Acyl thiokinase

FIGURE 4.5 Reaction scheme for conversion of octanoic acid into esters, E, acyl-thiokinase E2, aeyl-CoA-alcohol-transacylase E3, acyl-CoA reductase E4, alcohol-NAD oxydoreductase. (From Tressl, R., R Drawert, J. Agric. Food Chem., 21, p. 560, 1973. With permission.)... 

Reaction (1) is catalysed by three different acyl-thiokinases, specifically depending on the length of the fatty acyl chain, and although catalysed by a single enzyme, actually proceeds in the two-step sequence of reactions (3) and (4) ... 

Furthermore, a second type of ATP-dependent acyl thiokinase is located in the inner compartment of the mitochondrion (mitochondrial matrix). This enzyme uses endogenous CoASH and ATP formed through the oxidative phosphorylation machinery. 

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). 

After a LCFA enters a cell, it is converted to the CoA derivative by long-chain fatty acyl CoA synthetase (thiokinase) in the cytosol (see p. 174). Because 0-oxidation occurs in the mitochondrial matrix, the fatty acid must be transported across the mitochon drial inner membrane. Therefore, a specialized carrier transports the long-chain acyl group from the cytosol into the mitochondrial matrix. This carrier is carnitine, and the transport process is called the carnitine shuttle (Figure 16.16). 

It is likely that pyruvate, the product of the oxidative branch of the mitochondrial dismutation reaction, is further metabolised in cestodes to acetyl-CoA by oxidation with NAD+, as catalysed by the lipoamide-dependent mitochondrial pyruvate dehydrogenase complex. This enzyme has been reported in H. diminuta (935) and S. solidus (406). The acetyl-CoA is then hydrolysed to acetate. During this step, ATP synthesis may occur through the conservation of the acetyl-CoA energy-rich thioester bond by the combined action of an acyl-CoA transferase and a thiokinase (398) as follows ... 

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. 

Eugene Kennedy and Albert Lehninger showed in 1949 that fatty acids are oxidized in mitochondria. Subsequent work demonstrated that they are activated before they enter the mitochondrial matrix. Adenosine triphosphate (ATP) drives the formation of a thioester linkage between the carboxyl group of a fatty acid and the sulfhydryl group of CoA. This activation reaction takes place on the outer mitochondrial membrane, where it is catalyzed by acyl CoA synthetase (also called fatty acid thiokinase). 

Fatty acid + CoA + ATP acyl CoA + AMP + Acyl CoA synthetase [also called fatty acid thiokinase and... 

Free fatty acids are converted to fatty acyl-CoA by fatty acid thiokinase (Figure 5.6). The fatty acids are broken down, not when in their free form but in the tiiiol ester form. Breakdown of fatty acids results in step-by-step discharge of two-carb-on units as acetyl-CoA. This breakdown also results in the production of FADH2 and NADH + For each two-carbon imit liberated, one molecule each of FAD and NAD is reduced (Figure 5.7). 

The answer is d. (Murray, pp 230-267. Scriver, pp 2297-2326. Sack, pp 121-138. Wilson, pp 287-320.) Fatty acids must be activated before being oxidized. In this process, they are linked to CoA in a reaction catalyzed by thiokinase (also known as acyl CoA synthetase). ATP is hydrolyzed to AMP plus pyrophosphate in this reaction. In contrast, the enzyme thiolase cleaves off acetyl CoA units from p-ketoacyl CoA, while it forms thioesters during P oxidation. 

Acyl-activating enzyme. Acyl-CoA synthetase. Fatty acid thiokinase (long-chain). Lignoceroyl-CoA synthase. 

The importance of energy generation from acyl CoA hydrolysis, although feasible, is less well documented, although thiokinase activities have been measured in helminth extracts. In adult A. suum muscle, the intramitochondrial levels of free CoASH are very low and the transfer of the CoA moiety appears to be mediated by a number of distinct CoA transferases (62,77). Low free CoASH levels may be critical for the formation of branched-chain fatty acids, since the initial reaction in this sequence, catalyzed by propionyl CoA condensing enzyme, is potently inhibited by free CoASH (78). Low free... 

Fatty acids must be activated to acyl CoA derivatives before they can participate in 3-oxidation and other metabolic pathways (Fig. 23.2). The process of activation involves an acyl CoA synthetase (also called a thiokinase) that uses ATP energy to form the fatty acyl CoA thioester bond. In this reaction, the p bond of ATP is cleaved to form a fatty acyl AMP intermediate and pyrophosphate (PPi). Subsequent cleavage of PPi helps to drive the reaction. 

After entering the cytoplasm, the fatty acids are trapped in the cell via the action of thiokinase that yields fatty-acyl-CoA. The activated fatty acids are either oxidized after transport into the mitochondria or reesterified with glycerol to give triacylglycerols that are stored in lipid droplets in the cytoplasm. The esterification may be only partial, giving diacylglycerols that are intermediates in phospholipid synthesis. 

P-oxidation of fatty acids is a major metabolic process in which fatty acids are degraded in the mitochondria and peroxisome to produce energy [59, 60], P-oxidation occurs at the P-carbon (C-3) of the fatty acid. However, fatty acids must be activated for degradation before being P-oxidized, because negatively charged fatty acids cannot enter the plasma membrane. Activation of fatty acids are catalyzed by fatty acyl-CoA synthetase (FACS, or called thiokinases) to form fatty acyl-CoA thioester [61]. The net reaction of this activation process is ATP-dependent. 

The direct route of acyl coenzyme A synthesis from a free carboxylic acid is catalysed by a group of nucleoside triphosphate-requiring en mes, collectively known as thiokinases. The general mechanism, as exemplified for acetate activation by acetyl thiokinase, proceeds as follows. The carboxylic acid is first activated by acetyl adenylate formation with the displacement of pyrophosphate from ATP. While the initial reaction is fully reversible, subsequent action of pyrophosphatase drives the reaction... 

Thioesters of coenzyme A (D 11) are important intermediates in carboxylic acid metabolism. They are formed by acid-tbiol bgases (thiokinases) either via acyl phosphates ... 

Thiokinase catalyzes a reaction between the carboxyl group of the fatty acid and reduced CoA to yield acyl-CoA. ATP and magnesium are required in the reaction, and AMP and pyrophosphate are formed. The details of the mechanism of the reaction are not known, but Ingraham and Green have postulated the existence of a number of intermediates in the formation of which magnesium is assumed to play an essential role. 

Enzymes (1) Lipases (2) Fatty acid thiokinase (3) Acyl CoA dehydrogenase (4) Crotonase (5) jS-Hydroxyacyl CoA (6) )5-Ketoacyl thiolase (7) Citrate synthetase (8) Aconitase (9) Isocitrate lyase (10) Malate synthetase (11) Malate dehydrogenase (12) Catalase (13) Succinate dehydrogenase (14) Fumarase (15) Malate... 

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 ... 

Points worth noting are (1) All the reactants are acyl derivatives of CoA. (2) All the enzymes are localized within the mitochondria with those of the citrate cycle and electron-transport chain. This ensures efficient utilization of the acetyl-CoA released by the fatty acid oxidation. (3) Only one activation step is necessary, regardless of the length of the fatty acid chain. This uses two high-energy bonds, since ATP is broken down to AMP and pyrophosphate during the thiokinase reaction. 

Fatty acids are oxidized only in the form of fatty a< l-CoA derivatives, and mitochondria from mammalian tissues contain the full equipment of enzymes necessary for the synthesis and the degradation of fatty acyl-CoA. The enzymes involved in the oxidative process are located in the mitochondrial matrix, and the inner mitochondrial membrane sequesters the oxidative process from the rest of these organelles. On the contrary, the fatty acids activating enzymes (thiokinase) seem to be present in different compartments of the mitochondrion and widely distributed among the subcellular fractions. The significance of this may lie in the fact that the conditions required for fatty acyl-CoA oxidation differ from those required for other CoA—SH dependent pathways. 

The synthesis of acyl-CoA is accomplished by two different enzymatic reactions in the first an ATP-specific thiokinase is involved ... 

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