Fatty acid spiral

The oxidation of fatty acids within the Knoop-Lynen cycle occurs in the matrix. The Knoop-Lynen cycle includes four enzymes that act successively on acetyl-CoA. These are acyl-CoA dehydrogenase (FAD-dependent enzyme), enoyl-CoA hydratase, 3-hydroxyacyl-CoA dehydrogenase (NAD-dependent enzyme), and acetyl-CoA acyltrans-ferase. Each turn, or revolution, of the fatty acid spiral produces... 

FATTY ACID SYNTHESIS The Reversibility of the Fatty Acid Spiral... 

Fatty acids have a basic structure, R-COOH, and the important fatty acids have long chains with an even number of carbon atoms (C12-C20), which may be described as saturated (e.g., stearic acid, C18 l), monounsaturated (e.g., oleic acid, C18 2), and polysaturated (e.g., linoleic, C18 2, and linolenic, C18 3, acids). These long chain fatty acids are used to provide energy by beta-oxidation, which shortens the fatty acid by two carbon atoms with the production of acetyl CoA (known as the fatty acid spiral). 

A number of detailed pathways such as glycolysis, the fatty acid spiral, and the urea cycle will be presented. Don t let the numerous reactions in these pathways intimidate you. Although each reaction could be looked at individually, we have tried to help you focus your attention by emphasizing the overall importance of each pathway by identifying its purpose, cellular location, starting material, products, energy requirements, and regulation. 

The new fatty acyl compound enters the p-oxidation process at Step 1, and the sequence is repeated until the fatty acyl CoA is completely degraded to acetyl CoA (Figure 14.5). The fatty acyl CoA molecule that starts each run through the P-oxidation process is two carbons shorter than the one going through the previous run. For this reason, the p-oxidation pathway for fatty acid degradation to acetyl CoA is often called the fatty acid spiral (rather than cycle). 

Figure 14.5 The fatty acid spiral P-oxidation of stearic add.

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. 

Every trip through the fatty acid spiral produces one NADH + H molecule and one FADH2 molecule. Thus,... 

Why is the oxidation of fatty acids referred to as the fatty acid spiral rather than the fatty acid cycle ... 

Complete the following equations for one turn of the fatty acid spiral ... 

The acyl-CoA is reduced by acyl-CoA dehydrogenase (EC 1.3.99.3) (an FAD-enzyme) to 2,3-dehydro-acyl-CoA, This compound is hydrated, then oxidized to the 3-keto-acyl-CoA. The latter is cleaved (thio-lysed) to produce acetyl-CoA and a new acyl-CoA which is two carbons shorter than the original one. The new acyl-CoA is immediately subject to further p-oxidation. Thus each turn of this fatty acid spiral produces one molecule of acetyl-CoA, which is further oxidized in the tricarboxylic acid cycle. Complete oxidation of 1 molecule of Steroyl-CoA yields 148... 

Fig. 1. Fatty acid degradation. Fatty acid spiral. In the degradation of fatty acids with an odd-number of C-atoms, the final step produces acetyl-CoA and propionyl-CoA (Fig, 2.).

The free fatty acids are partially oxidised and converted to two carbon units in a remarkable series of reactions known as the j8-oxidation pathway or fatty acid spiral fFig. 4.9). A detailed considera-... 

Fig. 4.11. The conversion of fats to sucrose in germinating fatty seeds. The reactions of the fatty-acid spiral and glyoxylate cycle are shown to occur in the glyoxysome, the conversion of succinate to phosphoenolpyruvate in the mitochondrion and the remaining reactions at sites other than these.

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