Acid: CoA ligase

The subsequent cleavage of the thio-ester succinylCoA into succinate and coenzyme A by succinic acid-CoA ligase (succinyl CoA synthetase, succinic thiokinase) is strongly exergonic and is used to synthesize a phosphoric acid anhydride bond ( substrate level phosphorylation , see p. 124). However, it is not ATP that is produced here as is otherwise usually the case, but instead guanosine triphosphate (CTP). However, GTP can be converted into ATP by a nucleoside diphosphate kinase (not shown). 

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... 

Figure 3-4. The general phenylpropanoid pathway. The enzymes involved in this pathway are (a) phenylalanine ammonia lyase (PAL E.C. 4.3.1.5), (b) cinnamic acid 4-hydroxylase (C4H E.C. 1.14.13.11), and (J) 4-coumaric acid CoA ligase (4CL E.C. 6.2.1.12). (a) depicts tyrosine ammonia lyase activity in PAL of graminaceous species. The grey structures in the box represent an older version of the phenylpropanoid pathway in which the ring substitution reactions were thought to occur at the level of the hydroxycinnamic acids and/or hydroxycinnamoyl esters. The enzymes involved in these conversions are (c) coumarate 3-hydroxylase (C3H E.C. 1.14.14.1), (d) caffeate O-methyltransferase (COMT EC 2.1.1.68), (e) ferulate 5-hydroxylase (F5H EC 1.14.13), and (g) caffeoyl-CoA O-methyltransferase (CCoA-OMT EC 2.1.1.104). These enzymes are discussed in more detail in Section 10.

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. 

Bile acids are also conjugated by a similar sequence of reactions involving a microsomal bile acid CoA ligase and a soluble bile acid A-acyl-transferase. The latter has been extensively purified, and differences in acceptor amino acids, of which taurine is the most common, have been related to the evolutionary history of the species. 

Long-chain-fatty-acid-coA ligase R223-RXN 0.008890324... 

Although 4CLs were believed to be specific for plants, a cinnamic acid CoA-ligase has recently been cloned from Streptomyces coelicolor A3(2). The heterologously expressed enzyme predominantly accepted 4-coumarate and cinnamate and with lower affinity caffeate, while ferulate was not accepted. Mutations of amino acid residues in the substrate-binding pocket were able to alter the substrate affinity (Kaneko et al, 2003). This bacterial 4CL gene was used for the biotechnological production of flavonoids in E. coli (see, e.g. Miyahisa et al, 2006, and the literature cited therein). 

The loading module comprises three domains. The first (CL) shows homology to ATP-dependent carboxylic acid-CoA ligases, the second is a putative enoyl reductase (ER) and the third an ACP. The probable sequence of operations starts with the enoic acid 74 derived from shikimic acid which is reduced by the ER domain. The first domain will activate the carboxylic acid to an active acyl derivative ready for transfer to the thiol residue of the ACP. The final saturated product will end up attached to the ACP as a thioester derivative ready for transfer to the KS domain of the first chain extension module. The timing of the reduction in this sequence of operations cannot be predicted. 

Conjugation of the carboxylic group in cholic acid involves participation of a cholic acid CoA ligase and a cholyl-CoA glycine (taurine) acyltransferase [153-158]. Since the primary product of the thiolytic cleavage of the CoA ester of 3a,7a,12a-tri-hydroxy-24-oxo-5/8-cholestanoic acid is the CoA ester of cholic acid, it is probable that only the transferase may be involved in the primary formation of cholic acid in the liver. A description of the properties of the CoA synthetase and the transferase is given in Chapter 11. 

Pimaricin, produced by S. natalensis, is also a 26-membered antifungal polyene macrolide antibiotic. The formation of pimaricin backbone requires 12 condensation cycles in which five polyketide synthases are involved. The domain composition in the loading module of the pimaricin polyketide synthase is different from other polyketide synthases. The formation of pimaricin is predicted to start on PIMSO, a single module polyketide synthase with an N-terminal ATP-dependent carboxylic acid CoA ligase domain (CoL), and continue to the next polyketide synthase with the formation of aglycon. 

Fig. 12. Predicted domain organization and biosynthetic intermediates of the pimaricin synthase. Each circle represents an enzymatic domain as defined in Fig. 5. CoL, carboxylic acid CoA ligase. Continued)...

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. 

Extramitochondrial fatty acid CoA ligases are destined for lipid synthesis [40]. Ibuprofen is a substrate for this enzyme, and it has therefore been demonstrated that a number of 2-APAs, including ketoprofen, ibuprofen, and fenoprofen, can be stereoselectively incorporated into lipid tissues... 

Incorporation into adipose tissue is not a general phenomenon of all NSAIDs that undergo bioinversion. NSAIDs such as KE-748 that are substrates for short-chain fatty acid CoA ligase in the mitochondrial matrix are not incorporated into adipose tissue [49]. 

Knights, K.M. Jones, M.E. Inhibition kinetics of hepatic microsomal long chain fatty acid-CoA ligase by 2-arylpropionic acid non-steroidal antiinflammatory drugs. Biochem. Pharmacol. 1992, 44, 2415-2417. 

Fig. 2. Acylglycerols. Biosynthesis of triacylglycer-ols in liver and adipose tissue cells. EC 1.1.1.8 Glycerol-3-phosphate dehydrogenase (NAD "). EC 2.3.1.15 (j ycerol-3-phosphate acyltransferase. EC 2.3.1.20 Diacylglycerol acyltransferase. EC 2.3.1.51 1-AcylgIycerol-3-phosphate acyltransferase. EC 3.1.3.4 Phosphatidate phosphatase. EC 6.2.1.3 Long-chaln-fatty acid-CoA ligase.

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 ... 

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