Fatty 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) ... 

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. 

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)... 

Noy, N. Zakim, D. (1985) Biochim. Biophys. Acta, 833, 239-244. Substrate specificity of fatty-acyl-CoA ligase in liver microsomes. 

The carboxyl group of a fatty acid provides a point for chemical attack. The first step is a priming reaction in which the fatty acid is converted to a water-soluble acyl-CoA derivative in which the a hydrogens of the fatty acyl radicals are "activated" (step a, Fig. 17-1). This synthetic reaction is catalyzed by acyl-CoA synthetases (fatty acid CoA ligases). It is driven by the hydrolysis of ATP to AMP and two inorganic... 

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 ... 

The activating enzyme occurs in the mitochondria and belongs to a class of enzymes known as the ATP-dependent acid CoA ligases (AMP) but has also been known as acyl CoA synthetase and acid-activating enzyme. It appears to be identical to the intermediate chain length fatty acyl-CoA-synthetase. 

In firefly luciferase reaction, the luminescence activity is enhanced by addition of Coenzyme A (CoA) and this phenomenon is explained by release of product inhibition. Also, firefly luciferase shows the sequence similarity to mammalian fatty acyl-CoA synthetase (AcCoAS) and plant 4-coumarate CoA ligase (4CL). They are classified as an adenylation enzyme for synthesizing acyl-CoA derivatives fi om carboxylic acid compounds in the presence of CoA, ATP and Mg (Scheme 1). Furthermore, it was reported that the luminescence activity of firefly luciferase is inhibited competitively by various long-chain fatty acids. We have determined that firefly luciferase is a bi-functional enzyme, catalyzing both the luminescence reaction and fatty acyl-CoA synthetic reaction. ... 

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. 

It also appears that tiaprofenic acid, an NSAID that also undergoes inversion in rats, is not a substrate for purified microsomal rat liver long-chain acyl-CoA synthetase for which R-ibuprofen is a substrate [25]. This data may suggest that metabolic pathways involved in the inversion of tiaprofenic acid and possibly other 2-APA NSAIDs are different from those known for R-ibuprofen. It has been recently reported that in both an in vitro cell-free system and in rat liver homogenates the chiral inversion of ibuprofen was apparent when both CoA and ATP were present however, the NSAID KE-748 was not inverted [26]. To induce hepatic microsomal and outer mitochondrial long-chain fatty acid CoA ligase, rats were treated with clofibric acid [27]. Whereas chiral inversion of ibuprofen was enhanced significantly compared to controls, this was not the case for R(—)-KE-748. 

Fig. 2. Various reactions leading to acyl-CoA. The pyruvate dehydrogenase reaction is in fact a sequence of five reactions catalyzed by a three enzyrmes complex. It is the major pathway producing acetyl-CoA, a key molecule in many important metabolic pathways such as the oxidative degradation of glucides (citric acid cycle, also known as Krebs cycle ). Acetyl-CoA is also the starting point for the symthesis of fatty acids, through the fatty acid synthase reaction, which increases the length of the starting fatty acid (R) by two carbon atoms. Most of the other acyl-CoAs are obtained through the acyl-CoA ligase reaction.

Similarly the fatty acids must be activated by conversion to their CoA derivatives before they can be metabolized. Formation of the fatty acyl-CoA derivatives is catalysed by various/affy acid thiokinases (fatty acid CoA ligases) whose activity is linked with the breakdown of ATP to AMP and pyrophosphate, the liberated energy being used in the formation of the thiol ester bond ... 

As shown in Table 1, crude mitochondrial fractions prepared by differential centrifugation (as other slightly different conventional methods) appeared to exhibit pronounced aryl-ester hydrolase (EC 3.1.1.2) activity. As this activity was found specifically in miaosomes, mitochondrial fractions prepared conventionally are always contaminated by miCTOsomes. In mitochondrial fractions isolated on Percoll gradient under the conditions precisely described in the Materials and Methods section, the above microsomal activity was extremely low. The pattern was similar for acyl-CoA ligase (ACL), its specific activity was very high in microsomal fractions and almost negligible in Percoll-purified mitochondrial fractions (Fig. 1). Taking into account the specific activity of aryl-ester hydrolase in microsomal fractions, the amount of microsomal protein present in purified mitochondrial fractions could be estimated around 2% and was sufficient to account for the residual ACL activity in these fractions. It was, therefore, clear that a totally pure liver mitochondrial fraction could not esterify fatty adds with CoA. 

The fatty acid in its ionized form is activated in the long-chain-fatty-acid-CoA ligase outer surface of the eukaryotic outer mitochon- q substrates butyrate-CoA... 

Acyl-CoA synthetases are enzymes (i.e., ligases) that convert fatty acid molecules into acyl-Coenzyme A molecules for their subsequent oxidation. 

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