Thiamine pyrophosphate TPP

As shown in Eigure 18.17, thiamine is composed of a substituted thiazole ring joined to a substituted pyrimidine by a methylene bridge. It is the precursor of thiamine pyrophosphate (TPP), a coenzyme involved in reactions of carbo-... 

FIGURE 18.17 Thiamine pyrophosphate (TPP), the active form of vitamin is formed by the action of TPP-synthetase. 

Thiamine pyrophosphate (TPP) from the vitamin thiamine Lipoic acid... 

A novel and more general method to enable biocatalyzed conversion and synthesis of hydrophobic compounds involves the use of gel-stabilized aqueous-organic two-phase systems [8], Features, advantages, disadvantages, and perspectives of this method in asymmetric synthesis will be discussed in this chapter, illustrated for the stereoselective benzoin condensation and the reduction of ketones catalyzed by thiamine pyrophosphate (TPP)-dependent lyases and NAD(P)H-dependent alcohol dehydrogenases, respectively. 

Fig. 9. A schematic drawing of a possible mechanism for the reaction catalyzed by the pyruvate dehydrogenase complex. The three enzymes Elf E2, and E3 are located so that lipoic acid covalently linked to E2 can rotate between the active sites containing thiamine pyrophosphate (TPP) and pyruvate (Pyr) on Elt CoA on E2, and FAD on E3. Acetyl-CoA and GTP are allosteric effectors of E, and NAD+ is an inhibitor of the overall reaction.

Thiamin (vitamin B 6.16) consists of two heterocyclic rings (substitued pyrimidine and substituted thiazole), linked by a methylene bridge. Thiamin acts as a co-enzyme in the form of thiamin pyrophosphate (TPP 6.17)... 

The pyruvate decarboxylase reaction provides our first encounter with thiamine pyrophosphate (TPP) (Fig. 14-13), a coenzyme derived from vitamin B Lack of vitamin Bi in the human diet leads to the condition known as beriberi, characterized by an accumulation of body fluids (swelling), pain, paralysis, and ultimately death. 

Transketolase requires the cofactor thiamine pyrophosphate (TPP), which stabilizes a two-carbon car-banion in this reaction (Fig. 14—26a), just as it does in the pyruvate decarboxylase reaction (Fig. 14-13). Transaldolase uses a Lys side chain to form a Schiff base with the carbonyl group of its substrate, a ketose,... 

The combined dehydrogenation and decarboxylation of pyruvate to the acetyl group of acetyl-CoA (Fig. 16-2) requires the sequential action of three different enzymes and five different coenzymes or prosthetic groups—thiamine pyrophosphate (TPP), flavin adenine dinucleotide (FAD), coenzyme A (CoA, sometimes denoted CoA-SH, to emphasize the role of the —SH group), nicotinamide adenine dinucleotide (NAD), and lipoate. Four different vitamins required in human nutrition are vital components of this system thiamine (in TPP), riboflavin (in FAD), niacin (in NAD), and pantothenate (in CoA). We have already described the roles of FAD and NAD as electron carriers (Chapter 13), and we have encountered TPP as the coenzyme of pyruvate decarboxylase (see Fig. 14-13). 

Thiamine pyrophosphate (TPP) is the biologically active form of fre vitamin, formed by the transfer of a pyrophosphate group from ATP to thiamine (Figure 28.11). Thiamine pyrophosphate serves as a coen zyme in the formation or degradation of a-ketols by transketolase (Figure 28.12A), and in the oxidative decarboxylation of a-keto adds i (Figure 28.12B). 

Thiamine pyrophosphate (TPP) (37), a derivative of vitamin Bi (38), contains two substituted heterocycles, the pyrimidine and thiazolium rings, connected by a methylene bridge. The reactive portion of this coenzyme is the thiazolium ring, the pyrimidine portion (as well as the pyrophosphate group) being important in binding interactions with proteins... 

There are two 2-oxoacid dehydrogenase multienzyme complexes in E. coli. One is specific for pyruvate, the other for 2-oxoglutarate. Each complex is about the size of a ribosome, about 300 A across. The pyruvate dehydrogenase is composed of three types of polypeptide chains El, the pyruvate decarboxylase (an a2 dimer of Mr — 2 X 100 000) E2, lipoate acetyltransferase (Mr = 80 000) and E3, lipoamide dehydrogenase (an a2 dimer of Mr = 2 X 56 000). These catalyze the oxidative decarboxylation of pyruvate via reactions 1.6, 1.7, and 1.8. (The relevant chemistry of the reactions of thiamine pyrophosphate [TPP], hydroxyethylthiamine pyrophosphate [HETPPJ, and lipoic acid [lip-S2] is discussed in detail in Chapter 2, section C3.)... 

THIAMINE (Vitamin Bi). Some earlier designations for this substance included aneurin, antmeuntic factor, antibenben factor, and oryzamin. Thiamine is metabolically active as thiamine pyrophosphate (TPP). the formula of which is ... 

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. 

Transketolase is one of several enzymes that catalyze reactions of intermediates with a negative charge on what was initially a carbonyl carbon atom. All such enzymes require thiamine pyrophosphate (TPP) as a cofactor (chapter 10). The transketolase reaction is initiated by addition of the thiamine pyrophosphate anion to the carbonyl of a ketose phosphate, for example xylulose-5-phosphate (fig. 12.33). The adduct next undergoes an aldol-like cleavage. Carbons 1 and 2 are retained on the enzyme in the form of the glycol-aldehyde derivative of TPP. This intermediate condenses with the carbonyl of another aldolase. If the reactants are xylulose-5-phosphate and ribose-5-phosphate, the products are glyceraldehyde-3-phosphate and the seven-carbon ketose, sedoheptulose-7-phosphate (see fig. 12.33). 

In addition to the larger families of preparatively useful aldolases, some less common aldolases have been evaluated lately for preparative use. A range of mechanistically distinct enzymes, which are actually categorized as transferases but which also catalyze aldol-related additions through the aid of cofactors such as pyridoxal 5-phosphate (PLP), thiamine pyrophosphate (TPP), tetrahydro-folate (THF), or coenzyme A (CoA), are becoming more frequently applied in organic synthesis. Because of their emerging importance and/or commercial availability, a selection of these enzymes and examples of their synthetic utility will be included in further separate chapters. 

Decarboxylases (PDC, ADC) decarboxylase carboligation thiamine pyrophosphate (TPP) catalysis Bruhn, 1995... 

Thiamin is important as a component of the coenzyme thiamin pyrophosphate (TPP) (cocarboxylase). Good sources are lucerne, grains and yeast. A deficiency is less frequently encountered than deficiencies of other vitamins, since thiamin occurs in abundance in whole grains which make up the major part of poultry diets. A diet deficient in thiamin results in nervous disorders in both young and old birds, and eventual paralysis of the peripheral nerves (polyneuritis). 

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... 

Answer Thiamine is essential for the formation of thiamine pyrophosphate (TPP), one of the cofactors in the pyruvate dehydrogenase reaction. Without TPP, the pyruvate generated by glycolysis accumulates in cells and enters the blood. 

Answer Thiamine is required for the synthesis of thiamin pyrophosphate (TPP), a prosthetic group in the pyruvate dehydrogenase and a-ketoglutarate dehydrogenase complexes. A thiamin deficiency reduces the activity of these enzyme complexes and causes the observed accumulation of precursors. 

This reaction was found later in extracts of another bacterium by Wolfe and O Kane (113) who showed requirements for coenzyme A (CoA) and thiamine pyrophosphate (TPP). Further experiments (Mortlock, Valentine, and Wolfe (76)) established acetyl-CoA as the first stable 2-carbon compound formed in this reaction (Eq. 2). 

Pyruvate produced by the glycolytic pathway may be transported into the mitochondria (via an antiport with OH"), where it is converted to acetyl-CoA by the action of the enzyme complex pyruvate dehydrogenase. The pertinent enzyme activities are pyruvate dehydrogenase (PD), lipoic acid acetyltransferase, and dihydrolipoic acid dehydrogenase. In addition, several cofactors are utilized thiamine pyrophosphate (TPP), lipoic acid, NAD+, Co A, and FAD. Only Co A and NAD+ are used in stoichiometric amounts, whereas the others are required in catalytic amounts. Arsenite and Hg2+ are inhibitors of this system. The overall reaction sequence may be represented by Figure 18.5. The NADH generated may enter the oxidative phosphorylation pathway to generate three ATP molecules per NADH molecule reduced. The reaction is practically irreversible its AGq = -9.4 kcal/mol. 

Figure 7-2. Reactions of the pyruvate dehydrogenase (PDU) multienzyme complex (PDC). Pyruvate is decarboxylated by the PDH subunit (I ,) in the presence of thiamine pyrophosphate (TPP). The resulting hydroxyethyl-TPP complex reacts with oxidized lipoamide (LipS3), the prosthetic group of dehydrolipoamide transacetylase (Ii2), to form acetyl lipoamide. In turn, this intermediate reacts with coenzyme A (CoASH) to yield acetyl-CoA and reduced lipoamide [Lip(SH)2]. The cycle of reaction is completed when reduced lipoamide is reoxidized by the flavoprotein, dehydrolipoamide dehydrogenase (E3). Finally, the reduced flavoprotein is oxidized by NAD+ and transfers reducing equivalents to the respiratory chain via reduced NADH. PDC is regulated in part by reversible phosphorylation, in which the phosphorylated enzyme is inactive. Increases in the in-tramitochondrial ratios of NADH/NAD+ and acetyl-CoA/CoASH also stimulate kinase-mediated phosphorylation of PDC. The drug dichloroacetate (DCA) inhibits the kinase responsible for phosphorylating PDC, thus locking the enzyme in its unphosphory-lated, catalytically active state. Reprinted with permission from Stacpoole et al. (2003).

However, this reaction is in fact the sum of five reactions catalyzed by three enzymes and requiring five cofactors coenzyme A (CoA), NAD+, FAD, thiamine pyrophosphate (TPP), and lipoic acid (an 8-carbon acid with sulfhydryl groups at positions 6 and 8). The detailed steps of these reactions are shown in Fig. 12-7. 

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