FADH2, fatty acid oxidation yielding

The yield of ATP is reduced slightly for unsaturated fatty acids, since the additional metabolic reactions which enable them to be degraded by the (3-oxidation pathway either involve using NADPH or bypass an FADH2-producing reaction (see Fig. 4). 

We can now calculate the energy yield derived from the oxidation of a fatty acid. In each reaction cycle, an acyl CoA is shortened by two carbon atoms, and one molecule each of FADH2, NADH, and acetyl CoA is formed. 

The aerobic oxidation of a fatty acid generates a large number of ATP molecules. As previously described (see p. 315), the oxidation of each FADH2 during electron transport and oxidative phosphorylation yields approximately 1.5 molecules of ATP. Similarly, the oxidation of each NADH yields approximately 2.5 molecules of ATP. The yield of ATP from the oxidation of palmitoyl-CoA, which generates 7 FADH2 7 NADH, and 8 acetyl-CoA molecules to form C02 and H20 is calculated as follows ... 

Oxidation of fatty acids In mitochondria yields acetyl CoA, which enters the citric acid cycle, and the reduced coenzymes NADH and FADH2. Subsequent oxidation of these metabolites Is coupled to formation of ATP. 

STEP 1 Introduction of a double bond. The /3-oxidation pathway begins when fatty acid forms a thiol ester with coenzyme A to give a fatty acyl CoA. Two hydrogen atoms are then removed from carbons 2 and 3 by an acyl CoA dehydrogenase enzyme to yield an ,/3-unsaturated acyl CoA. This kind of oxidation—the introduction of a conjugated double bond into a carbonyl compound—occurs frequently in biochemical pathways and is usually carried out by the coenzyme flavin adenine dmucleotide iFAD). Reduced FADH2 is the by-product. 

Four rounds of [3 oxidation of a fatty acid with a trans-A ° double bond would yield a trans-A -enoyl CoA derivative. This compound is the natural intermediate formed by an acyl CoA dehydrogenase. It would be hydrated by enoyl CoA hydratase to form the L-3-hydroxyacyl CoA derivative. For the fatty acid with a cis-A double bond, four rounds of (3 oxidation would produce a ds-A double bond, which would not serve as a substrate for enoyl CoA hydratase. An isomerase would convert this bond into the trans-A configuration to allow subsequent metabolism. Since the double bond already exists in the fatty acids and does not arise from (3 oxidations, one less FADH2 would be formed. Consequently, approximately 1.5 fewer ATP would be produced for each pre-existing double bond. 

As a stearoyl CoA molecule (18 carbons) passes through the p-oxidation spiral, 9 acetyl CoA, 8 FADH2, and 8 NADH molecules are produced. Acetyl CoA produced in the fatty acid spiral can enter the citric acid cycle (followed by the electron transport chain), where each molecule of acetyl CoA results in the production of 10 ATP molecules. In addition, when the FADH2 and NADH molecnles enter the electron transport chain, each FADH2 yields 1.5 ATP molecules, and each NADH yields 2.5 molecules. The calculations are summarized in I Table 14.1, which shows a total of 120 molecules of ATP formed from the 18-carbon fatty acid. 

If one acetyl -CoA involved in TCA cycle gives = 10 ATPs ATPs due 8 acetyl-CoA = 8 X 10 = 80 ATPs due to 7 FADH2 = 1.5 x 7= 10.5 ATPS due 7 (NADH + H ) = 2.5 x 7.5 = 17.5. The total of number ATPs produced 108. During the initiation of the y - oxidation pathway a 2 ATPs converts into a 2 AMP and 2 Pi for the activation of fatty acid. So, net ATPs produced by palmitic acid are 106. These calculations assume that mitoehondrial oxidative phosphorlation produces 1.5 ATPS/FADH2 oxidized and 2.5 ATP/NADH2 oxidized. The Guanosine-5 -triphosphate (GTP) produced directly in the acid citric cycle yields ATP in the reaetion eatalyzed by nucleoside diphosphate kinase [99, 130]. 

Figure 9. The y6-oxidation of saturated fatty acids involves a cycle of four enzyme-catalyzed reactions. Each cycle produces single molecules of FADH2, NADH, and acetyl-CoA and yields a fatty acid shortened by two carbons.

Once y0-oxidation is complete, the terminal two carbons of the fatty acid chain are then released as acetyl CoA. Oxidation and cleavage of the fatty acid continue until it is entirely converted to acetyl CoA. The conversion of a saturated fatty acid with 18 carbon atoms to 9 acetyl CoA produces 8 NADH and 8 FADH2. The acetyl CoA is burned by the citric acid cycle to generate more ATP. The high caloric content of fats pays off to cells in the yield of ATP. 

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