The Catabolism of Fats

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. 

The citric acid cycle (third stage of catabolism) is a series of eight reactions in which the acetyl group of each molecule of acetyl-CoA—formed by the catabolism of fats, carbohydrates, and amino acids—is converted to two molecules of CO2 (Figure 25.5). 

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. 

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.

The TCA cycle, strictly speaking, has only one input fuel— acetyl-CoA. Catabolism of carbohydrates and fats leads to the production of acetyl-CoA, so that the TCA cycle is ideally suited to serve as the major oxidative sequence in the catabolism of those types of compounds. However, degradation of the amino acids that result from the hydrolysis of protein produces a number of intermediates, among which are a-ketoglutarate, succinyl-CoA, and oxaloacetate (chapter 22). a-Ketoglutarate and succinyl-CoA, can be oxidized... 

Lipolysis is considered to be an important biochemical event during cheese ripening and the current knowledge have been discussed in detail (Collins et al., 2003, 2004 McSweeney and Sousa, 2000). The formation of short-chain FFAs by the lipolysis of milk fat by lipases is a desirable reaction in many cheese types (e.g., mold-ripened cheeses). The catabolism of FFAs, which is a secondary event in the ripening process, leads to the formation of volatile flavor compounds such as lactones, thioesters, ethyl esters, alkanols, and hydroxyl fatty acids. The contributions of lipolysis to the flavor of bacterially ripened cheeses are limited. 

Synthesizing fats is an efficient way for organisms to store energy. The catabolism of 1 g of fat yields about 38 kJ of energy, whereas the catabolism of 1 g of protein or carbohydrate yields about 17 kJ of energy. 

The final step in fatly acid catabolism involves the attack of a molecule of coenzyme A at the 3-carbon of the fatty add, resulting in the discharge of a two-carbon unit (acetyl group). The sole breakdown product of even-carbon-num-bered fatty acids, such as palntihc acid, is acclyl-CoA. A small fraction of the fatty acids encountered in the diet have an odd number of carbons. The catabolism of these fatty acids yields a number of molecules of acctyl-CoAplus one molecule of propionyl oA. 

List the sequence of intermediates involved in the catabolism of glycerol from hydrolyzed fats to yield acetyl CoA. 

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