Acetoacetyl CoA thiolase

Ketone body synthesis occurs only in the mitochondrial matrix. The reactions responsible for the formation of ketone bodies are shown in Figure 24.28. The first reaction—the condensation of two molecules of acetyl-CoA to form acetoacetyl-CoA—is catalyzed by thiolase, which is also known as acetoacetyl-CoA thiolase or acetyl-CoA acetyltransferase. This is the same enzyme that carries out the thiolase reaction in /3-oxidation, but here it runs in reverse. The second reaction adds another molecule of acetyl-CoA to give (i-hydroxy-(i-methyl-glutaryl-CoA, commonly abbreviated HMG-CoA. These two mitochondrial matrix reactions are analogous to the first two steps in cholesterol biosynthesis, a cytosolic process, as we shall see in Chapter 25. HMG-CoA is converted to acetoacetate and acetyl-CoA by the action of HMG-CoA lyase in a mixed aldol-Claisen ester cleavage reaction. This reaction is mechanistically similar to the reverse of the citrate synthase reaction in the TCA cycle. A membrane-bound enzyme, /3-hydroxybutyrate dehydrogenase, then can reduce acetoacetate to /3-hydroxybutyrate. 

The acetoacetyl-CoA then reacts with CoASH to yield two molecules of acetyl-CoA, catalyzed by acetoacetyl-CoA thiolase... 

The enzymes responsible for their metabolism, d-P-hydroxybutyrate dehydrogenase, acetoacetate-succinyl-CoA transferase and acetoacetyl-CoA-thiolase, are present in... 

Beis A., Zammit V. A. and Newsholme V. A. (1980) Activities of 3-hydroxybutyrate dehydrogenase, 3-oxoacid CoA-transferase and acetoacetyl-CoA thiolase in relation to ketone body utilisation in muscles from vertebrates and invertebrates. Eur. J. Biochem. 104, 209-215. 

Porubek DJ, Grillo MP, Olsen RK, et al. Toxic metabolites of valproic acid inhibition of rat liver acetoacetyl-CoA thiolase by 2-n-propyl-4-pentenoic acid (A4-VP A) and related branched chain carboxylic acids. In Levy RH, Penry JK, eds. Idiosyncratic Reactions to Valproate Clinical Risk Patterns and Mechanisms of Toxicity. New York Raven Press, 1991 53-58. 

Figure 10-5. Intrahepatic metabolism of free fatty acids (FFA). CPT I, CPT II, carnitine palmitoyltransferase I, II, respectively LCFA, long-chain fatty acid VLDL, very low-density lipoprotein. 1, Long-chain acyl-CoA synthase 2, acetoacetyl-CoA thiolase 3, hydrox-ymethylglutaryl-CoA synthase 4, hydroxymethylglutaryl-CoA lyase 5, 3-hydroxybutyrate dehydrogenase 6, acetyl-CoA carboxylase 7, fatty acid synthase 8, glycerolphosphate acyltransferase Reprinted with permission from Girard et al. (1992).

Most tissues oxidize the acetyl-CoA produced during P-oxidation to C02 and water via the TCA cycle. During fasting, however, the liver utilizes the intermediates of the TCA cycle as gluconeogenic substrates. Under these conditions, the Ever converts acetyl-CoA to ketone bodies (acetoacetate and P-hydroxybutyrate) (Figure 32-5). Most other peripheral tissues can oxidize ketone bodies by the pathway shown in the figure. After entering the mitochondria, acetoacetate reacts with succinyl-CoA to form acetoacetyl-CoA, a reaction that is catalyzed by 3-oxoacid-CoA transferase. Alternatively, acetoacetyl-CoA is formed by direct activation of acetoacetate by the enzyme acetoacetyl-CoA synthetase. Acetoacetyl-CoA is then cleaved to form two molecules of acetyl-CoA by acetoacetyl-CoA thiolase.As noted earlier in... 

The first enzyme in the pathway is acetoacetyl-CoA thiolase which catalyzes the condensation of two molecules of acetyl-CoA. 

HMG-CoA reductase, HMG-CoA synthase, acetoacetyl-CoA thiolase and mevalonate kinase activities all increase when CHO cells are incubated in delipi-dated serum [210], However, the rise in HMG-CoA reductase activity precedes that of these other enzymes [210]. 

A number of other inhibitors of fatty acid oxidation have been reported in the literature [81,86, 140-142] including 2-bromopalmitoyl-CoA, bromoacetyl-CoA and S-methanesulphonyl-CoA decanoylcamitine, all inhibitors of carnitine palmitoyltransferase, 4-pentenoic acid, an inhibitor of 3-ketoacyl-CoA thiolase and acetoacetyl-CoA thiolase, and hypoglycin, a compound isolated from the Jamaican ackee fruit (Blighia sapida) which potently inhibits several acyl-CoA dehydrogenases. The toxicity associated with these compounds or their expense have precluded further development as hypoglycaemic agents. 

Mitochondria contain three classes of thiolases (i) acetoacetyl-CoA thiolase or acetyl-CoA acetyltransferase, which is specific for acetoacetyl-CoA (C4) as a substrate ... 

Fig. 2 takes a closer look at the cholesterol biosynthetic pathway. Sterols are synthesized from the two-carbon building block, acetyl-CoA. The soluble enzyme acetoacetyl-CoA thiolase interconverts acetyl-CoA and acetoacetyl-CoA, which are then condensed by... 

Acetoacetyl-CoA thiolase catalyzes the initial step in cholesterol biosynthesis, the condensation of two molecules of acetyl-CoA into forming acetoacetyl-CoA, which is then followed b y its condensatio n with a third molecule of acetyl-CoA to produce the six-carbon compound HMG-CoA via HMG-CoA synthase [12-15]. HMG-CoA is converted to mevalonate by HMG-CoA reductase. HMG-CoA reductase requires two... 

Acetoacetyl CoA is cleaved to two molecules of acetyl CoA by acetoacetyl CoA thiolase, the same enzyme involved in p-oxidation. The principal fate of this acetyl CoA is oxidation in the TCA cycle. 

The specificity of the inhibition by mevinolin through its high affinity for HlK-CoA reductase may also serve as a model to develop highly effective and specific artificial biocides and to stinulate research in this topic. For example, citrinin, another antibiotic having a bicyclic structure produced by Pentctllium citrinum has been shown to inhibit sterol biosynthesis (143) at the site of acetoacetyl-CoA thiolase (EC 2.3.2.9) and HMG-CoA... 

Quite a few of the enzymes involved in the biosynthesis of HMG-CoA have been studied in C. roseus. Acetoacetyl-CoA thiolase (AACT, acetyl-CoA acetyl-CoA C-transferase, EC 2.1.3.9) catalyzing the first step, the Claisen-type condensation of two molecules of acetyl-CoA, was partially purified from a cell suspension culture of C. roseus (78). The enzyme consists... 

Acetoacetyl-CoA thiolase (EC 2.3.1.9) catalyses the reversible condensation of 2 molecules of acetyl-CoA yielding acetoacetyl-CoA and CoASH. The chemical equilibrium of the reaction strongly favours acetyl-CoA formation (K=5.0 x 10 at... 

Bromooctanoate, by becoming converted to 2-bromo-3-ketooctanoyl-CoA, irreversibly inhibits the 3-ketothiolase (EC 2.3.1.6) activity (Raaka and Lowenstein, 1979). 4-Bromooctanoate does likewise by becoming converted to 4-bromo-3-ketobutyryl-CoA (Olowe, 1981). Arsenite restrains p-oxi-dation by inhibiting acetoacetyl-CoA thiolase (EC 2.3.1.9) (Rein et a/., 1979). The inhibitory effect of pent-4-enoate on the activities of 3-ketoacyl-... 

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