8-Ketoacyl thiolase

Masamune, S., Walsh, C. T., Sinskey, A. J., Peoples, O. P. Poly-(R)-3-hydroxybutyrate (PHB) biosynthesis mechanistic studies on the biological Claisen condensation catalyzed by -ketoacyl thiolase. PureAppl. Chem. 1989, 61,303-312. 

The oxidation of the l( —) hydroxy fatty acyl-CoA is catalyzed by ketoacyl thiolases. An enzyme has been isolated from beef and sheep liver mitochondria that catalyzes such a reaction on fatty ketoacyl with chain lengths of four to eighteen carbons. The enzyme molecule contains SH groups that are essential for activity. The cell probably contains more than one thiolase. A thiolase specific for acetoacetic acid has been found and is discussed in the section on ketosis. 

The degradation reactions of methylbutyryl CoA are again catalyzed by the enzymes that are involved in the oxidation of the straight-chain fatty acids acyl dehydrogenase, crotonase, hydroxyacyl dehydrogenase. Only the last step of the sequence of reactions requires a specialized enzyme, ketoacyl thiolase. The products of that reaction are propionic acid and acetyl-CoA. 

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... 

Very recently interest has focused on still earlier control sites in the sterol pathway. According to the important findings of Lane e. al. (3), beta-ketoacyl thiolase and HMG-CoA synthetase occur in the cytoplasm as well as in the mitochondria. A cytoplasmic provision of acetoacetyl-CoA and HMG-CoA would first of all make sterol synthesis autonomous and independent of the mitochondrial supply of these precursors. At the same time HMG-CoA would lose its status as the common intermediate in both keto-genesis and cholesterol synthesis. It seems significant in this context that the citric acid analogue, hydroxycitrate, not only blocks hepatic cholesterogenesis but also inhibits ATP citrate lyase, the enzyme that supplies extramitochondrial acetyl-CoA. 

FIGURE 25.12 Elongation of fatty acids in mitochondria is initiated by the thiolase reaction. The /3-ketoacyl intermediate thus formed undergoes the same three reactions (in reverse order) that are the basis of /3-oxidation of fatty acids. Reduction of the /3-keto group is followed by dehydration to form a double bond. Reduction of the double bond yields a fatty acyl-CoA that is elongated by two carbons. Note that the reducing coenzyme for the second step is NADH, whereas the reductant for the fourth step is NADPH. 

Step 4 of Figure 29.3 Chain Cleavage Acetyl CoA is split off from the chain in the final step of /3-oxidation, leaving an acyl CoA that is two carbon atoms shorter than the original. The reaction is catalyzed by /3-ketoacyl-CoA thiolase and is mechanistically the reverse of a Claisen condensation reaction (Section 23.7). In the forward direction, a Claisen condensation joins two esters together to form a /3-keto ester product. In the reverse direction, a retro-Claisen reaction splits a /3-keto ester (or /3-keto thioester) apart to form two esters (or two thioesters). 

Uchicda, Y., Izai, K., Orii, T., Hashimoto, T. (1992). Novel fatty acid p-oxidation enzymes in rat liver mitochondria. II. Purification and properties of enoyl-coenzyme A (CoA) hydratase/3-hy-droxyacyl-CoA dehydrogenase/3-ketoacyl-CoA thiolase trifunctional protein. J. Biol. Chem. 267, 1034-1041. 

Inherited defects in the enzymes of (3-oxidation and ketogenesis also lead to nonketotic hypoglycemia, coma, and fatty hver. Defects are known in long- and short-chain 3-hydroxyacyl-CoA dehydrogenase (deficiency of the long-chain enzyme may be a cause of acute fetty liver of pr nancy). 3-Ketoacyl-CoA thiolase and HMG-CoA lyase deficiency also affect the degradation of leucine, a ketogenic amino acid (Chapter 30). 

This enzyme [EC 2.3.1.16], also known as 3-ketoacyl-CoA thiolase, transfers an acyl group from an acyl-CoA to acetyl-CoA to form free coenzyme A and 3-oxoa-cyl-CoA. 

Desaturation of alkyl groups. This novel reaction, which converts a saturated alkyl compound into a substituted alkene and is catalyzed by cytochromes P-450, has been described for the antiepileptic drug, valproic acid (VPA) (2-n-propyl-4-pentanoic acid) (Fig. 4.29). The mechanism proposed involves formation of a carbon-centered free radical, which may form either a hydroxy la ted product (alcohol) or dehydrogenate to the unsaturated compound. The cytochrome P-450-mediated metabolism yields 4-ene-VPA (2-n-propyl-4pentenoic acid), which is oxidized by the mitochondrial p-oxidation enzymes to 2,4-diene-VPA (2-n-propyl-2, 4-pentadienoic acid). This metabolite or its Co A ester irreversibly inhibits enzymes of the p-oxidation system, destroys cytochrome P-450, and may be involved in the hepatotoxicity of the drug. Further metabolism may occur to give 3-keto-4-ene-VPA (2-n-propyl-3-oxo-4-pentenoic acid), which inhibits the enzyme 3-ketoacyl-CoA thiolase, the terminal enzyme of the fatty acid oxidation system. 

The fourth and last step of the /3-oxidation cycle is catalyzed by acyl-CoA acetyltransferase, more commonly called thiolase, which promotes reaction of /3-ketoacyl-CoA with a molecule of free coenzyme A to split off the carboxyl-terminal two-carbon fragment of the original fatty acid as acetyl-CoA The other product is the coenzyme A thioester of the fatty acid, now shortened by two carbon atoms (Fig. 17-8a). This reaction is called thiolysis, by analogy with the process of hydrolysis, because the /3-ketoacyl-CoA is cleaved by reaction with the thiol group of coenzyme A... 

CoA-requiring cleavage of the resulting /3-ketoacyl-CoA by thiolase, to form acetyl-CoA and a fatty acyl-CoA shortened by two carbons. The shortened fatty acyl-CoA then reenters the sequence. 

In extraliepatic tissues, d-/3-hydroxybutyrate is oxidized to acetoacetate by o-/3-hydroxybutyrate dehydrogenase (Fig. 17-19). The acetoacetate is activated to its coenzyme A ester by transfer of CoA from suc-cinyl-CoA, an intermediate of the citric acid cycle (see Fig. 16-7), in a reaction catalyzed by P-ketoacyl-CoA transferase. The acetoacetyl-CoA is then cleaved by thiolase to yield two acetyl-CoAs, which enter the citric acid cycle. Thus the ketone bodies are used as fuels. 

Variation on a theme. Thiolase is homologous in structure to the condensing enzyme. On the basis of this observation, propose a mechanism for the cleavage of 3-ketoacyl CoA by CoA. 

In the last step of P-oxidation, p-ketoacyl CoA reacts with coenzyme A in the presence of the enzyme, thiolase. The products of this reaction are acetyl CoA and an acyl CoA containing two carbons less than the original acyl CoA molecule that underwent oxidation. 

The bond between the a- and p-carbons of the P-ketoacyl CoA is cleaved by a thiolase that requires coenzyme A Acetyl CoA is produced from the two carbons at the carboxyl end of the original fatty acyl CoA, and the remaining carbons form a fatty acyl CoA that is two carbons shorter than the original fatty acyl CoA. 

MGKAT deficiency. 602199 Medium-chain 3-ketoacyl-CoA thiolase <1 100,000 Cardiomyopathy, liver disease 1... 

Ketoacyl-CoA thiolase (/6-ketothiolase) catalyzes a thiolytic cleavage, has broad specificity, and yields acetyl-CoA and acyl-CoA shortened by two carbon atoms. The reaction is highly exergonic... 

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