Oxidation reverse Claisen reaction

Reverse Claisen reaction in biochemistry -oxidation of fatty acids... 

Perhaps the most important example of the reverse Claisen reaction in biochemistry is that involved in the P-oxidation of fatty acids, used to optimize energy release from storage fats, or fats ingested as food (see Section 15.4). In common with most biochemical sequences, thioesters rather than oxygen esters are utilized (see Box 10.8). 

The P-oxidation sequence involves three reactions, dehydrogenation, hydration, then oxidation of a secondary alcohol to a ketone, thus generating a P-ketothioester from a thioester. We shall study these reactions in more detail later (see Section 15.4.1). The P-ketothioester then suffers a reverse Claisen reaction, initiated by nucleophilic attack of the thiol coenzyme A (see Box 10.8). 

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). 

The reverse of the above reaction is a key step in the oxidative degradation of fatty acids. This reverse Claisen condensation (catalyzed by thiolase) involves the cleavage of a carbon-carbon bond of a /3-keto ester of coenzyme A by another molecule of coenzyme A to give a new acyl derivative (RCO—SCoA) and ethanoyl (acetyl) derivative (CH3CO—SCoA) ... 

The formation of the poly-P-keto chain could be envisaged as a series of Claisen reactions, the reverse of which are involved in the 3-oxidation sequence for the metabolism of fatty acids (see page 18). Thus, two molecules of acetyl-CoA could participate in a Claisen condensation giving acetoacetyl-CoA, and this reaction could be repeated to generate a poly-P-keto ester of appropriate chain length (Figure 3.1). However, a study of the enzymes involved in fatty acid biosynthesis showed this simple rationalization could not be correct, and a more complex series of... 

The final step in the /3-oxidation cycle is the cleavage of the /3-ketoacyI-CoA. This reaction, catalyzed by thiolase (also known as j8-ketothiolase), involves the attack of a cysteine thiolate from the enzyme on the /3-carbonyI carbon, followed by cleavage to give the etiolate of acetyl-CoA and an enzyme-thioester intermediate (Figure 24.17). Subsequent attack by the thiol group of a second CoA and departure of the cysteine thiolate yields a new (shorter) acyl-CoA. If the reaction in Figure 24.17 is read in reverse, it is easy to see that it is a Claisen condensation—an attack of the etiolate anion of acetyl-CoA on a thioester. Despite the formation of a second thioester, this reaction has a very favorable A).q, and it drives the three previous reactions of /3-oxidation. 

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 first reaction (1), catalyzed by thiolase, involves a Claisen condensation of two acetyl CoA s (essentially a reversal of the last reaction of beta-oxidation) to give acetoacetyl CoA - almost the final product The problem now is to remove the CoASH. 

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