Sedoheptulose phosphate

The transaldolase functions primarily to make a useful glycolytic substrate from the sedoheptulose-7-phosphate produced by the first transketolase reaction. This reaction (Figure 23.35) is quite similar to the aldolase reaction of glycolysis, involving formation of a Schiff base intermediate between the sedohep-tulose-7-phosphate and an active-site lysine residue (Figure 23.36). Elimination of the erythrose-4-phosphate product leaves an enamine of dihydroxyacetone, which remains stable at the active site (without imine hydrolysis) until the other substrate comes into position. Attack of the enamine carbanion at the carbonyl carbon of glyceraldehyde-3-phosphate is followed by hydrolysis of the Schiff base (imine) to yield the product fructose-6-phosphate. 

One of the steps in the pentose phosphate pathway for glucose catabolism is the reaction of sedoheptulose 7-phosphate with glyceraldehyde 3-pho phate in the presence of a transaldolase to yield erythrose 4-phosphate and fructose 6-phosphate. 

Transketolase (TKase) [EC 2.2.1.1] essentially catalyzes the transfer of C-2 unit from D-xylulose-5-phosphate to ribose-5-phosphate to give D-sedoheptulose-7-phosphate, via a thiazolium intermediate as shown in Fig. 16. An important discovery was that hydroxypyruvate works as the donor substrate and the reaction proceeds irreversibly via a loss of carbon dioxide (Fig. 17). In this chapter, we put emphasis on the synthesis with hydroxypyruvate, as it is the typical TPP-mediated decarboxylation reaction of a-keto acid. ... 

Transfer of dihydroxyacetone moiety from sedoheptulose 7-phosphate onto glyceraldehyde 3-phosphate. This reaction is reversible and is catalyzed by transaldolase according to the scheme ... 

Transaldolase transfers C3 units from sedoheptulose 7-phosphate, a ketose with seven C atoms, to the aldehyde group of glyc-eraldehyde 3-phosphate. 

T) Ribulose 1,5-bisphosphate (2 Carbon dioxide (3) 3-Phosphoglycerate (4 1,3-Bisphosphoglycerate (5 Clyceraldehyde 3-phosphate Dihydroxyacetone phosphate (7) Fructose 1,6-bisphosphate Fructose 6-phosphate (9) Erythrose 4-phosphate Sedoheptulose 1,7-bisphosphate ( Sedoheptulose 7-phosphate Xylulose 5-phosphate Ribose 5-phosphate Ribulose 5-phosphate ( ) Glucose 6-phosphate... 

Ribose 5-phosphate ) Xylulose 5-phosphate (1 Sedoheptulose 7-phosphate Glyceraldehyde 3-phosphate (1 Erythrose 4-phosphate Fructose 6-phosphate Fructose 1,6-bisphosphate Glycerone-3-phosphate... 

This enzyme [EC 2.7.1.11], also known as phosphohexo-kinase and phosphofructokinase 1, catalyzes the reaction of ATP with D-fructose 6-phosphate to produce ADP and D-fructose 1,6-bisphosphate. Both D-tagatose 6-phosphate and sedoheptulose 7-phosphate can act as the sugar substrate. UTP, CTP, GTP, and ITP all can act as the nucleotide substrate. This enzyme is distinct from that of 6-phosphofructo-2-kinase. See also ATP GTP Depletion... 

This enzyme [EC 2.2.1.2], also known as dihydroxy acetone transferase and glycerone transferase, catalyzes the reversible reaction of sedoheptulose 7-phosphate with D-glyceraldehyde 3-phosphate to produce D-erythrose 4-phosphate and o-fructose 6-phosphate. 

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. 

Formation of a possible precursor of 9, namely, o-glycero-D-manno-heptose 7-phosphate, from D-sedoheptulose 7-phosphate was demonstrated in Salmonella typhimurium.238... 

The hydroxyethylthiamine pyrophosphates are potent nucleophiles and may add to carbonyl compounds to form carbon-carbon bonds. A good illustration of carbon-carbon bond making and breaking occurs in the reactions of transketo-lase. The enzyme contains tightly bound thiamine pyrophosphate and shuttles a dihydroxyethyl group between D-xylulose 5-phosphate and D-ribose 5-phosphate to form D-sedoheptulose 7-phosphate and D-glyceraldehyde 3-phosphate (equations 2.55 and 2.56). 

A highly purified FDPase from the slime mold Polysphondylium pallidum has been shown (92), to hydrolyze both FDP and SDP, at nearly equal rates, to yield fructose 6-phosphate and sedoheptulose 7-phosphate, respectively. In other respects the purified enzyme was remarkably similar to that isolated from Candida utilis it was completely inactive at pH 7.5 or 8.0, and showed a pH optimum at 9.2. In the presence of low concentrations of EDTA a second pH optimum appeared at pH 7.5. Unlike the Candida FDPase, however, the Polysphondylium enzyme was not inhibited by AMP at any pH. The levels of enzyme which could be extracted from the cells did not change significantly during the various stages of differentiation, and its activity could not be related to catabolic or anabolic processes which characterize these stages. 

The transaldolase-catalyzed conversion of fructose-6-phosphate and erythrose-4-phosphate to glyceraldehyde-3-phosphate and sedoheptulose-7-phosphate. This is a two-step conversion. The first step is similar to the aldolase reaction except that the dihydroxyacetone produced is held at the catalytic site while the aldose product diffuses away and is replaced by another aldose molecule. The second step involves an aldol condensation. 

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). 

The transketolase-catalyzed conversion of xylulose-5-phosphate and ribose-5-phosphate to glyceraldehyde-3-phosphate and sedoheptulose-7-phosphate. Although the aldolase and ketolase reactions superficially resemble each other, they proceed by very different mechanisms. This is because in the aldolase reaction the carbon adjacent to a carbonyl... 

Transferases 2.2.1.1 Sedoheptulose 7-phosphate D-glyceraldehyde 3-phosphate transketolase TK... 

Ribulose-5-phosphate (3.13) can be converted to ribose-5-phosphate (3.14) and xylulose 5-phosphate (3.15), by the enzymes ribose-5-phosphate isomerase and ribulose 5-phosphate 3-epimerase, respectively. The two pentose-phosphate molecules, 3.14 and 3.15, are converted to a C3 and a C7 sugar-phosphate, glyceraldehyde 3-phosphate (3.4) and sedoheptulose-7-phosphate (3.16), respectively, via the action of atransketolase. 

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