Acetyl coenzyme from fatty acid oxidation

Though the reactions in fatty acid biosynthesis resemble the reversal of the analogous reactions in oxidation, fatty acid synthesis is distinct from fatty acid oxidation (Figure 18.23). For example, acyl groups are carried by acyl carrier protein in fatty acid synthesis, instead of coenzyme A. Furthermore, reducing equivalents come from NADPH and energy is provided by ATP. Overall, the biosynthesis of palmitate from 8 acetyl-CoAs requires 7 ATPs and 14 NADPHs. 

Although fatty acids are both oxidized to acetyl-CoA and synthesized from acetyl-CoA, fatty acid oxidation is not the simple reverse of fatty acid biosynthesis but an entirely different process taking place in a separate compartment of the cell. The separation of fatty acid oxidation in mitochondria from biosynthesis in the cytosol allows each process to be individually controlled and integrated with tissue requirements. Each step in fatty acid oxidation involves acyl-CoA derivatives catalyzed by separate enzymes, utihzes NAD and FAD as coenzymes, and generates ATP. It is an aerobic process, requiring the presence of oxygen. 

The actual pathway by which fatty acid oxidation occurred was established by Lynen (1952-1953). Its unique and characteristic reaction was the thioclastic attack by coenzyme A on the B-ketoacyl CoA derivative, splitting off the 2C fragment, acetyl CoA. Free coenzyme A was very difficult to isolate and although it was synthesized in Todd s laboratory in Cambridge in the mid-1950s, much of the early work from Lynen s laboratory utilized A-acetyl cysteamine as a not very efficient (ca.1%) coenzyme A analogue. It carried the essential thiol group of the B-mercaptoethylamine end of CoA and could be used in most, but not all, of the steps in the spiral. 

The metabolic functions of pantothenic acid in human biochemistry are mediated through the synthesis of CoA. Pantothenic acid is a structural component of CoA. which is necessary for many important metabolic processes. Pantothenic acid is incorporated into CoA by a. series of five enzyme-catalyzed reactions. CoA is involved in the activation of fatty acids before oxidation, which requires ATP to form the respective fatty ocyl-CoA derivatives. Pantothenic acid aI.so participates in fatty acid oxidation in the final step, forming acetyl-CoA. Acetyl-CoA is also formed from pyruvate decarboxylation, in which CoA participates with thiamine pyrophosphate and lipoic acid, two other important coenzymes. Thiamine pyrophosphate is the actual decarboxylating coenzyme that functions with lipoic acid to form acetyidihydrolipoic acid from pyruvate decarboxylation. CoA then accepts the acetyl group from acetyidihydrolipoic acid to form acetyl-CoA. Acetyl-CoA is an acetyl donor in many processes and is the precursor in important biosyntheses (e.g.. those of fatty acids, steroids, porphyrins, and acetylcholine). 

Oleate is an abundant 18-carbon monounsaturated fatty acid with a cis double bond between C-9 and C-10 (denoted A ). In the first step of oxidation, oleate is converted to oleoyl-CoA and, like the saturated fatty acids, enters the mitochondrial matrix via the carnitine shuttle (Fig. 17-6). Oleoyl-CoA then undergoes three passes through the fatty acid oxidation cycle to yield three molecules of acetyl-CoA and the coenzyme A ester of a A, 12-carbon unsaturated fatty acid, cis-A -dodecenoyl-CoA (Fig. 17-9). This product cannot serve as a substrate for enoyl-CoA hydratase, which acts only on trans double bonds. The auxiliary enzyme A, A -enoyl-CoA isomerase isomerizes the ci5-A -enoyl-CoA to the fra/J5-A -enoyl-CoA, which is converted by enoyl-CoA hydratase into the corresponding L-/3-hydroxyacyl-CoA (fra/75-A -dodecenoyl-CoA). This intermediate is now acted upon by the remaining enzymes of /3 oxidation to yield acetyl-CoA and the coenzyme A ester of a 10-carbon saturated fatty acid, decanoyl-CoA. The latter undergoes four more passes through the pathway to yield five more molecules of acetyl-CoA. Altogether, nine acetyl-CoAs are produced from one molecule of the 18-carbon oleate. 

The Krebs cycle is a series of enzymatic reactions that catalyzes the aerobic metabolism of fuel molecules to carbon dioxide and water, thereby generating energy for the production of adenosine triphosphate (ATP) molecules. The Krebs cycle is so named because much of its elucidation was the work of the British biochemist Hans Krebs. Many types of fuel molecules can be drawn into and utilized by the cycle, including acetyl coenzyme A (acetyl CoA), derived from glycolysis or fatty acid oxidation. Some amino acids are metabolized via the enzymatic reactions of the Krebs cycle. In eukaryotic cells, all but one of the enzymes catalyzing the reactions of the Krebs cycle are found in the mitochondrial matrixes. 

Very little is known about the oxidative reactions which lead to the production of acetyl-coenzyme A from fatty acids in animal tissues, but, since it is likely that these oxidations utilize the same electron transfer system as Krebs cycle oxidations, and since fatty acid oxidation takes place in the same enzyme preparations which are used for the study of oxidative phosphorylation, it would seem likely on a priori grounds that the over-all P/0 ratio for fatty acid oxidation must be similar to that observed for the complete oxidation of pyruvate. 

Glyoxylate cycle A modification of the Krebs cycle, which occurs in some bacteria. Acetyl coenzyme A is generated directly from oxidation of fatty acids or other lipid compounds. 

As a rule, the anabolic pathway by which a substance is made is not the reverse of the catabolic pathway by which the same substance is degraded. The two paths must differ in some respects for both to be energetically favorable. Thus, the y3-oxidation pathway for converting fatty acids into acetyl CoA and the biosynthesis of fatty acids from acetyl CoA are related but are not exact opposites. Differences include the identity of the acvl-group carrier, the stereochemistry of the / -hydroxyacyl reaction intermediate, and the identity of the redox coenzyme. FAD is used to introduce a double bond in jS-oxidalion, while NADPH is used to reduce the double bond in fatty-acid biosynthesis. 

Each turn of the P-oxidation spiral splits off a molecule of acetyl-CoA. The process involves four enzymes catalysing, in turn, an oxidation (to form a double bond), a hydration, another oxidation (forming a ketone from a secondary alcohol) and the transfer of an acetyl group to coenzyme A (Figure 7.12). The process of P-oxidation operates as a multienzyme complex in which the intermediates are passed from one enzyme to the next, i.e. there are no free intermediates. The number of molecules of ATP generated from the oxidation of one molecule of the long-chain fatty acid pal-mitate (C18) is given in Table 7.4. Unsaturated fatty acids are also oxidised by the P-oxidation process but require modification before they enter the process (Appendix 7.3). 

The leaving group is the enolate anion of acetyl-CoA, and the reaction thus cleaves off a two-carbon fragment from the original fatty acyl-CoA. Since the nucleophile is coenzyme A, the other product is also a coenzyme A ester. In fact, the reaction generates a new fatty acyl-CoA, shorter by two carbons, which can re-enter the P-oxidation cycle. Most natural fatty acids have an even number of carbons, so the process continues until the original fatty acid chain is cleaved completely to acetyl-CoA fragments. 

The oxidation of aciy lic acid can be rationalized in terms of the endogenous catabolism of propionic acid, in which acrylyl coenzyme A is an intermediate. This pathway is analogous with fatty acid 3-oxidation, common to all species and, unlike the corresponding pathway in plants, does not involve vitamin 8,2. 3-Hydroxypropionic acid has been found as an intennediate in the metabolism of acrylic acid in vitro in rat liver and mitochondria (Finch Frederick, 1992). The CO2 excreted derives from the carboxyl carbon, while carbon atoms 2 and 3 are converted to acetyl coenzyme A, which participates in a variety of reactions. The oxidation of acrylic acid is catalysed by enzymes in a variety of tissues (Black Finch, 1995). In mice, the greatest activity was found in kidney, which was five times more active than liver and 50 times more active than skin (Black et al., 1993). 

Following removal of one acetyl-CoA unit from palmitoyl-CoA, the coenzyme A thioester of the shortened fatty acid (now the 14-carbon myristate) remains. The myristoyl-CoA can now go through another set of four /3-oxidation reactions, exactly analogous to the first, to yield a second molecule of acetyl-CoA and lauroyl-CoA, the coenzyme A thioester of the 12-carbon laurate. Altogether, seven passes through the j8-oxidation sequence are required to oxidize one molecule of palmitoyl-CoA to eight molecules of acetyl-CoA (Fig. 17-8b). The overall equation is... 

It is an acyl-CoA of the type mentioned in Section 1 and can also be formed from acetate, ATP, and coenzyme A. Although the human diet contains some acetic acid, the two major sources of acetyl-CoA in our bodies are the oxidative decarboxylation of pyruvate (Eq. 10-6) and the breakdown of fatty acid chains. Let us consider the latter process before examining the further metabolism of acetyl-CoA. 

The TCA cycle. In each turn of the cycle, acetyl-CoA from the glycolytic pathway or from /3 oxidation of fatty acids enters and two fully oxidized carbon atoms leave (as C02). ATP is generated at one point in the cycle, and coenzyme molecules are reduced. The two C02 molecules lost in each cycle originate from the oxaloacetate of the previous cycle rather than from incoming acetyl from acetyl-CoA. This point is emphasized by the use of color. 

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