Acyl CoA

Due to the presence of an ADP-like moiety in the coenzyme-A part of acyl-CoA, it 

CoA and an ATP-regenerating system were added to the mitochondrial incubation (----------------), the 

It has been criticized that the effect of acyl-CoA is unspecific and due to a general detergent action. However, acyl-CoA does not induce permeability in parallel with Cl permeability [85], and there are clearly effects of palmitoyl-CoA on brown fat mitochondria at concentrations below those that yield (definitely unspecific ) uncoupling in rat liver mitochondria. Further, the uncoupling effects of free fatty acids are clearly — at low concentrations — better if acyl-CoA formation is allowed to proceed than when it is hampered [81] (Fig. 10.14). 

whereas there is no doubt that acyl-CoAs will interact unspecifically with the membrane if added at very high concentrations ((6) on Fig. 10.13), the possibility remains that acyl-CoAs may interact in a more specific way at low concentrations with thermogenin, and thus be the physiological mediators of thermogenesis. 

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The primary transporter of cholesterol in the blood is low density Hpoprotein (LDL). Once transported intraceUularly, cholesterol homeostasis is controlled primarily by suppressing cholesterol synthesis through inhibition of P-hydroxy-P-methyl gluterate-coenzyme A (HMG—CoA) reductase, acyl CoA—acyl transferase (ACAT), and down-regulation of LDL receptors. An important dmg in the regulation of cholesterol metaboHsm is lovastatin, also known as mevinolin, MK-803, and Mevacor, which is an HMG—CoA reductase inhibitor (Table 5). 

FIGURE 18.24 Acyl transfer from acyl-CoA to a nucleophile is more favorable than transfer of an acyl group from an oxygen ester. 

FIGURE 21.V The fatty acyl-CoA dehydrogenase reaction, emphasizing that the reaction involves reduction of enzyme-bonnd FAD (indicated by brackets). 

This is a crucial point because (as we will see) proton transport is coupled with ATP synthesis. Oxidation of one FADHg in the electron transport chain results in synthesis of approximately two molecules of ATP, compared with the approximately three ATPs produced by the oxidation of one NADH. Other enzymes can also supply electrons to UQ, including mitochondrial 5w-glyc-erophosphate dehydrogenase, an inner membrane-bound shuttle enzyme, and the fatty acyl-CoA dehydrogenases, three soluble matrix enzymes involved in fatty acid oxidation (Figure 21.7 also see Chapter 24). The path of electrons from succinate to UQ is shown in Figure 21.8. 

FIGURE 24.7 The acyl-CoA synthetase reaction activates fatty acids for /3-oxidation. The reaction is driven by hydrolysis of ATP to AMP and pyrophosphate and by the subsequent hydrolysis of pyrophosphate. 

FIGURE 24.8 The mechanism of the acyl-CoA synthetase reaction involves fatty acid carboxylate attack on ATP to form an acyl-adenylate intermediate. The fatty acyl CoA thioester product is formed by CoA attack on this intermediate. 

All of the other enzymes of the /3-oxidation pathway are located in the mitochondrial matrix. Short-chain fatty acids, as already mentioned, are transported into the matrix as free acids and form the acyl-CoA derivatives there. However, long-chain fatty acyl-CoA derivatives cannot be transported into the matrix directly. These long-chain derivatives must first be converted to acylearnitine derivatives, as shown in Figure 24.9. Carnitine acyltransferase I, located on the outer side of the inner mitochondrial membrane, catalyzes the formation of... 

FIGURE 24.11 The acyl-CoA dehydrogenase reaction. The two electrons removed in this oxidation reaction are delivered to the electron transport chain in the form of reduced coenzyme Q (UQH9). 

FIGURE 24.12 The mechanism of acyl-CoA dehydrogenase. Removal of a proton from the u-C is followed by hydride transfer from the /3-carbon to FAD. 

FIGURE 24.13 The subunit structure of medium chain acyl-CoA dehydrogenase from pig liver mitochondria. Note the location of the bound FAD (red). (Adapted from Kim, J-T., and Wiz, J., 1988. Structure of the medium-chain acyl-CoA clchyclro-genase from pig liver mitochonciria at 3-A resolution. Proceedings of the National Academy of Sciences, USA 85 6671-668. )... 

A Metabolite of Hypoglycin from Akee Fruit Inhibits Acyl-CoA Dehydrogenase... 

The unripened fruit of the akee tree contains hypoglycin, a rare amino acid (Figure 24.14). Metabolism of hypoglycin yields methylenecydopropylacetyl-CoA (MCPA-CoA). Acyl-CoA dehydrogenase will accept MCPA-CoA as a substrate,... 

FIGURE 24.14 The conversion of hypoglycin from akee fruit to a form that inhibits acyl-CoA dehydrogenase. 

The final step in the /3-oxidation cycle is the cleavage of the /3-ketoacyI-CoA. This reaction, catalyzed by thiolase (also known as j8-ketothiolase), involves the attack of a cysteine thiolate from the enzyme on the /3-carbonyI carbon, followed by cleavage to give the etiolate of acetyl-CoA and an enzyme-thioester intermediate (Figure 24.17). Subsequent attack by the thiol group of a second CoA and departure of the cysteine thiolate yields a new (shorter) acyl-CoA. If the reaction in Figure 24.17 is read in reverse, it is easy to see that it is a Claisen condensation—an attack of the etiolate anion of acetyl-CoA on a thioester. Despite the formation of a second thioester, this reaction has a very favorable A).q, and it drives the three previous reactions of /3-oxidation. 

FIGURE 24.17 The mechanism of the thiolase reaction. Attack by an enzyme cysteine thiolate group at the /3-carbonyl carbon produces a tetrahedral intermediate, which decomposes with departure of acetyl-CoA, leaving an enzyme thioester intermediate. Attack by the thiol group of a second CoA yields a new (shortened) acyl-CoA. 

In essence, this series of four reactions has yielded a fatty acid (as a CoA ester) that has been shortened by two carbons, and one molecule of acetyl-CoA. The shortened fatty acyl-CoA can now go through another /3-oxidation cycle, as shown in Figure 24.10. Repetition of this cycle with a fatty acid with an even number of carbons eventually yields two molecules of acetyl-CoA in the final step. As noted in the first reaction in Table 24.2, complete /3-oxidation of palmitic acid yields eight molecules of acetyl-CoA as well as seven molecules of FADHg and seven molecules of NADFI. The acetyl-CoA can be further metabolized in the TCA cycle (as we have already seen). Alternatively, acetyl-CoA can also be used as a substrate in amino acid biosynthesis (Chapter 26). As noted in Chapter 23, however, acetyl-CoA cannot be used as a substrate for gluco-neogenesis. 

Polyunsaturated fatty acids pose a slightly more complicated situation for the cell. Consider, for example, the case of linoleic acid shown in Figure 24.24. As with oleic acid, /3-oxidation proceeds through three cycles, and enoyl-CoA isomerase converts the cA-A double bond to a trans-b double bond to permit one more round of /3-oxidation. What results this time, however, is a cA-A enoyl-CoA, which is converted normally by acyl-CoA dehydrogenase to a trans-b, cis-b species. This, however, is a poor substrate for the enoyl-CoA hydratase. This problem is solved by 2,4-dienoyl-CoA reductase, the product of which depends on the organism. The mammalian form of this enzyme produces a trans-b enoyl product, as shown in Figure 24.24, which can be converted by an enoyl-CoA isomerase to the trans-b enoyl-CoA, which can then proceed normally through the /3-oxidation pathway. Escherichia coli possesses a... 

FIGURE 24.24 The oxidation pathway for polyunsaturated fatty adds, illustrated for linoleic add. Three cycles of /3-oxidation on linoleoyl-CoA yield the cis-A, d.s-A intermediate, which is converted to a tran.s-A, cis-A intermediate. An additional round of /S-oxi-dation gives d.s-A enoyl-CoA, which is oxidized to the trans-A, d.s-A species by acyl-CoA dehydrogenase. The subsequent action of 2,4-dienoyl-CoA reductase yields the trans-A product, which is converted by enoyl-CoA isomerase to the tran.s-A form. Normal /S-oxida-tion then produces five molecules of acetyl-CoA. 

Peroxisomal /3-Oxidation Requires FAD-Dependent Acyl-CoA Oxidase... 

Eder, M., Krantle, R, Dong, Y, et al., 1997. Characterization of human and pig kidney long-chain-acyl-CoA dehydrogena.se.s and dieir role in beta-oxidation. European Journal of Biochemistry 245 600—607. 

FIGURE 25.3 In the acetyl-CoA carboxylase reaction, the biotin ring, on its flexible tether, acquires carboxyl groups from carbonylphos-phate on the carboxylase subunit and transfers them to acyl-CoA molecules on the transcarboxylase subunits. 

Dephospho-acetyl-CoA carboxylase (Low [citrate] activates, high [fatty acyl-CoA] inhibits)... 

FIGURE 25.5 The activity of acetyl-CoA carboxylase is modulated by phosphorylation and dephosphoryladon. The dephospho form of the enzyme is activated by low [citrate] and inhibited only by high levels of fatty acyl-CoA. 

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