Thiamin pyrophosphate-dependent decarboxylation

In the case of decarboxylations, the negative charge produced in the decarboxylation step may be neutralized by some distal positive charge. This occurs in the case of Schiff base-dependent decarboxylations, thiamin pyrophosphate-dependent decarboxylations, and in a few other cases. Such systems are often referred to as electron sinks. ... 

This is the most studied TPP-dependent enzyme, and it is likely that mechanisms of other thiamin pyrophosphate-dependent decarboxylations are similar. Editor s note Other aspects of thiamin pyrophosphate chemistry are presented in Chapter 7 by Kluger.)... 

The quinone ring is derived from isochorismic acid, formed by isomerization of chorismic acid, an intermediate in the shikirnic acid pathway for synthesis of the aromatic amino acids. The first intermediate unique to menaquinone formation is o-succinyl benzoate, which is formed by a thiamin pyrophosphate-dependent condensation between 2-oxoglutarate and chorismic acid. The reaction catalyzed by o-succinylbenzoate synthetase is a complex one, involving initially the formation of the succinic semialdehyde-thiamin diphosphate complex by decarboxylation of 2-oxoglutarate, then addition of the succinyl moiety to isochorismate, followed by removal of the pyruvoyl side chain and the hydroxyl group of isochorismate. 

The decarboxylation of branched chain keto acids is apparently carried out by the same decarboxylase (EC 4.1.1.1, Mg and thiamine pyrophosphate dependent) which is active on pyruvate in yeast (154, 167) and orange juice (168, 169). The latter enzyme acts on the keto acids derived from valine and isoleucine at about 15% of the pyruvate rate (169). Its activity declines rapidly (80% in 1 hour) in juice (pH 3.4). The malty flavor defect produced by S. lactis var. maltigenes, however, appears to be caused by a specific decarboxylase activity not found in other strains (158, 160) since simultaneous decarboxylation of pyruvate and a-ketoisocaproate was almost additive (163) while a-ketoisocaproate and a-ketoisovalerate competed with each other. Decarboxylation of... 

The first step of this reaction, decarboxylation of pyruvate and transfer of the acetyl group to lipoic acid, depends on accumulation of negative charge on the carbonyl carbon of pyruvate. This is facilitated by the quaternary nitrogen on the thiazolium group of thiamine pyrophosphate. As shown in (c), this cationic... 

Intermediates of this type have the necessary chemical reactivity for cleaving the bonds indicated in figure 10.1b and c. The decarboxylated product of the pyruvate adduct shown in equation (2) is resonance-stabilized by the thiazolium ring (fig. 10.2a). This intermediate may be protonated to a-hydroxyethyl thiamine pyrophosphate (fig. I0.2d) alternatively, it may react with other electrophiles, such as the carbonyl groups of acetaldehyde or pyruvate, to form the species in figure 10.2b and c or it may be oxidized to acetyl-thiamine pyrophosphate (fig. 10.2e). The fate of the intermediate depends on the reaction specificity of the enzyme with which the coenzyme is associated. 

Mechanism of thiamine pyrophosphate action. Intermediate (a) is represented as a resonance-stabilized species. It arises from the decarboxylation of the pyruvate-thiamine pyrophosphate addition compound shown at the left of (a) and in equation (2). It can react as a carbanion with acetaldehyde, pyruvate, or H+ to form (b), (c), or (d), depending on the specificity of the enzyme. It can also be oxidized to acetyl-thiamine pyrophosphate (TPP) (e) by other enzymes, such as pyruvate oxidase. The intermediates (b) through (e) are further transformed to the products shown by the actions of specific enzymes. 

The a-keto acid decarboxylases such as pyruvate (E.C. 4.1.1.1) and benzoyl formate (E.C. 4.1.1.7) decarboxylases are a thiamine pyrophosphate (TPP)-dependent group of enzymes, which in addition to nonoxidatively decarboxylating their substrates, catalyze a carboligation reaction forming a C-C bond leading to the formation of a-hydroxy ketones.269-270 The hydroxy ketone (R)-phenylacetylcarbinol (55), a precursor to L-ephedrine (56), has been synthesized with pyruvate decarboxylase (Scheme 19.35). BASF scientists have made mutations in the pyruvate decarboxylase from Zymomonas mobilis to make the enzyme more resistant than the wild-type enzyme to inactivation by acetaldehyde for the preparation of chiral phenylacetylcarbinols.271... 

Thiamine pyrophosphate (TPP) is derived from thiamine (vitamin Bl) via an ATP-dependent pyrophosphorylation. TPP is the coenzyme for all decarboxylations of ot-keto acids. Steps in the process are depicted in Figure 14.6, summarized below. 

Thiamin pyrophosphate (vi tamln Bi) Is essential for pyruvate dehydrogenase and similar large multlenzyme complexes which oxidatively decarboxylate Oi-ketoaolds. RNI 0.4 mg/1000 kcal (depends on energy Intake)... 

The decarboxylation of pyruvic acid is an example of a more general type of biochemical reaction the decarboxylation of a-keto acids. The reaction is complex and occurs in several consecutive steps. The intermediates have been identified, but little is known of the enzymes involved. The reaction starts with the complexion of pyruvic acid with one molecule of enzyme-bound thiamine pyrophosphate. This is followed by decarboxylation of pyruvic acid and the formation of an intermediate, 2-acetylthiamine pyrophosphate, in which the aldehyde carbon of the acetyl is bound to the carbon 2 of the thiozole ring of the thiamine pyrophosphate. In the second step, the aldehyde is oxidized, the disulfide bond of enzyme-bound lipoic acid is reduced, and the free enzyme-bound thiamine pyrophosphate is restored. The tWrd step of the reaction involves the transacylation from reduced lipoic acid to CoA. Finally, lipoic acid is reoxidized by the catalytic activity of an NAD-dependent flavoprotein, lipoic dehydrogenase (see Fig. 1-14). 

One important subgroup of the lyases are the decarboxylases. The decarboxylation of amino acids is assisted by pyridoxal phosphate as a prosthetic group, whereas in the decarboxylation of pyruvate to acetaldehyde, thiamine pyrophosphate (TPP) plays that role. Oxidative decarboxylation, lastly, depends on the cooperation of no fewer than five cofactors thiamine pyrophosphate, lipoic acid, coenzyme A, flavin-adenine dinucleotide, and nicotinamide-adenine dinucleotide. 

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