Oxidation is the major pathway for the catabolism of saturated fatty acids. It involves the successive removal of two-carbon fragments from the carboxyl end of the fatty acyl CoA (Champe, Harvey, & Ferrier, 2005). The first three steps of this ? -oxidation spiral are the following (Murray, Granner, Mayes, & Rodwell, 2000): Dehydrogenation or oxidation – the removal of two hydrogen atoms from the 2(? )- and 3(? )-carbon atoms, catalyzed by acyl-CoA dehydrogenase, yielding 1 FADH2; Hydration – the addition of water to saturate the double bond, forming 3-hydroxyacyl-CoA, catalyzed by ?
2-enoyl-CoA hydrase (also known as enoyl-CoA hydratase); and Dehydrogenation or oxidation – a further dehydrogenation on the 3-carbon, catalyzed by the enzyme L(+)-3-hydroxyacyl-CoA dehydrogenase, producing 1 NADH. The end-products of this pathway, after a few more steps and successive reentry of acyl-CoA into the cycle, are acetyl CoA, NADH and FADH2. The citric acid cycle is the energy-producing pathway “where the oxidative metabolism of carbohydrates, amino acids, and fatty acids converge, their carbon skeletons being converted to CO2 and H2O” (Champe, Harvey, & Ferrier, 2005).
Although it is technically a cycle, the “last” three steps are identified as follows (Murray, Granner, Mayes, & Rodwell, 2000): Dehydrogenation or oxidation – succinate, in the presence of FAD, is converted by the enzyme succinate dehydrogenase into fumarate and FADH2; Hydration – the addition of water to fumarate to produce L-malate, catalyzed by fumarate hydratase (also known as fumarase); and Dehydrogenation or oxidation – the conversion of malate and NAD+ by the enzyme malate dehydrogenase to oxaloacetate, NADH and H+.
The chemical conversions involved in the citric acid cycle that eventually produce carbon dioxide and water generate a form of energy that is usable by aerobic organisms (http://en. wikipedia. org/wiki/Citric_acid_cycle). Obviously, the first three steps of the ? -oxidation pathway and the last three steps of the citric acid cycle involve the same reactions. Both occur in the mitochondria of the cell. In both cycles, the first dehydrogenation (also called oxidation, depending on the source) involves a flavoprotein coenzyme with a FAD prosthetic group, and FADH2 is thus yielded.
The hydration step in ? -oxidation and the citric acid cycle involve hydratase enzymes. The second dehydrogenation step in both cycles require the NAD+ coenzyme and produce NADH + H+. That having been said, the main difference between these otherwise similar reactions is that they have different substrates and yield different products. The last step in the ? -oxidation pathway is the splitting of ? -ketoacyl-CoA by the enzyme ? -ketothiolase (http://themedicalbiochemistrypage. org/fatty-acid-oxidation. html).
This reaction produces an acyl-CoA derivative that contains two carbons less than the original acyl-CoA molecule that that underwent oxidation, as well as acetyl-CoA. The acyl-CoA thus formed reenters the ? -oxidation spiral, while the acetyl-CoA is oxidized to carbon dioxide and water through the citric acid cycle. The first step of the citric acid cycle is the initial condensation of acetyl-CoA with oxaloacetate to form citrate, in a reaction catalyzed by citrate synthase (Murray, Granner, Mayes, & Rodwell, 2000). A carbon-to-carbon bond is created between the methyl group of acetyl-CoA and the carbonyl carbon of oxaloacetate.
Acetyl-CoA transfers its two-carbon acetyl group to oxaloacetate, which initially contains four carbons, and a six-carbon molecule is then formed as citrate (http://en. wikipedia. org/wiki/Citric_acid_cycle). Both reactions involve acetyl-CoA. However, acetyl-CoA is the product in ? -oxidation, whereas it is one of the substrates in the citric acid cycle. Coenzyme A is required in the last step of ? -oxidation, whereas it is one of the by-products in the first step of the citric acid cycle. Finally, in ? -oxidation, two carbons are removed per cycle, whereas the citric acid cycle involves the addition of two carbons.
References Champe, P. C. , Harvey, R. A. , & Ferrier, D. R. (2005). Lippincott’s Illustrated Reviews: Biochemistry. Philadelphia: Lippincott Wiliams & Wilkins. Citric Acid Cycle. (2008). In Wikipedia, the free encyclopedia. Retrieved October 7, 2008, from Wikipedia: http://en. wikipedia. org/wiki/Citric_acid_cycle Fatty Acid Oxidation. (2008). In The Medical Biochemistry Page. Retrieved October 7, 2008, from: http://themedicalbiochemistrypage. org/fatty-acid-oxidation. html Murray, R. K. , Granner, D. K. , Mayes, P. A. , & Rodwell, V. W. (2000). Harper’s Biochemistry (25th ed. ). New York: McGraw-Hill.
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