Palmityl CoA

Summary of the energy yield from the oxidation of palmityl CoA (16 carbons). CC = acetyl CoA. 

Glycolipid synthesis begins on the cytosolic face of the ER (34) with the condensation of a serine residue and a palmityl-CoA to form 3-dehydrosphinganine, which is hy-droxylated at the 4 oxygen, A-acylated, and unsaturated between C4 and C5 in a trans-fashion to form ceramide (Cer)... 

Coenzyme A (CoA) derivatives of bile acids were prepared [33,34] by a modification of the mixed anhydride procedure for the synthesis of palmityl CoA. A yield of 70% was calculated from the quantity of cholylhydroxamate formed after treatment of the CoA derivative with hydroxylamine. By modification of this procedure and purification of the products by chromatography on Sephadex LH-20, aqueous solutions of CoA derivatives of cholic, chenodeoxycholic, deoxycholic, and lithocholic acids were pbtained, frozen at - 70°C, and shown to be free from hydrolysis for several months [35]. 

The total energy yield from the oxidation of 1 mole of palmityl CoA to 8 moles of acetyl CoA is therefore 28 moles of ATP 1.5 for each of the 7 FAD(2H), and 2.5 for each of the 7 NADH. To calculate the energy yield from oxidation of 1 mole of palmitate, two ATP need to be subtracted from the total because two high-energy phosphate bonds are cleaved when palmitate is activated to palmityl CoA. 

Palmitic acid is 16 carbons long, with no double bonds, so it requires 7 oxidation spirals to be completely converted to acetyl-CoA. After 7 spirals, there are 7 FAD(2H), 7 NADH, and 8 acetyl-CoA. Each NADH yields 2.5 ATP, each FAD(2H) yields 1.5 ATP, and each acetyl-CoA yields 10 ATP as it is processed around the TCA cycle. This then yields 17.5 + 10.5 + 80.5 = 108 ATP. However, activation of palmitic acid to palmityl-CoA requires two high-energy bonds, so the net yield is 108 - 2, or 106 moles of ATP. 

Fig. 33.11. Regulation of acetyl Co A carboxylase. This enzyme is regulated allosterically, both positively and negatively, by phosphorylation (circled P) and dephosphorylation, and by diet-induced induction (circled t). It is active in the dephosphorylated state when citrate causes it to polymerize. Dephosphorylation is catalyzed by an insulin-stimulated phosphatase. Low energy levels, via activation of an AMP-dependent protein kinase, cause the enzyme to be phosphorylated and inactivated. The ultimate product of fatty acid synthesis, palmitate, is converted to its CoA derivative palmityl CoA, which inhibits the enzyme. A high-calorie diet increases the rate of transcription of the gene for acetyl CoA carboxylase, whereas a low-calorie diet reduces transcription of this gene.

After synthesis on the fatty acid synthase complex, palmitate is activated, forming palmityl CoA. Palmityl CoA and other activated long-chain fatty acids can be... 

The synthesis of sphingolipids begins with the formation of ceramide (Fig. 33.32). Serine and palmityl CoA condense to form a product that is reduced. A very-long-chain fatty acid (usually containing 22 carbons) forms an amide with the amino group, a double bond is generated, and ceramide is formed. 

The palmitate produced by the synthase complex is converted to palmityl CoA and elongated and desaturated to form other fatty acyl CoA molecules, which are converted to triacylglycerols. These triacylglycerols are packaged and secreted into the blood as VLDL. 

Acetyl-CoA carboxylase is also inhibited by long-chain fatty acyl-CoA, and such inhibition is accompanied by enzyme depolymerization (91, 92, 95). Binding of 1 mole of palmityl-CoA per mole of rat liver acetyl-CoA carboxylase inhibits the enzyme (95). The Ti for palmityl-CoA, about 5 nM, is far lower than the critical micellar concentration of the thioester this indicates that the inhibition may be physiologically significant. If the allosteric control mechanisms of citrate promoted "substrate activation or fatty acyl-CoA mediated "feed back inhibition of fatty acid synthesis function at all under in vivo conditions, they must function as a dual mechanism (90). 

Purified acetyl-CoA carboxylase (ACC) which had been dialyzed in Trie buffer without citrate was incubated with 0.24 fj.M palmityl-CoA in buffer containing 50 laM Tris-HCl, pH 7.5,1 mAf DTT, 1 mM theophylline for 15 minutes at 37°C. Following this incubation, additional ligands were added as indicated, and the reaction mixtures were further incubated for 30 minutes at 37°C. The carboxylase was assayed for 3 minutes in the absence of citrate and BSA. 

Limited studies indicate that the phosphorylated forms of the en-zjrme are more susceptible to inactivation or inhibition by various negative eflFectors such as palmityl-CoA, avidin, and ATP than the dephosphorylated forms 17,21), whereas the dephosphorylated forms require less citrate for maximum activation 13, 17, 21, 42, 57). For example, the phosphorylated form has a Km for citrate of 2.4 mM whereas the Km of the active dephosphorylated form is 0.2 mAf. Since the citrate concentration in the cell is only about 0.6 mM, and most of this citrate is localized in the mitochondria, it would appear that only the nonphosphorylated form will be active under normal physiological conditions. Thus, the covalent modification mechanism makes the allosteric control mechanism functional at physiological concentrations of cellular metabolites. Such changes in the properties of acetyl-CoA carboxylase also occur under in vivo conditions as reported by Witters etal. 130) using isolated hepatocytes treated with insulin or glucagon. 

A number of experiments support the causative relationship between the polymer-protomer transition and the carboxylase activity under both in vivo and in vitro conditions. Polymer-protomer transitions in vitro in the presence of the activator, citrate, or inactivators, such as palmityl-CoA, are the earliest examples of the relationship between the activity and the quaternary structure of the carboxylase... 

Acetyl CoA can be converted to fatty acyl CoA by one of two routes. One utilises 3-hydroxyacyl CoA dehydrogenase and the other, which proceeds via malonyl CoA, uses acetyl CoA carboxylase. The former route is considered to be reversible and 3-hydroxyacyl CoA dehydrogenase is therefore an enzyme of fatty acid oxidation also. When formed, fatty acyl CoA can be incorporated into other lipids as shown in Figure 2.11. Palmityl CoA, however, has other important utilisation routes leading to the synthesis of sphingomyelins (phospholipid) or ceramide hexosides. The latter represent an important junction of lipid and hexose metabolism and are precursors of the gangliosides which contain hexose, hexosamine, and A -acetylneuraminic acid (referred to briefly under glycolipid synthesis above). 

There are three known mechanisms for the synthesis of SEs in animal tissue (Fig. 3). It seemed likely that one of these might apply in the spinach leaf. Palmityl-CoA served as a donor, but several compounds were labeled before SE, and these could have served as the actual acyl donor (Garcia and Mudd, 1978a). Radioactive PC was a good acyl donor, but product analysis showed after short times that DG was the predominant material. When DG was isolated from reaction mixtures, it proved to be an excellent acyl donor. Subsequent work with chemically prepared DG has confirmed the sugges-... 

There is abundant evidence indicating that a natural hydrophobic inhibitor of acetyl-CoA carboxylase is present in crude enzyme extracts of liver and adipose tissue [128,129,182,192,236-238]. The activating effect of (+)-palmityl carnitine on fatty acid synthesis in crude liver extracts and on impure acetyl-CoA carboxylase preparations has tentatively been ascribed to the displacement of hydrophobic inhibitors such as fatty acids or fatty acyl-CoA derivatives [129,182,192,236-238]. Inhibition of rat liver acetyl-CoA carboxylase by added palmityl-CoA can be reversed in part by (+)-palmityl carnitine [236], but not by citrate. This activating effect does not appear to be specific with respect to (+)-palmityl carnitine in that cetyl trimethylammonium ion is also effective [192]. Furthermore, impure preparations of acetyl-CoA carboxylase from adipose tissue or rat liver are markedly activated by serum albumin [123,129,238] or extensive dilution of the enzyme preparation prior to assay [129,182]. On the other hand, none of these agents [(+)-palmityl carnitine, serum albumin, or dilution], which activate the impure carboxylase, have an activating effect on the homogeneous acetyl-CoA carboxylases from adipose tissue or liver [129,182, 239]. It is evident that an inhibitory substance, apparently hydrophobic in nature, is removed either by purification of the enzyme or by the agents or treatments mentioned above. 

Several hypolipidemic drugs (2-methyl-2-phenoxypropionate derivatives) have been found to inhibit acetyl-CoA carboxylase [240,241]. All appear to inhibit competitively with respect to acetyl-CoA and tricarboxylic acid activator and noncompetitively with respect to ATP and HCOs . These inhibitors, like malonyl-CoA and palmityl-CoA, are antagonistic to the activator in that they tend to reverse the activator-promoted aggregation of the carboxylase (see Fig. 7). Confirmation of the inhibitory effect of these drugs on lipogenesis at the carboxylase-catalyzed step in vivo would lend substantial support to the proposed regulatory role of the carboxylase. 

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