TPP-dependent reactions

TPP thus plays a very important role in carbohydrate metabolism. The parent alcohol thiamine is one of the B group vitamins, namely vitamin Bi dietary 

The conversion of pyravic acid into acetyl-CoA is conveniently written according to the equation 

The proton in the thiazolium ring is relatively acidic (pATa about 18) and can be removed by even weak bases to generate the carbanion or ylid (see Box 11.8). An ylid (also ylide) is a species with positive and negative charges on adjacent atoms this ylid is an ammonium ylid with extra stabilization from the sulfur atom. 

This ylid can act as a nucleophile, and is also a reasonable leaving group. Addition to the carbonyl group of pyruvic acid is followed by decarboxylation, the positive nitrogen in the ring acting 

In oxidative decarboxyiation of pyruvate to acetyi-CoA, the enzyme-bound disulfide-containing coenzyme lipoic acid is also involved. The electron-rich enamine intermediate, instead of accepting a proton, is used to attack a sulfur in the lipoic acid moiety. This leads to fission of the S-S bond, and thereby effectively reduces the lipoic acid fragment. Regeneration of the TPP ylid via the reverse aldol-type 

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TPP-dependent reactions in detail in Chapter 15. At this stage, we should merely examine the structure of thiamine, and correlate its properties with our knowledge of heterocycles. 

The proposed catalytic mechanism of the ferredoxin oxidoreductase [32] is shown in Fig. 4, a similar mechanism existing for the analogous citric acid cycle enzyme, 2-oxoglutarate oxidoreductase. In outline, the 2-oxoacid is decarboxylated in a TPP-dependent reaction to give an hydroxyalkyl-TPP. From this, one electron is abstracted and transferred to the enzyme-bound iron-sulphur cluster, generating a free-radical-TPP species. This intermediate can then interact direct with coenzyme-A to form acyl-CoA, the iron-cluster receiving the second electron. In each case, ferredoxin serves to re-oxidise the enzyme s redox centre. 

One of the steps in the pentose phosphate pathway for glucose metabolism is the TPP-dependent reaction of xylulose 5-phosphate with rihose 5-phosphate to give glyceraldehyde 3-phosphate and sedoheptulose 7-phosphate. 

Biosynthesis of the 8-lactam antibiotic clavulanic acid begins with a TPP-dependent reaction between D-glyceraldehyde 3-phosphate and arginine. 

TPP-dependent enzymes are involved in oxidative decarboxylation of a-keto acids, making them available for energy metabolism. Transketolase is involved in the formation of NADPH and pentose in the pentose phosphate pathway. This reaction is important for several other synthetic pathways. It is furthermore assumed that the above-mentioned enzymes are involved in the function of neurotransmitters and nerve conduction, though the exact mechanisms remain unclear. 

TPP-dependent enzymes catalyze either simple decarboxylation of a-keto acids to yield aldehydes (i.e. replacement of C02 with H+), or oxidative decarboxylation to yield acids or thioesters. The latter type of reaction requires a redox coenzyme as well (see below). The best known example of the former non-oxidative type of decarboxylation is the pyruvate decarboxylase-mediated conversion of pyruvate to acetaldehyde and C02. The accepted pathway for this reaction is shown in Scheme 10 (69MI11002, B-70MI11003, B-77MI11001>. 

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... 

Transketolase is a TPP-dependent enzyme found in the cytosol of many tissues, especially hver and blood cells, in which principal carbohydrate pathways exist. In the pentose phosphate pathway, which additionally supplies reduced nicotinamide-adenine dinucleotide phosphate (NADPH) necessary for biosynthetic reactions, this enzyme catalyzes the reversible transfer of a glycoaldehyde moiety from the first two carbons of a donor ketose phosphate to the aldehyde carbon of an aldose phosphate. 

It has been postulated that the 2-ketoglutarate undergoes a TPP-dependent decarboxylation with the formation of succinic semialdehyde anion of TPP (13) and a requirement for TPP in the reaction was shown. ° The mechanism of decarboxylation is identical to that catalyzed by the first enzyme of the... 

In animals TPP-dependent decarboxylation reactions are essential to the production of energy needed for cell metahohsm. In these reactions a-ketoacids are converted to acyl CoA molecules and carbon dioxide. The reactions (e.g., the conversion of pyruvate to acetyl CoA) are an important part of the breakdown of carbohydrates, and of the conversion of several classes of molecules (carbohydrates, fats, and proteins) to energy, carbon dioxide, and water in the citric acid cycle. In other organisms, in addition to its participation in the above reactions, TPP is a required coenzyme in alcohol fermentation, in the carbon fixation reactions of photosynthesis, and in the hiosynthesis of the amino acids leucine and valine. 

It was recognized in 1953 that TK is a TPP-dependent enzyme [1, 2]. Since the work of Breslow who succeeded in directly localizing the catalytic site of thiamine [3] a considerable knowledge on the mechanism of TPP catalyzed reactions has accumulated [4-6], Very little, however, is known about the interaction of TPP with its corresponding apoenzyme, which will be the subject of this paper. 

Hydroxy acids H0-(CH2)n-C02H with n>3 react with DEAD and TPP to afford the corresponding lactones. This procedure can be utilized in the preparation of macrolide antibiotics and related compounds. Macrolactonization by the use of DEAD-TPP depends on the reaction conditions and the structure of the seco acids. Thus dropwise addition of the hydroxy acid (18) over a period of 10 h to a mixture of DEAD (7.7 equiv) and TPP (7.5 equiv) affords the lactone (19) in 59% yield as well as the unwanted di-lactide (20) in a yield of <1%. On the other hand, the reaction of... 

The reaction of hydroxyethyl-TPP with the oxidized form of lipoic acid yields the energy-rich thiol ester of reduced lipoic acid and results in oxidation of the hydroxyl-carbon of the two-carbon substrate unit (c). This is followed by nucleophilic attack by coenzyme A on the carbonyl-carbon (a characteristic feature of CoA chemistry). The result is transfer of the acetyl group from lipoic acid to CoA. The subsequent oxidation of lipoic acid is catalyzed by the FAD-dependent dihydrolipoyl dehydrogenase and NAD is reduced. 

Water soluble iron porphyrins [Fem(TPPS)(H20) ]3-330 and [Fem(TMPy)(H20)2]5+ 331 332 (TPPS = maso-tetrakis(/ -sulfonatophenyl)porphyrin, TMPyP = / /e.vo-tetrakis(7V-methyl-4-pyridi-nium)porphyrin331 or maso-tetrakis (A -methyl-2-pyridinium)porphyrin332 dications) act as effective electrocatalysts for the reduction of nitrite to ammonia in aqueous electrolytes (Equation (64) Ei/2= 0.103 V vs. SCE at pH 7), with NH2OH or N20 also appearing as products depending on the reaction conditions. Nitric oxide then ligates to the iron(III) porphyrin to form a nitrosyl complex [Fen(P)(NO+)] (P = porphyrin) as intermediate. 

Studies in this laboratory (69) of the water soluble ferri-heme model Fem(TPPS) in aqueous solution have shown that this species also undergoes reductive nitrosylation in solutions that are moderately acidic (pH 4-6) (Eq. (32)). The rate of this reaction includes a buffer dependent term indicating that the reaction of the Fem(TPPS)(NO) complex with H20 is subject to general base catalysis. The reaction depicted in Eq. (33) is not observable at pH values < 3, since the half-cell reduction potential for the nitrite anion (Eq. (1)) is pH dependent, and Eq. (33) is no longer thermodynamically favorable. 

Epoxidation with PhIO in the presence of manganese(III) tetraphenylporphyrine chloride [Mn(TPP)Cl] has been extensively studied.244 245,307 The main characteristic of these reactions is the loss of stereochemistry at the double bond with product distributions depending on the porphyrin structure 134... 

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