TPP ylide

O Nucleophilic addition of thiamin diphosphate (TPP) ylide to pyruvate gives an alcohol addition product. 

Step 4 of Figure 29.12 Oxidative Decarboxylation The transformation of a-ketoglutarate to succinyl CoA in step 4 is a multistep process just like the transformation of pyruvate to acetyl CoA that we saw in Figure 29.11. In both cases, an cv-keto acid loses CO2 and is oxidized to a thioester in a series of steps catalyzed by a multienzyme dehydrogena.se complex. As in the conversion of pyruvate to acetyl CoA, the reaction involves an initial nucleophilic addition reaction to a-ketoglutarate by thiamin diphosphate ylide, followed by decarboxylation, reaction with lipoamide, elimination of TPP ylide, and finally a transesterification of the dihydrolipoamide thioester with coenzyme A. 

Wittig reaction, the TPP ylide is a nucleophile and adds to the ketone carbonyl group of pyruvate to yield an alcohol addition product. 

O Cleavage of the adduct in a retro-aldol reaction gives 1-deoxy-D-xylulose 5-phosphate and regenerates TPP ylide. 

Decarboxylation of p-hydroxyphenylpyruvate begins with nucleophilic addition of TPP ylide to the ketone carbonyl group, followed by loss of CO2 to give an enamine in the usual way. But whereas the enamine formed from pyruvate decarboxylation reacts with lipoamide to give a thioester and regenerated TPP ylide, the enamine from p-hydroxyphenylpyruvate decarboxylation is simply protonated to give an aldehyde plus TPP ylide. The mechanism is shown in Figure 25.8. 

The hydrogen bonded to the imine carbon of TPP is relatively acidic (p Tg = 12.7) compared to a hydrogen attached to other sp carbons, because the ylide formed when the proton is removed is stabilized by the adjacent positively charged nitrogen. The TPP ylide is a good nucleophile. 

After the proton is removed, the TPP ylide adds to the carbonyl carbon of the a-keto acid. An acid side chain of the enzyme increases the electrophilicity of the carbonyl carbon. 

Protonation of the enamine on carbon and a subsequent elimination reaction forms acetaldehyde and regenerates the TPP ylide. 

The first enzyme in the complex catalyzes the reaction of the TPP ylide with pyruvate to form an enamine identical to the one formed by pyruvate decarboxylase and by the enzyme in Problems 9 and 10. 

The TPP ylide adds to the carbonyl carbon of the acetaldehyde. Removal of a proton forms the same enamine that is formed by both pymvate decarboxylase and the pyruvate dehydrogenase complex— the only difference in the reactions is that a proton, instead of a carboxyl group, is removed from the substrate. The enamine then reacts with lipoate just as it does in the pyruvate dehydrogenase complex. The result is that the offending acetaldehyde is converted to acetyl-CoA. 

Notice the similar function of TPP in all TPP-requiring enzymes. In each reaction, the TPP ylide adds to a carbonyl carbon of the substrate and allows a bond to that carbon to be broken because the electrons left behind can be delocalized into the thiazoUum ring. The acyl group is then transferred—to a proton in the case of pyruvate decarboxylase, to coenzyme A (via lipoate) in the pyruvate dehydrogenase system, and to a carbonyl group in Problems 9,10, and 11. 

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