3-Ketoacyl-CoA esters

Inhibition of the 3-ketoacyl-CoA thiolase step (with the likely accumulation of 3-ketoacyl-CoA esters) or the 3-hydroxyacyl-CoA dehydrogenase, with accumulation of 3-hydroxyacyl-CoA esters, could lead to feedback inhibition of Poxidation (see Fig. 1). The 3-hydroxyacyl-CoA dehydrogenases, enoyl-CoA hydratases and short-medium-and long- chain acyl-CoA dehydrogenases can be all be inhibited by 3-ketoacyl-CoA esters. The enoyl-CoA hydratases catalyse an equilibrium but can be inhibited by their 3-hydroxyacyl-CoA products. Finally, the acyl-CoA dehydrogenases are subject to inhibition by their 2-enoyl-CoA products and, hence, feedback inhibition has been viewed as potentially very important in the regulation and control of P-oxidation flux. 

Hence, were disposal of acetyl-CoA to keh enesis, the tricarhoxylic acid cycle or to acetyl-carnitine inhibited, feedback inhibition of P-oxidation would result and it has been suggested that this may control p-oxidation in cardiac mitochondria. However, although 3-ketoacyl-CoA esters are readily observed as intermediates of peroxisomal P-oxidation, we have never observed accumulation of 3-ketoacyl-CoA esters in mitochondrial incubations even at maximal P-oxidation flux and under conditions in which acetyl-CoA accumulates. Therefore, the accumulation of 3-ketoacyl-CoA esters may be strongly prevented and an excess of thiolase activity puBs P-oxidation as the... 

We conclude that P-oxidation flux can be controlled by the [NAD ]/[NADH] and [acyl-CoA]/[CoA] ratios in intact mitochondrion. The gross intramitochondrial [NAD ]/[NADH] ratio may not exert control directly over P-oxidation because of the channelling of NAD(H) between 3-hydroxyacyl-CoA dehydrogenases and complex I. Although control of P-oxidation, by CoA acylation or acetylation and feedback inhibition via the 3-ketoacyl-CoA thiolases, is possible it appears to be unlikely to have much impact in intact mitochondrion because (i) 3-ketoacyl-CoA esters are not observed as intermediates of mitochondrial P-oxidation (ii) the K for CoASH of the 3-ketoacyl-CoA thiolases is comparable to that of mitochondrial CoASH-dependent dehydrogenases and much lower than that of CPT II. However, further work characterising the dependence of mitochondrial P-oxidation on these conserved-moiety cycles is necessary. 

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). 

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. 

Hydroxyacid esters are contained in several subtropical fruits like pineapple (26), passion fruit (27) and mango (2 ). 3-Hydroxyacid derivatives are formed as intermediates during de novo synthesis and P -oxidation of fatty acids, but the two pathways lead to opposite enantiomers. S-(+)-3-Hydroxyacyl-CoA-esters result from stereospecific hydration of 2,3-trans-enoyl-CoA during P -oxidation R-(-)-3-hydroxyacid derivatives are formed by reduction of 3-ketoacyl-S-ACP in the course of fatty biosynthesis. Both pathways may be operative in the production of chiral 3-hydroxyacids and 3-hydroxyacid esters in tropical fruits. 

In extrahepatic tissues, D-/3-hydroxybutyrate is oxidized to acetoacetate by D-/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 /3-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. 

FIGURE 25.7 The pathway of palmhate synthesis from acetyl-CoA and malonyl-CoA. Acetyl and malonyl building blocks are introduced as acyl carrier protein conjugates. Decarboxylation drives the /3-ketoacyl-ACP synthase and results in the addition of two-carbon units to the growing chain. Concentrations of free fatty acids are extremely low in most cells, and newly synthesized fatty acids exist primarily as acyl-CoA esters. 

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