Acyl-CoA ligases

We now explore the remarkable process by which a long-chain saturated fatty acid is converted into two-carbon units (acetate), which can be oxidized to C02 and H20 via the tricarboxylic acid cycle and the electron-transport chain. Fatty acids that enter cells are activated to their CoA derivatives by the enzyme acyl-CoA ligase and transported into the mitochondria for /3 oxidation as we discuss later in this chapter. 

Fatty acids taken into cells are first activated in the cytosol by reaction with CoA and ATP to yield fatty acyl-CoA in a reaction catalyzed by acyl-CoA ligase ... 

Fatty acids are utilized as fuels by most tissues, although the brain, red and white blood cells, the retina, and adrenal medulla are important exceptions. Catabolism of fatty acids requires extramitochondrial activation, transport into mitochondria, and then oxidation via the /3-oxidative pathway. The initial step is catalyzed by fatty acyl-CoA synthetase (also called thiokinase and fatty acyl-CoA ligase), as shown in Equation (19.5). The product, fatty acyl-CoA, then exchanges the CoA for carnitine, as shown in Equation (19.6) ... 

The conjugation of carboxylic acid xenobiotics with amino acids occurs in both liver and kidney and is catalyzed by an enzyme system located in the mitochondria. Conjugation requires initial activation of the xenobiotic to a Co A derivative in a reaction catalyzed by acyl CoA ligase. The acyl CoA subsequently reacts with an amino acid, giving rise to acylated amino acid conjugate and CoA. 

If we consider the requirements for the coupling of the lipid and 13 amino acids, followed by ring closure, the multi-subunit NRPS contains 45 enzymatic functions (13 condensation (C) domains, 13 adenylation (A) domains, 13 thiolation (T) domains, three epimerase (E) domains, one thioesterase (Te) domain, one acyl-CoA ligase and one ACP Figure 14.2). 

Fatty acids must be activated in the cytoplasm in order to enter the mitochondrion (where the /S-oxidation pathway occurs (Figure 2.7)). Activation is catalysed by fatty acyl-CoA ligase (also called acyl-CoA synthetase or thiokinase). The net result of this activation process is the consumption of 2 molar equivalents of ATP. 

Ulrich Gehring and Feodor Lynen Acyl-CoA Ligases... 

Peroxisomal membrane possesses an acyl-CoA ligase activity that is specific for very long-chain fatty acids. Mitochondria apparently cannot activate long-chain fatty acids such as tetracosanoic (24 0) and hexacosanoic (26 0). Peroxisomal carnitine acyltransferases catalyze the transfer of these molecules into peroxisomes, where they are oxidized to form acetyl-CoA and medium-chain acyl-Co A molecules (i.e., those possessing between 6 and 12 carbons). Medium-chain acyl-Co As are further degraded via /3-oxidation within mitochondria. 

Bremer and Gloor [40] concluded that enzymes for both reactions were present in hepatic microsomes, but recent studies with microsomes of rat [40-42] and human liver [43] have confirmed the presence of only one enzyme, CoA ligase. The assay system, essentially that for long-chain acyl-CoA ligase [42,43], includes 50 mM NaF, a phosphate buffer (pH 7.5), the enzyme preparation, and constituents of Eqn. 1. Product formation was linear up to 12 min with added protein (between 0.1 and 1.2 mg) from a crude microsomal fraction. Sterol carrier protein [44], cysteine or nicotinamide [38,40] were without effect. This rate-limiting enzyme in the two-step sequence catalyzing conjugation of bile acids exhibits a diurnal variation such that the time of maximum enzyme activity coincides with predicted maximum activity of cholesterol 7a-hydroxylase and the time of maximal biosynthesis of bile acids [45]. The enzyme has not been purified. 

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... 

Rifamycin B, produced by Amycolatopsis mediterranei, is one of the most notable members of the ansamycin family [36, 37, 64, 65] (Fig. 14). It has been used clinically in a synthetically modified form called rifampicin and it is still one of the first-line therapies effective in the treatment of tuberculosis and other mycobacterial infections. The starter unit for rifamycin polyketide assembly is part of the chromophore and is derived from 3-amino-5-hydroxybenzoic acid. Five polyketide synthases are involved in the formation of rifamycin chromophore and the first polyketide synthase contains at the N terminus the loading domain for 3-amino-5-hydroxybenzoic acid, which consists of an acyl-CoA ligase linked to ACP, and module 1-3. The rifamycin polyketide synthase lacks a TE domain at the C terminus. The release of polyketide chain from polyketide synthase and the formation of amide to generate the macrocyclic lactam will be catalyzed by RifF, which is very similar to arylamine A-acetyltransferase. 

Fatty acid + ATP + CoASH <=> Fatty acyl-CoA + AMP + PPi (catalyzed by Fatty acyl-CoA Ligase). 

Enzymes that act on acyl-CoAs include thiolase, fatty acyl-CoA ligase, fatty acyl-CoA dehydrogenase, enoyl-CoA hydratase, 3-hydroxyacylCoA dehydrogenase, enoyl-CoA isomerase, and 2,4 dienoyl-CoA reductase. 

Fatty acyl-CoA ligases (specific for short, medium, or long chain fatty acids) catalyze formation of fatty acyl thioester conjugate with coenzyme A (Diagram)... 

Fig. 31.33 Mechanism of the chiral inversion of / )- or (S)-aryl-2-propionic acids. Reactions 1, 2 and 3 are catalysed by acyl-CoA ligases, acyl-CoA hydrolases and acyl-CoA epimerases, respectively.

Reaction mechanism of formation of acyl-CoA intermediates by the acyl-CoA ligases... 

Fig. 31.34 Reaction scheme of the acyl-CoA formation catalysed by the acyl-CoA ligases. (A) Reaction equation (B) suggested ordered mechanism. AMP, adenosine monophosphate PPi, pyrophosphate (adapted from Vessey and...

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 ligases (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 palh-ways22,37,52-54 summarized in Table 32.6. 

Purity of Mitochondrial Fractions and Acyl-CoA Ligase Activity... 

Figure 1. Acyl-CoA ligase activity in microsomal fractions (MCS), crude mitochondrial fractions (CMF) and highly purified mitochondrial fraction (HPMF). Values are expressed as nmol of palmih l-CoA (palm-CoA) synthesized/min for the indicated amounts of protein. Results which were obtained from subcellular fractions of the liver of one rat, are refnesentative of fractions isolated from four rats.

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