is not known. It is likely to be variable and probably small in hnmans. 

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