Fat Metabolism

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Fat Catabolism Triacylglycerol (fat) consists of three fatty acids linked to glycerol (Chapter 2). Fat accounts for the major portion (approximately 80 percent) of the energy stored in the body (Table 4-8). Under resting conditions, approximately half the energy used by such tissues as muscle, liver, and kidneys is derived from the catabolism of fatty acids.

Although most cells store small amounts of fat, the majority of the body's fat is stored in specialized cells known as adipocytes. Almost the entire cytoplasm of these cells is filled with a single large fat droplet. Clusters of adipocytes form adipose tissue, most of which is in deposits underlying the skin. The function of adipocytes is to synthesize and store triacylglycerols during periods of food uptake and then, when food is not being absorbed from the intestinal tract, to release

TABLE 4-8 Fuel Content of a 70-kg Person

Total-Body Content, kg

Energy Content, kcal/g

Total-Body Energy Content kcal %




140,000 78




38,000 21




2,000 1

fatty acids and glycerol into the blood for uptake and use by other cells to provide the energy for ATP formation. The factors controlling fat storage and release from adipocytes will be described in Chapter 18. Here we will emphasize the pathway by which fatty acids are catabolized by most cells to provide the energy for ATP synthesis, and the pathway for the synthesis of fatty acids from other fuel molecules.

Figure 4-27 shows the pathway for fatty acid ca-tabolism, which is achieved by enzymes present in the mitochondrial matrix. The breakdown of a fatty acid is initiated by linking a molecule of coenzyme A to the carboxyl end of the fatty acid. This initial step is accompanied by the breakdown of ATP to AMP and two P;.

The coenzyme-A derivative of the fatty acid then proceeds through a series of reactions, known as beta oxidation, which split off a molecule of acetyl coen-zyme A from the end of the fatty acid and transfer two pairs of hydrogen atoms to coenzymes (one pair to FAD and the other to NAD+). The hydrogen atoms from the coenzymes then enter the oxidative-phosphorylation pathway to form ATP.

When an acetyl coenzyme A is split from the end of a fatty acid, another coenzyme A is added (ATP is not required for this step), and the sequence is repeated. Each passage through this sequence shortens the fatty acid chain by two carbon atoms until all the carbon atoms have been transferred to coenzyme A molecules. As we saw, these molecules then enter the Krebs cycle to produce CO2 and ATP via the Krebs cycle and oxidative phosphorylation.

How much ATP is formed as a result of the total catabolism of a fatty acid? Most fatty acids in the body contain 14 to 22 carbons, 16 and 18 being most common. The catabolism of one 18-carbon saturated fatty acid yields 146 ATP molecules. In contrast, as we have seen, the catabolism of one glucose molecule yields a maximum of 38 ATP molecules. Thus, taking into account the difference in molecular weight of the fatty acid and glucose, the amount of ATP formed from the catabolism of a gram of fat is about 2j times greater

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

Protein Activity and Cellular Metabolism CHAPTER FOUR

Protein Activity and Cellular Metabolism CHAPTER FOUR

Number Atp Lipid Metabolism


Pathway of fatty acid catabolism, which takes place in the mitochondria. The energy equivalent of two ATP is consumed at the start of the pathway.


Pathway of fatty acid catabolism, which takes place in the mitochondria. The energy equivalent of two ATP is consumed at the start of the pathway.

than the amount of ATP produced by catabolizing 1 gram of carbohydrate. If an average person stored most of his or her fuel as carbohydrate rather than fat, body weight would have to be approximately 30 percent greater in order to store the same amount of usable energy, and the person would consume more energy moving this extra weight around. Thus, a major step in fuel economy occurred when animals evolved the ability to store fuel as fat. In contrast, plants store almost all their fuel as carbohydrate (starch).

Fat Synthesis The synthesis of fatty acids occurs by reactions that are almost the reverse of those that degrade them. However, the enzymes in the synthetic pathway are in the cytosol, whereas (as we have just seen) the enzymes catalyzing fatty acid breakdown are in the mitochondria. Fatty acid synthesis begins with cytoplasmic acetyl coenzyme A, which transfers its acetyl group to another molecule of acetyl coenzyme A to form a four-carbon chain. By repetition of this process, long-chain fatty acids are built up two carbons at a time, which accounts for the fact that all the fatty acids synthesized in the body contain an even number of carbon atoms.

Once the fatty acids are formed, triacylglycerol can be synthesized by linking fatty acids to each of the three hydroxyl groups in glycerol, more specifically, to a phosphorylated form of glycerol called «-glycerol phosphate. The synthesis of triacylglycerol is carried out by enzymes associated with the membranes of the smooth endoplasmic reticulum.

Compare the molecules produced by glucose ca-tabolism with those required for synthesis of both fatty

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

PART ONE Basic Cell Functions acids and «-glycerol phosphate. First, acetyl coenzyme A, the starting material for fatty acid synthesis, can be formed from pyruvate, the end product of glycolysis. Second, the other ingredients required for fatty acid synthesis—hydrogen-bound coenzymes and ATP— are produced during carbohydrate catabolism. Third, a-glycerol phosphate can be formed from a glucose intermediate. It should not be surprising, therefore, that much of the carbohydrate in food is converted into fat and stored in adipose tissue shortly after its absorption from the gastrointestinal tract. Mass action resulting from the increased concentration of glucose intermediates, as well as the specific hormonal regulation of key enzymes, promotes this conversion, as will be described in Chapter 18.

It is very important to note that fatty acids, or more specifically the acetyl coenzyme A derived from fatty acid breakdown, cannot be used to synthesize new molecules of glucose. The reasons for this can be seen by examining the pathways for glucose synthesis (see Figure 4-26). First, because the reaction in which pyru-vate is broken down to acetyl coenzyme A and carbon dioxide is irreversible, acetyl coenzyme A cannot be converted into pyruvate, a molecule that could lead to the production of glucose. Second, the equivalent of the two carbon atoms in acetyl coenzyme A are converted into two molecules of carbon dioxide during their passage through the Krebs cycle before reaching oxaloacetate, another takeoff point for glucose synthesis, and therefore cannot be used to synthesize net amounts of oxaloacetate.

Thus, glucose can readily be converted into fat, but the fatty acid portion of fat cannot be converted to glucose. However, the three-carbon glycerol backbone of fat can be converted into an intermediate in the glu-coneogenic pathway and thus give rise to glucose, as mentioned earlier.

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Essentials of Human Physiology

Essentials of Human Physiology

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