Acyl-CoA thioesters

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. 

Reaction 3 is analogous to the dehydrogenation of fatty acyl-CoA thioesters (see Figure 22—3). In isovaleric acidemia, ingestion of protein-rich foods elevates isovalerate, the deacylation product of isovaleryl-CoA. Figures 30-20, 30-21, and 30-22 illustrate the subsequent reactions unique to each amino acid skeleton. 

Defects of mitochondrial transport interfere with the movement of molecules across the inner mitochondrial membrane, which is tightly regulated by specific translocation systems. The carnitine cycle is shown in Figure 42-2 and is responsible for the translocation of acyl-CoA thioesters from the cytosol into the mitochondrial matrix. The carnitine cycle involves four elements the plasma membrane carnitine transporter system, CPT I, the carnitine-acyl carnitine translocase system in the inner mitochondrial membrane and CPT II. Genetic defects have been described for each of these four steps, as discussed previously [4,8,9]. 

Boelsterli UA. Xenobiotic acyl glucuronides and acyl CoA thioesters as protein-reactive metabolites with the potential to cause idiosyncratic drug reactions. Curr Drug Metab 2002 3(4) 439-450. 

Once inside cells, fatty acids are activated at the outer mitochondrial membrane by conversion to fatty acyl-CoA thioesters. Fatty acyl-CoA to be oxidized enters mitochondria in three steps, via the carnitine shuttle. 

Figure 1 Polyketide biosynthesis. Polyketide backbones are formed via condensations from acyl-CoA thioesters of carboxylic acids. The (3-ketone which results from each condensation can undergo a series of reductive steps analogous to fatty acid biosynthesis. However, either none or only some of the reductive activities may occur in a given cycle. This allows PKSs to generate diversity through selection of priming and extender units, variation of the reductive cycle, and stereoselectivity. (ACP, acyl carrier protein AT, acyl transferase KS, ketosynthase DH, dehydratase ER, enoylreductase KR, ketoreductase TE, thioesterase.) The structure depicted in the lower right-hand corner is representative of the possible structural variations that can arise during polyketide biosynthesis.

Whereas we have no intention to describe in this review the various aspects of halobacterial metabolism, we would like to mention several unique features of their metabolic system. The conversions of the two 2-oxoacids (pyruvate and oxoglutarate) to their corresponding acyl-CoA thioesters are crucial steps in the two pathways described above. In most eukaryotes and aerobic eubacteria these reactions are catalyzed by the 2-oxoacid dehydrogenase multienzyme complexes that use NAD+ as the final electron acceptor. These complexes are... 

Acetyl-CoA is formed from CoA and acetate by the enzyme acetyl-CoA synthetase, an ADP-forming ligase. Phosphotrans-acetylase forms acetyl-CoA from CoA and acetyl-phosphate, which in turn is formed from acetate and ATP catalyzed by acetate kinase. Other enzymes that can form acetyl-CoA from CoA and other acetyl group donors include ATP citrate lyase and thiolase. Longer chain acyl-CoA thioesters are typically formed from CoA and a fatty acid catalyzed by ligases generally known as acyl-CoA synthetases. 

MCAD is one of the best studied flavoprotein dehydrogenases (10). In this enzyme, the pro-R a-hydrogen of the acyl-CoA thioester is removed by the catalytic base Glu376 and the pro-R 3-hydrogen of the substrate is transferred directly to flavin N5 as a hydride (11) (Fig. 2a). MCAD is inactivated by a range of acyl-CoA derivatives. One such compound is methylenecyclopropylacetyl-CoA which acts as a suicide inhibitor by forming a covalent adduct with flavin N5 (see Further Reading for more information). 

Hepatic lipase is involved in the metabolism of high-density lipoproteins and intermediate density lipoproteins (IDLs), converting the HDL2 fraction to HDL3 and generating LDLs from IDLs. The enzyme appears to have broad specificity it hydrolyzes tri-, di-, and mono-acylglycerols, acyl-CoA thioesters, and even phospholipids. hHL is secreted by the liver parenchymal cells and does not require any cofactors for its activity. 

The a,-dehydrogenation is catalyzed by an FAD protein and is analogous to the dehydrogenation of straight-chain acyl-CoA thioesters in jd-oxidation of fatty acids (Chapter 18). Methylenecyclopropylacetyl-CoA derived from the plant toxin hypoglycin (Chapters 15 and 18), which inhibits this step in )S-oxidation, also inhibits it in the catabolism of branched-chain amino acids. 

R—COOH) forming an acyl-CoA thioester (R—CO—S— CoA) as the metabolic intermediate and as a cofactor. The reaction requires ATP and is catalyzed by various acyl-CoA synthetases also known as acyl-CoA Ugases (Table 32.5) of overlapping substrate specificity. The acyl-CoA conjugates thus formed are seldom excreted, but they can be isolated and characterized relatively easily in in vitro studies. In the present context, the interest of acyl-CoA conjugates is then-further transformation by a considerable variety of path-ways22.37,52 54 summarized in Table 32.6. 

FIGURE 32.10 A few examples of reactions of acetylation (a), and of some reactions consecutive to the formation of xenobiotic acyl-CoA conjugates (b). The substrates are salicylic acid (19), (R)-ibuprofen (21), and valproic acid whose intermediate acyl-CoA thioester is shown here (22). The arrows point to the target sites. 

Hertz, R., Magenheim, J., Berman, I., and Bar-Tana, J. (1998) Fatty acyl-CoA thioesters are ligands of hepatic nuclear factor-4alpha. Nature 392, 512-516. 

ACADs catalyze the oxidation of acyl-CoA thioesters, forming the corresponding enoyl-CoA ester and reduced flavin " (Equation (8)). 

Medium-chain acyl-CoA dehydrogenase also donates electrons to FTF and this reaction has been studied kinetically as well. Medium-chain acyl-CoA dehydrogenase is reduced by two electrons by an acyl-CoA thioester (see Section 7.03.2). The electrons are then passed one at a time to FTF reforming oxidized dehydrogenase. The product of the reductive half-reaction, enoyl-CoA, is not released until the enzyme has been fully oxidized. " ... 

The a-oxoamine synthases family is a small group of fold-type I enzymes that catalyze Claisen condensations between amino acids and acyl-CoA thioesters (Figure 16). Members of this family are (1) 8-amino-7-oxononanoate (AON) synthase (AONS), which catalyzes the first committed step in the biosynthesis of biotine, (2) 5-aminolevulinate synthase (ALAS), responsible for the condensation between glycine and succinyl-CoA, which yields aminolevulinate, the universal precursor of tetrapyrrolic compounds, (3) serine palmitoyltransferase (SPT), which catalyzes the first reaction in sphingolipids synthesis, and (4) 2-amino-3-ketobutyrate CoA ligase (KBL), involved in the threonine degradation pathway. With the exception of the reaction catalyzed by KLB, all condensation reactions involve a decarboxylase step. 

The acyl-CoA ligases (EC 6.2.1.- often also referred to as acyl-CoA synthetases, or ACSs) catalyze the reversible nucleoside triphosphate-dependent formation of acyl-CoA thioesters from CoA and a free carboxylic acid. Two mechanistic types can be distinguished in this group of enzymes the first uses ATP to activate... 

CoA-transferase enzymes (EC 2.8.3.—) catalyze the reversible transfer of CoA between an existing acyl-CoA thioester and a free carboxylic acid. No cofactors are involved in the reaction and no prior activation of the carboxylic acid is required (Equation (18)). 

Degradation of fatty acids proceeds via an inducible set of enzymes that catalyze the pathway of P-oxidation [18]. P-Oxidation occurs via repeated cycles of reactions that are essentially the reverse of the reactions of fatty acid synthesis (Fig. 8). However, three major differences distinguish the two pathways. First, P-oxidation utilizes acyl-CoA thioesters and not acyl-ACPs. Second, the P-hydroxy intermediates have the opposite stereochemistry (L in P-oxidation and d in synthesis). Finally, the enzymes of P-oxidation share no homology with those of synthesis. 

The metabolism of fatty acids requires their prior activation by conversion to fatty acyl-CoA thioesters. The activating enzymes are ATP-dependent acyl-CoA synthetases, which catalyze the formation of acyl-CoA by the following two-step mechanism in which E represents the enzyme ... 

Fatty acyl-CoA thioesters that are formed at the outer mitochondrial membrane cannot directly enter the mitochondrial matrix where the enzymes of P-oxidation are located, because CoA and its derivatives are unable to pass rapidly through the inner mitochondrial membrane. Instead, carnitine carries the acyl residues of acyl-CoA thioesters across the inner mitochondrial membrane. The carnitine-dependent translocation of fatty acids across the inner mitochondrial membrane is schematically shown in Fig. 1 [8]. The reversible... 

In the mitochondrial matrix, carnitine palmitoyltransferase II (CPT II) catalyzes the reversible transfer of acyl residues with 10-18 carbon atoms between carnitine and CoA to form acyl-CoA thioesters that are the substrates of P-oxidation [4]. CPT II purified from mitochondria of bovine heart and rat liver has a subunit molecular mass of approximately 70 kDa. The crystal stmcture of CPT II revealed the presence of two antiparallel helices that are absent from soluble carnitine acyltransferases and are believed to facilitate the association with the inner mitochondrial membrane (M. Henning, 2006). 

The ACDs constitute a family of flavin-containing enzymes with at least nine members that catalyze the a,/3-oxidation of fatty acyl-CoA thioesters (see Chapter 7.03). Interest in the structure and mechanism of ACDs stems in part from their potential role in diseases such as sudden infant death syndrome. Most ACDs are homotetramers binding one molecule of flavin adenine dinucleotide (FAD) per subunit, and MCAD, from the mitochondrial /3-oxidation pathway, is the most heavily studied family member. Several reviews on the structure and mechanism of the ACDs have appeared, and the reader s attention is drawn to Thorpe and Kim, Kim and Miura, and Ghisla and Thorpe. ... 

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