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Chapter: Medical Physiology: Lipid Metabolism

Use of Triglycerides for Energy: Formation of Adenosine Triphosphate

About 40 per cent of the calories in a typical American diet are derived from fats, which is almost equal to the calories derived from carbohydrates.

Use of Triglycerides for Energy: Formation of Adenosine Triphosphate

About 40 per cent of the calories in a typical American diet are derived from fats, which is almost equal to the calories derived from carbohydrates. Therefore, the use of fats by the body for energy is as important as the use of carbohydrates is. In addition, many of the carbohy-drates ingested with each meal are converted into triglycerides, then stored, and used later in the form of fatty acids released from the triglycerides for energy.

Hydrolysis of Triglycerides. The first stage in using triglyc-erides for energy is their hydrolysis into fatty acids and glycerol. Then, both the fatty acids and the glycerol are transported in the blood to the active tissues, where they will be oxidized to give energy. Almost all cells—with some exceptions, such as brain tissue and red blood cells—can use fatty acids for energy.

Glycerol, on entering the active tissue, is immediately changed by intracellular enzymes into glycerol-3-phosphate, which enters the glycolytic pathway forglucose breakdown and is thus used for energy. Before the fatty acids can be used for energy, they must be processed further in the following way.

Entry of Fatty Acids into Mitochondria. Degradation and oxi-dation of fatty acids occur only in the mitochondria. Therefore, the first step for the use of fatty acids is their transport into the mitochondria. This is a carrier-mediated process that uses carnitine as the carrier sub-stance. Once inside the mitochondria, fatty acids split away from carnitine and are degraded and oxidized.

Degradation  of  Fatty  Acids  to  Acetyl  Coenzyme  A  by  Beta- Oxidation.  The fatty acid molecule is degraded in the mitochondria by progressive release of two-carbon seg-ments in the form of acetyl coenzyme A (acetyl-CoA). This process, which is shown in Figure 68–1, is called the beta-oxidation process for degradation of fatty acids.

To understand the essential steps in the beta-oxidation process, note that in equation 1 the first step is combination of the fatty acid molecule with coenzyme A (CoA) to form fatty acyl-CoA. In equations 2, 3, and 4, the beta carbon (the second carbon from the right) of the fatty acyl-CoA binds with an oxygen molecule—that is, the beta carbon becomes oxidized.

Then, in equation 5, the right-hand two-carbon portion of the molecule is split off to release acetyl-CoA into the cell fluid. At the same time, another CoA mol-ecule binds at the end of the remaining portion of the fatty acid molecule, and this forms a new fatty acyl-CoA molecule; this time, however, the molecule is two carbon atoms shorter because of the loss of the first acetyl-CoA from its terminal end.

Next, this shorter fatty acyl-CoA enters into equation 2 and progresses through equations 3, 4, and 5 to release still another acetyl-CoA molecule, thus shortening the original fatty acid molecule by another two carbons. In addition to the released acetyl-CoA molecules, four atoms of hydrogen are released from the fatty acid mol-ecule at the same time, entirely separate from the acetyl-CoA.

Oxidation of Acetyl-CoA. The acetyl-CoA moleculesformed by beta-oxidation of fatty acids in the mitochondria enter immediately into the citric acid cycle, combining first with oxaloacetic acid to form citric acid, which then is degraded into carbon dioxide and hydrogen atoms. The hydrogen is subse-quently oxidized by the chemiosmotic oxidative systemof the mitochondria. The net reaction in the citric acid cycle for each molecule of acetyl-CoA is the following:

        Thus, after initial degradation of fatty acids to acetyl-CoA, their final breakdown is precisely the same as that of the acetyl-CoA formed from pyruvic acid during the metabolism of glucose. And the extra hydrogen atoms are also oxidized by the samechemiosmotic oxidativesystem of the mitochondria that is used in carbohydrateoxidation, liberating large amounts of adenosine triphosphate (ATP).

Tremendous Amounts of ATP Are Formed by Oxidation of Fatty Acids. In Figure 68–1, note that the 4 separate hydrogenatoms released each time a molecule of acetyl-CoA is split from the fatty acid chain are released in the forms FADH2, NADH, and H+. Therefore, for every stearic fatty acid molecule that is split to form 9 acetyl-CoA molecules, 32 extra hydrogen atoms are removed. In addition, for each of the 9 molecules of acetyl-CoA that are subsequently degraded by the citric acid cycle, 8 more hydrogen atoms are removed, making another 72 hydrogens. This makes a total of 104 hydrogen atoms eventually released by the degradation of each stearic acid molecule. Of this group, 34 are removed from the degrading fatty acids by flavoproteins, and 70 are removed by nicotinamide adenine dinucleotide (NAD+) as NADH and H+.

These two groups of hydrogen atoms are oxidized in the mitochondria, but they enter the oxidative system at different points, so that 1 molecule of ATP is synthesized for each of the 34 flavoprotein hydrogens, and 1.5 molecules of ATP are synthesized for each of the 70 NADH and H+ hydro-gens. This makes 34 plus 105, or a total of 139 molecules of ATP formed by the oxidation of hydrogen derived from each molecule of stearic acid. Another 9 molecules of ATP are formed in the citric acid cycle itself (sepa-rate from the ATP released by the oxidation of hydro-gen), one for each of the 9 acetyl-CoA molecules metabolized. Thus, a total of 148 molecules of ATP are formed during the complete oxidation of 1 molecule of stearic acid. However, two high-energy bonds are con-sumed in the initial combination of CoA with the stearic acid molecule, making a net gain of 146 molecules of ATP.

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