Fatty Thiokinase

Synthesis of PHAMCL from fatty acids such as octanoic acid or from the corresponding alkanes such as octane was first detected in P. oleovorans [119]. The alkanes are oxidized to the fatty acids the latter are activated by thiokinases and then degraded via the fatty acid /1-oxidation pathway. Obviously intermediates of this pathway accumulate under conditions favorable for the synthesis of PHA and are subsequently converted into substrates for the PHA synthase. Many reactions for the conversion of an intermediate of the -oxidation cycle into R-(-)-3-hydroxyacyl-CoA were considered. These were ... 

The generation of GTP by succinate thiokinase is another example of substrate-level phosphorylation (see p. 100). [Note Succinyl CoA is also produced from propionyl CoA derived from the metabolism of fatty acids with an odd number of carbon atoms (see p. 191), and from metabolism of several amino acids (see p. 264). 

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). 

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. 

It starts however on the cytosolic side With fatty acids coming from triglyceride Activated in the thiokinase way To thioester of CoA. 

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... 

What are the effects of the other metabolites of ethanol Liver mitochondria can convert acetate into acetyl CoA in a reaction requiring ATP. The enzyme is the thiokinase that normally activates short-chain fatty acids. 

The conversion of ethanol to acetic acid, by sequential action of the ethanol-oxidizing system of the ER and acetaldehyde dehydrogenase, is shown in Figure 4.70. The conversion of acetic acid to a cetyl-Co A, as catalyzed by thiokinase, is shown in Figure 4,71. Thiokinase accepts short-chain fatty acids, as well as acetic acid, as its substrate. Acetj l-CoA is a substrate of the Krebs cycle. 

Free fatty acids are converted to fatty acyhCoA by fatty acid thiokinase (Figure 5-6). The fatty acids are broken down, not when in their free form but in the thiol ester form. Breakdown of fatty acids results in step-by-step discharge of two-carbon units as acetyl-CoA. This breakdown also results in the production of FADH2 and NADH + For each two-carbon unit liberated, one molecule each of FAD and NAD is reduced (Figure 5.7). 

FIGURE 4.71 Short-chain fatty acid reactions catalyzed by thiokinase. 

The products of the isoleucine catabolic pathway are propionyl-CoA and ace-tyl-CoA valine catabolism produces one molecule of propionyl-CoA and two molecules of carbon dioxide. Propionyl-CoA is further cataboli25ed to succinyl-CoA, an intermediate of the Krebs cycle (Figure 8.7). This pathway is also used for catabolism of the short-chain fatty acid propionic acid, after its conversion to the thiol ester form by thiokinase. The first step in propionyl-CoA breakdown is catalyzed by propionyl-CoA carboxylase, a biotin-requiring enzyme. The second step is catalyzed by methylmalonyl-CoA mutase, a vitamin Bi2-requiring enzyme. 

A short-chain fatty add thiokinase catalyzes the ATP-dependent conversion of propionic acid to propionyl-CoA. Which cells of the gastrointestinal tract might be expected to have high levels of short-chain thiokinase and of methylmalonyl-CoA mutase. Hint see the section on Dietary Fiber in Chapter 3. 

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. 

The first reaction in the fatty acid oxidation sequence is the thioesterification of the acid by CoA-SH. This involves bond formation between the CoA-S and the fatty acid and is therefore endergonic it is achieved by linking it to the simultaneous hydrolysis of ATP to AMP. The enzyme which does the job is a thiokinase (27). 

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. 

Roughan etal 2) compared acetate, pyruvate and malonate as potential precursors and found that acetate was about three times better than pyruvate while malonate was not used at all. Consistent with this result is the report of Kuhn elal Q) who found that acetate concentration in plant tissue is in the order of mM and that acetate thiokinase is localized in the plastld. Nevertheless, alternative substrates can not be entirely excluded at this stage. Schulze-Siebert etal (4) have reported that when chloroplasts are incubated with bicarbonate, pyruvate accumulates in the chloroplast. Furthermore Williams and Randall (5) have reported that pyruvate dehydrogenase of pea chloroplasts has an activity of 6-9 pmol/h/mg chlorophyll. It is therefore conceivable that the acetyl CoA used in the first steps of fatty acid synthesis is derived from pyruvate. 

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. 

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