Acetyl CoA acetyltransferase

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. 

Hydroxymethyl Glutarate Cycle. At the first step of this cycle, condensation of two acetyl-CoA molecules takes place, with the participation of acetyl CoA acetyltransferase ... 

Figure 7.17 The pathway of ketone body oxidation hydroxybutyrate to acetyl-CoA. Hydroxybutyrate is converted to acetoacetate catalysed by hydroxybutyrate dehydrogenase acetoacetate is converted to acetoacetyl-CoA catalysed by 3-oxoacid transferase and finally acetoacetyl-CoA is converted to acetyl-CoA catalysed by acetyl-CoA acetyltransferase, which is the same enzyme involved in synthesis of acetoacetyl-CoA.

Acetyl CoA acetyltransferase, a key enzyme of ketogenesis, and 3-oxo-acyl CoA thiolase, involved in -oxidation, bind CoA by formation of a disulfide bond to cysteine, a reaction that can be reversed by glutathione and other sulfhydryl reagents. The physiological significance of this reaction with CoA, which inactivates the enzymes, is not clear (Quandt and Huth, 1984, 1985 Schwerdt and Huth, 1993). 

Quandt L and Huth W (1985) On the mechanism of the chemical modification of the mitochondrial acetyl-CoA acetyltransferase by co enzyme A. Biochimica etBiophysica Acta 829, 103-8. 

Schwerdt G and Huth W (1993) Turnover and transformation of mitochondrial acetyl-CoA acetyltransferase into CoA-modifled forms. Biochemical Journal 292, 915-19. Scientific Committee for Food (1993) Nutrient and Energy Intakes for the European Community. Luxemburg Commission of the European Communities. 

Schwerdt G and Huth W (1993) Uirnover and transformation of mitochondrial acetyl-CoA acetyltransferase into CoA-modifled forms. Biochemical Journal 292,915-19. 

Willadsen and Eggerer (75) have studied the stereochemistry of the enzyme acetyl CoA acetyltransferase, a key enzyme in both the terminal step in C-3 oxidation of fatty acids and the initial step in the biosynthesis of terpenes and steroids. The enzyme, when incubated separately with (2S)-[2-2Hi,2-3Hi]aceto-acetyl CoA and the (2R) isomer gave two moles of acetyl CoA as depicted in Scheme 17. Eggerer et al. (76) utilized the enzyme enoyl CoA hydratase to convert properly labeled crotonyl CoA, via syn addition, to the doubly isotopically labeled 3-hydroxyacyl CoA derivatives needed in this study. A discussion of this unique type of hydration has been presented by Rose (9). The labeled... 

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. 9. Biosynthesis of platelet-activating factor (PAF) via the remodeling pathway. Lyso-PAF, the immediate precursor of PAF, can be formed from l-alkyl-2-acyl-sn-glycero-3-phosphocholine through the direct action of (I) PLAj or (II) CoA-independent transacylase. The lysoplasmenylethanolamine (or other potential ethanolamine- and choline-containing lysoglycerophospholipids) is thought to be generated by (III) a PLAj that exhibits a high degree of selectivity for substrates with an atachidonoyl moiety at the sn-2 position. The transacylase (II) appears to possess both acyl transfer and PLAj hydrolytic activities. Lyso-PAF produced by either the transacylation (II) or direct PLAj (I) action can be acetylated to form PAF by (IV) an acetyl-CoA acetyltransferase.

The hydroxymethylglutaryl-CoA pathway, the major route involved in acetoacetate production, requires the participation of acetyl-CoA ace-tyltransferase (EC 2.3.1.9) for the conversion of acetyl-CoA to acetoacetyl-CoA, and of the hydroxymethylglutaryl-CoA synthase (EC 4.1.3.5) which catalyzes the reaction acetoacetyl-CoA + acetyl-CoA = 3-hydroxy-3-methylglutaryl-CoA + CoA. The activity of acetyl-CoA acetyltransferase is strongly inhibited by acetoacetyl-CoA and CoA, both of which decrease its affinity for acetyl-CoA (Huth et al., 1978). Acetoacetyl-CoA inhibits the activity of 3-hydroxy-3-methylglutaryl-CoA synthase (Reed et a/., 1975) also. For both the above steps, acetyl-CoA is a substrate and CoA is a product. One would expect, therefore, an elevation in acetyl-CoA/CoA ratio to favor acetoacetate production and many data support it (Sauer and Erfle, 1966 Lopes-Cardozo et a/., 1975 Siess et a/., 1976). 

Acetyl CoA acetyltransferase 2 hydroxymethylglutaryl CoA synthase 3 hydroxymethyl-glutaryl CoA reductase 4 mevalonate kinase 5 phosphomevalonate kinase 6 pyrophospho-mevalonate decarboxylase... 

AAV40815 ORF33 Acyl-CoA thiolase, acetyl-CoA acetyltransferase 1456668-1457939... 

Haywood et al. purified 3-ketothiolase (acetyl-CoA acetyltransferase) from a glucoseutilizing strain of R. eutropha [147]. They found it to consist of two distinct constitutive isoenzymes, 3-ketothiolase A and B, each with its own substrate specificity. 3-Ketothiolase A is active with only with four- or five-carbon 3-ketoacyl-CoAs, and must solely be responsible for PHA synthesis in R. eutropha, as this micro-organism does not accumulate PHAs with hydroxyacid repeating units of more than five carbon atoms. 3-Ketothiolase B has a broader specificity (four- to ten-carbon 3-ketoacyl-CoAs), and Haywood and colleagues have speculated that it may have a function other than PHA accumulation. 

Mevalonic acid itself is a product of acetate metabolism. Three molecules of acetate coenzyme A, produced by the citric acid cycle, are used to form mevalonic acid (Scheme 5.1). Two molecules undergo a Claisen condensation via acetyl-CoA-acetyltransferase enzyme [EC 2.3.1.9] to produce acetoacetyl-CoA, and a third is incorporated in a stereospecific aldol addition to the formation of p-hydroxy-p-methylglutaryl-CoA (HMG-CoA) by the aid of HMG-CoA synthase [EC 2.3.3.10]. The first Claisen reaction was found to involve formation of Cys-89 acetyl-5-enzyme reaction intermediate [9]. Then, Cys-378 residue on the active site of the enzyme activates a second molecule of acetyl-CoA to initiate the condensation reaction (Fig. 5.4) [11]. Similarly, in HMG-CoA synthases (S. aureus HMG-CoA synthase), Cysl 11/129 are the crucial residues of covalent attach to acetyl-CoA to produce acetyl-enzyme thioester with the subsequent loss of coenzyme A (Fig. 5.4). Glu79/95 residues are responsible for the enolization of acetyl-enzyme intermediate in order to react with acetoacetyl-CoA, which is bound to His233/264 residues [12]. 

Ornithine acetyltransferase has been partially purified from Chlamydomonas (Staub and Denes, 1966) and Chlorella (Morris and Thompson, 1975). The enzyme from Chlamydomonas was purified 60-fold and exhibited a broad pH optimum from 7.5 to 9. The values (pH 7.5) for glutamate and acetylomithine were 13 and 5.5 ta.M, respectively. The enzyme was specific for L-gluta-mate but would use other acetyl donors. The enzyme was not assayed for acetyl-CoA acetyltransferase activity. The reaction was reversible with a =... 

The enzyme from Chlorella possesses both acetyl-CoA acetyltransferase and acetylomithine acetyltransferase activity although the acetyl-CoA-dependent activity was more labile (Morris and Thompson, 1975). The enzyme purified over 180-fold had a pH optimum between 8 and 8.5. The values for glutamate, acetyl-CoA, and acetylomithine were 3, 3.2, and 0.2 mM, respectively. The enzyme was inhibited by arginine (A = 0.94 mM). Again, activity was unstable and no cofactor was required. The acetyl-CoA-dependent activity was stabilized by thiols and reduced by para-chloromercuribenzoate (PCMB) while the acetylomithine-dependent activity was not affected. These results suggest that free thiol is essential for activity of the acetyl-CoA acetyltransferase. 

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