Fatty acyl coenzyme synthesis

Thioesters play a paramount biochemical role in the metabolism of fatty acids and lipids. Indeed, fatty acyl-coenzyme A thioesters are pivotal in fatty acid anabolism and catabolism, in protein acylation, and in the synthesis of triacylglycerols, phospholipids and cholesterol esters [145], It is in these reactions that the peculiar reactivity of thioesters is of such significance. Many hydrolases, and mainly mitochondrial thiolester hydrolases (EC 3.1.2), are able to cleave thioesters. In addition, cholinesterases and carboxylesterases show some activity, but this is not a constant property of these enzymes since, for example, carboxylesterases from human monocytes were found to be inactive toward some endogenous thioesters [35] [146], In contrast, allococaine benzoyl thioester was found to be a good substrate of pig liver esterase, human and mouse butyrylcholinesterase, and mouse acetylcholinesterase [147],... 

Several pharmacogenomic studies, mainly in animal models, have characterized the molecular mechanisms contributing to the lipidlowering effect of fibrates. Hepatic transcription profiling of clofibrate or gemfibrozil-treated WT rats demonstrated increased expression of many genes involved in beta-oxidation, as well as FFA and cholesterol synthesis, including fatty acyl-Coenzyme A oxidase [45, 191, 192], acetyl-Coenzyme A acetyltransferase... 

Synthesis of phosphatidylcholine. The rate-limiting reaction is that catalyzed by cytidylyltransferase (reaction 2) which appears to be active only when attached to the endoplasmic reticulum, although it is also found free in the cytosol. Cytidylyltransferase is inactivated by a cAMP-dependent protein kinase and activated by a phosphatase. Translocation to the endoplasmic reticulum can be stimulated by substrates such as fatty acyl Coenzyme A (CoA). Choline deficiency can result in deposition of triacylglycerol in liver and reduced phospholipid synthesis. Enzymes (1) choline kinase ... 

In the final stages of this fat synthesis, fatty acyl coenzyme A molecnles react with a-glycero-phosphoric acid to give phosphatidic acid, which is a chiral compound by virtue of the asymmetric carbon C (11.98). Phosphatidic acid can now be hydrolysed to yield a 1,2 diacylglycerol which will react with another molecule of fatty acyl coenzyme A to give a triacylglycerol according to (11.99). 

These finding brought triglyceride synthesis in line with the synthesis of phospholipids, which was shown by Smith et al. (1957) to proceed by removal of the phosphate from phosphatidic acid, with the formation of an a )3-diglyceride. Diglyceride can now be converted into phospholipid, by cytidine-diphosphate-choline or ethanolamine (Kennedy and Weiss 1956) or into neutral triglycerides by the addition of one more fatty acid, derived from fatty acyl-coenzyme A. (Weiss and Kennedy 1956, 1960). 

Bishop, JE, and Hajra, AK. A method for the chemical synthesis of C-labeled fatty acyl coenzyme A s of high specific activity. Analytical Biochemistry 1980 106 344-350... 

The first step in the activation of a fatty acid— either for energy-yielding oxidation or for use in the synthesis of more complex lipids—is the formation of its thiol ester (see Fig. 17-5). The direct condensation of a fatty acid with coenzyme A is endergonic, but the formation of fatty acyl-CoA is made exergonic by stepwise removal of two phosphoiyl groups from ATP. First, adenylate (AMP) is transferred from ATP to the carboxyl group of the fatty acid, forming a mixed anhydride... 

This three-step process for transferring fatty acids into the mitochondrion—esterification to CoA, transesterification to carnitine followed by transport, and transesterification back to CoA—links two separate pools of coenzyme A and of fatty acyl-CoA, one in the cytosol, the other in mitochondria These pools have different functions. Coenzyme A in the mitochondrial matrix is largely used in oxidative degradation of pyruvate, fatty acids, and some amino acids, whereas cytosolic coenzyme A is used in the biosynthesis of fatty acids (see Fig. 21-10). Fatty acyl-CoA in the cytosolic pool can be used for membrane lipid synthesis or can be moved into the mitochondrial matrix for oxidation and ATP production. Conversion to the carnitine ester commits the fatty acyl moiety to the oxidative fate. 

Conversion of a free fatty acid to its activated form A fatty add must be converted to its activated form (attached to coenzyme A) before it can participate in TAG synthesis. This reaction, illustrated in Figure 15.6 (see p. 175), is catalyzed by a family of fatty acyl Co A synthetases (thiokinases). 

Oxidation of a-amino acids to keto acids catalysed by D- and L-amino acid oxidases Oxidation of NADH via the cytochrome system catalyzed by cytochrome reductase Energy production via the TCA or Krebs cycle catalyzed by succinate dehydrogenase Fatty acid oxidation catalyzed by acyl-coenzyme A dehydrogenases Synthesis of fatty acids from acetate (80,81)... 

As a result of the reduced activity of the mutase in vitamin B12 deficiency, there is an accumulation of methyhnalonyl CoA, some of which is hydrolyzed to yield methylmalonic acid, which is excreted in the urine. As discussed in Section 10.10.3, this can be exploited as a means of assessing vitamin B12 nutritional status. There may also be some general metabolic acidosis, which has been attributed to depletion of CoA because of the accumulation of methyl-malonyl CoA. However, vitamin B12 deficiency seems to result in increased synthesis of CoA to maintain normal pools of metabolically useable coenzyme. Unlike coenzyme A and acetyl CoA, neither methylmalonyl CoA nor propionyl CoA (which also accumulates in vitamin B12 deficiency) inhibits pantothenate kinase (Section 12.2.1). Thus, as CoA is sequestered in these metabolic intermediates, there is relief of feedback inhibition of its de novo synthesis. At the same time, CoA may be spared by the formation of short-chain fatty acyl carnitine derivatives (Section 14.1.1), which are excreted in increased amounts in vitamin B12 deficiency. In vitamin Bi2-deficient rats, the urinary excretion of acyl carnitine increases from 10 to 11 nmol per day to 120nmolper day (Brass etal., 1990). 

The final step in fatt acid synthesis is the dischar ge of the fatty acid from the sulfhydryl group of fatty acid synthase. This discharge involves the attack of a molecule of coenzyme A, resulting in the release of the fatly acid as fatty acyl-CoA, as shown in Figure 5.14. 

In muscle, most of the fatty acids undergoing beta oxidation are completely oxidized to C02 and water. In liver, however, there is another major fate for fatty acids this is the formation of ketone bodies, namely acetoacetate and b-hydroxybutyrate. The fatty acids must be transported into the mitochondrion for normal beta oxidation. This may be a limiting factor for beta oxidation in many tissues and ketone-body formation in the liver. The extramitochondrial fatty-acyl portion of fatty-acyl CoA can be transferred across the outer mitochondrial membrane to carnitine by carnitine palmitoyltransferase I (CPTI). This enzyme is located on the inner side of the outer mitochondrial membrane. The acylcarnitine is now located in mitochondrial intermembrane space. The fatty-acid portion of acylcarnitine is then transported across the inner mitochondrial membrane to coenzyme A to form fatty-acyl CoA in the mitochondrial matrix. This translocation is catalyzed by carnitine palmitoyltransferase II (CPTII Fig. 14.1), located on the inner side of the inner membrane. This later translocation is also facilitated by camitine-acylcamitine translocase, located in the inner mitochondrial membrane. The CPTI is inhibited by malonyl CoA, an intermediate of fatty-acid synthesis (see Chapter 15). This inhibition occurs in all tissues that oxidize fatty acids. The level of malonyl CoA varies among tissues and with various nutritional and hormonal conditions. The sensitivity of CPTI to malonyl CoA also varies among tissues and with nutritional and hormonal conditions, even within a given tissue. Thus, fatty-acid oxidation may be controlled by the activity and relative inhibition of CPTI. 

Elongation of the acyl group to make fatty acids longer than 16 carbons (palmitate) occurs apart from palmitate synthesis. Whereas palmitate synthesis occurs in the cytosol, elongation occurs in both the mitochondria and endoplasmic reticulum (ER). The ER is the dominant system. Elongation in the ER differs from cytosolic synthesis in employing coenzyme A (instead of acyl carrier protein) and separate enzymes (instead of a complex). The condensation reaction occurs between malonyl-CoA and an acyl-CoA to form a / -ketoacyl-CoA (see here and here). Two enzymes catalyze this step in the endoplasmic reticulum, one of which is specific for unsaturated fatty acyl-CoAs. 

Figure 4 (A) Coordinated interaction of members of the condensing (KAS) enzyme family results in biosynthesis of fatty acids in E. coli. The genes encoding each KAS as well as major destinations of the fatty acyl products are shown. Lipoic acid is a precursor of the coenzyme lipoamide. Lipid A consists of p-hydroxymyristate linked to saccharides in the cell membrane. PL are the membrane phospholipids. (B) How do KASes interact with one another and the other members of the FAS complex ACP, acyl carrier protein KR, p-ketoacyl-ACP reductase DH, P-hydroxyacyl-ACP dehydrase ER, enoylacyl-ACP reductase MAL TR, malonyl-CoA ACP transacylase TE, thioesterase AC TR acetyl-CoA ACP transacylase whose contribution to fatty acid synthesis is uncertain since the discovery and characterization of KAS III [33,38].

Intermediates in fatty acid synthesis are linked covalently to the suifhydryl groups of special proteins, the acyl carrier proteins. In contrast, fatty acid breakdown intermediates are bound to the —SH group of coenzyme A. Fatty acid synthesis occurs in the cytosol, whereas fatty acid degradation takes place in mitochondria. 

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