Thiamine pyrophosphate pentose phosphate

A number of lyases are known which, unlike the aldolases, require thiamine pyrophosphate as a cofactor in the transfer of acyl anion equivalents, but mechanistically act via enolate-type additions. The commercially available transketolase (EC 2.2.1.1) stems from the pentose phosphate pathway where it catalyzes the transfer of a hydroxyacetyl fragment from a ketose phosphate to an aldehyde phosphate. For synthetic purposes, the donor component can be replaced by hydroxypyruvate, which forms the reactive intermediate by an irreversible, spontaneous decarboxylation. 

Thiamine pyrophosphate Is an essential coenzyme for several critical metabolic enzymes—PDH, a-ketoglutarate dehydrogenase, and transketolase of the pentose phosphate pathway. 

FIGURE 20-12 TPP as a cofactor for transketolase. Transketolase transfers a two-carbon group from sedoheptulose 7-phosphate to glyceraldehyde 3-phosphate, producing two pentose phosphates (step in Fig. 20-10). Thiamine pyrophosphate serves as a temporary carrier of the two-carbon unit and as an electron sink (see Fig. 14-13) to facilitate the reactions. 

Vitamin B1 (thiamine) has the active form, thiamine pyrophosphate. It is a cofactor of enzymes catalyzing the conversion of pyruvate to acetyl CoA, a-ketoglutarate to succinyl CoA, and the transketolase reactions in the pentose phosphate pathway. A deficiency of thiamine causes beriberi, with symptoms of tachycardia, vomiting, and convulsions. In Wernicke-Korsakoff syndrome (most common in alcoholics), individuals suffer from apa thy, loss of memory, and eye movements. There is no known toxicity for this vitamin. 

Thiamine pyrophosphate also plays a key role in the biosynthetic reactions that build (or degrade) pentoses from hexoses. We have mentioned these reactions previously in connection with the Calvin cycle (Section 20-9) and the pentose-phosphate pathway (Section 20-10C). 

Fig. 1. The reductive pentose phosphate cycle (RPP). The solid lines indicate reactions of the RPP cycle. The number of lines per arrow indicates the number of times each reaction occurs for one complete turn of the cycle in which three molecules of COj are converted to one molecule of G3P. Each reaction of the cycle occurs at least once. The double dashed lines indicate the principal reactions removing intermediate compounds of the cycle for biosynthesis. Abbreviations RuBP, ribulose 1,5-bis-phosphate PGA, 3-phosphoglycerate DPGA, 1,3-diphosphoglycerate, FBP, fructose 1,6-bisphos-phate F6P, fructose 6-phosphate SBP, sedoheptulose 1,7-bisphosphate S7P, sedoheptulose 7-phosphate Xu5P, xylulose 5-phosphate R5P, ribose 5-phosphate Ru5P, ribulose 5-phosphate TPP, thiamine pyrophosphate. From Ref. 1.

The third phase of the Calvin cycle is the regeneration of ribulose 1,5-bisphosphate, the acceptor of CO2 in the first step. The problem is to construct a five-carbon sugar from six-carbon and three-carbon sugars. A transketolase and an aldolase play the major role in the rearrangement of the carbon atoms. The transketolase, which we will see again in the pentose phosphate pathway (Section 20.2.3). requires the coenzyme thiamine pyrophosphate (TPP) to transfer a two-carbon unit (CO-CH2OH) from a ketose to an aldose. 

FIGURE 9.71 Pentose phosphate pathway. The pentose phosphate pathway is used for the metabolism of various sugars. It is required for the biosynthesis of ribose 5-phosphate (a component of ATP, GTP, CTP, UTP, TTP, and the nucleic acids). The pentose phosphate pathway is used for the reduction of NADP. Thiamin pyrophosphate is a cofactor for two enzymes of the pathway, as indicated by TPP. The circled groups are the two-carbon units transferred by TPP. 

Thiamine pyrophosphate (Figure 4-8A) is involved in decarboxylation of a-keto acids and is the cofactor for the transketolase of the pentose phosphate pathway. 

In the pentose phosphate pathway, thiamine pyrophosphate is required for the action of... 

A. The transketolase of the pentose phosphate pathway requires thiamine pyrophosphate. 

Thiamine pyrophosphate is also an important cofactor for the transketolase reactions in the pentose phosphate pathway of carbohydrate metabolism (Fignre 15-3). These reactions are important in the reversible transformation of pentoses into the glycolytic intermediates fructose 6-phosphate and glyc-eraldehyde 3-phosphate. Again, it is the reactive carbon on the thiazole ring of TPP that reacts with a ketose phosphate (xylnlose 5-phosphate) to canse the release of an aldose phosphate with two fewer carbons (glyceraldehyde 3-phosphate). The TPP-bonnd glycoaldehyde unit is then transferred to a different aldose phosphate (ribose 5-phosphate or erythrose 4-phosphate) to produce a ketose phosphate that has two carbons more (sedoheptulose 7-phosphate or fructose 6-phosphate). 

Thiamine pyrophosphate is also an important cofactor for many dehydrogenase reactions as well as the transketolase reactions in the pentose phosphate pathway of carbohydrate metabolism. 

The pentose phosphate pathway (PPP) is the major pathway for recycling nicotinamide adenine dinucleotide (NAD) to nicotinamide adenine dinucleotide phosphate hydrogen (NADPH) and for the production of ribose-5-phosphate that is needed for the synthesis of nucleotides. The function of the PPP depends on the synthesis of nicotinamide-adenine dinucleotide phosphate (NADP) and thiamin pyrophosphate, a coenzyme... 

TK is one of the enzymes involved in the oxidative pentose phosphate pathway, and requires the cofactors thiamine pyrophosphate (TPP)12191 and Mg2+[218). It reversibly transfers the C1-C2 ketol unit from D-xylulose 5-phosphate to D-ribose 5-phosphate, and generates D-sedoheptulose 7-phosphate and D-Gly 3-P. D-Erythrose 4-phosphate also functions as an acceptor of the ketol unit from D-xylulose 5-phosphate, to produce Fru 6-P and D-Gly 3-P (Fig. 14.2-1). TK from baker s yeast is commercially available, and the enzyme can also be isolated from spinach[220, 2211 TK from E. coli has been overexpressed and prepared on a large scale12221. In ketol transfer reactions,... 

Many alcoholics such as Al Martini develop thiamine deficiency because alcohol inhibits the transport of thiamine through the intestinal mucosal cells. In the body, thiamine is converted to thiamine pyrophosphate (TPP). TPP acts as a coenzyme in the decarboxylation of a-keto acids such as pyruvate and a-ketoglutarate (see Fig. 8.11) and in the utilization of pentose phosphates in the pentose phosphate pathway. As a result of thiamine deficiency, the oxidation of a-keto acids is impaired. Dysfunction occurs in the central and peripheral nervous system, the cardiovascular system, and other organs. 

The cardiomyopathy is directly related to a reduction in the normal biochemical function of the vitamin thiamine in heart muscle. Inhibition of the a-keto acid dehydrogenase complexes causes accumulation of a-keto acids in heart muscle (and in blood), resulting in a chemically-induced cardiomyopathy. Impairment of two other functions of thiamine may also contribute to the cardiomyopathy. Thiamine pyrophosphate serves as the coenzyme for transketolase in the pentose phosphate pathway, and pentose phosphates accumulate in thiamine deficiency. In addition, thiamine triphosphate (a different coenzyme form) may function in Na conductance channels. 

Recall Does thiamine pyrophosphate play a role in the reactions of the pentose phosphate pathway If so, what is that role ... 

Thiamine pyrophosphate is a cofactor necessary for the function of transketolase, an enzyme that catalyzes one of the reactions in the nonoxidative part of the pentose phosphate pathway. 

Transketolase. Another group of enzymes that catalyze the stereospecific formation and cleavage of carbohydrates in vivo are the transketolases and transaldolases. Transketolase (E.C. 2.2.1.1) is a thiamin pyrophosphate (TPP) dependent enzyme that catalyzes the transfer of a hydroxyketo group from a ketose phosphate to an aldose phosphate in the pentose pathway (Scheme 12) (36). 

The position of phosphoketolase in pentose metabolism. P = phosphate. TPP = thiamin pyrophosphate. 

Transketolase (EC 2.2.1.1) an enzyme that catalyses transketolation, an important process of carbohydrate metabolism, especially in the Pentose phosphate cycle (see) and Calvin cycle (see). T. has been found in a wide variety of cells and tissues, including mammalian liver, green plants and many bacterial species. The enzyme contains divalent metal cations and the coenzyme, thiamin pyrophosphate. Transketolation involves transfer of a C2-unit (often called active glycolaldehyde or a ketol moiety) from a ketose to Cl of an aldose. Only ketoses with L-configuration at C3 and preferably irons configuration on the next carbon (i.e. Cl, 2, 3 and preferably 4 as in fructose) can serve as donors of the C2-unit. The acceptor is always an aldose. Thins-ketolation is reversible. Details of the reaction in which xylulose S-phosphate serves as the donor of... 

Thiamin pyrophosphate (or thiamin diphosphate) is a coenzyme involved in (1) the oxidative decarboxylation of pyruvate to acetyl coenzyme A (enzyme pyruvate dehydrogenase), (2) the oxidative decarboxylation of a-ketoglutarate to succinyl coenzyme A (a-ketoglutarate dehydrogenase) in the tricarboxylic add cycle, (3) the pentose phosphate pathway (transketolase) and (4) the synthesis of branched-chain amino acids such as valine (branched-chain ketoacid dehydrogenase) in bacteria, yeasts and plants. 

Evidence from a number of sources indicated that pentose phosphates were metabolized in a series of reactions that resulted in the formation of hexose monophosphates and hexose diphosphates. Several enzyme steps are involved in these transformations. The reaction between D-ribulose 5-phosphate and D-ribose 5-phosphate to form D-sedoheptulose 7-phosphate and D-glyceraldehyde 3-phosphate is catalyzed by an enzyme known as transketolase (91). This enzyme is found in plant, animal, and bacterial cells. Thiamine pyrophosphate (TPP) and Mg ions are required as cofactors. The mechanism of the reaction was suggested (92) as shown in reaction (28). 

Thiamine pyrophosphate has also recently been shown to exert a coenzyme function in the metabolism of pentose phosphate. - The formation of sedoheptulose phosphate from ribulose-5-phosphate by a highly purified enzyme preparation from spinach, which loses activity upon precipitation of the protein with ammonium sulfate at a low pH, was found to be almost completely reactivated by the addition of thiamine pyrophosphate. ... 

In the cleavage of pentose phosphate a 2-carbou fragment, presumably glycolaldehyde, is expected to be formed and also to react in the reverse reaction. Glycolaldehyde neither accumulates nor does it react with these enzyme preparations. It is now postulated that an active glycolaldehyde is formed in the reactions of pentose-phosphate metabolism. This may be conjugated with thiamine pyrophosphate. 

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