Transketolase-Catalyzed Reaction

The reactions catalyzed by transketolases are also extremely unique because of the following reason. If one sees only the starting materials and the product, the carbonyl carbon of a ketone is working as a nucleophile, which cannot happen in ordinary chemical reactions (Fig. 24). 

Transfer of glycolic aldehyde from xylulose 5-phosphate onto ribose 5-phosphate or the first transketolase reaction. The next reaction, which is catalyzed by transketolase, involves the pentose phosphates produced by the foregoing reaction (the transferable moiety is shown in the box) ... 

Transfer of glycolic aldehyde from xylulose 5-phosphate onto erythrose 4-phosphate or the second transketloase reaction. This reaction is related to the first transketolase reaction and is catalyzed by the same enzyme. The only distinction is that erythrose 4-phosphate acts as an acceptor for glycolic aldehyde ... 

Transketolase catalyzes the reversible transfer of a hydroxyacetyl fragment from a ketose to an aldehyde. Because the ketose products formed by transketolase reactions are not acceptors for a consecutive transformation by the same enzyme, we have investigated the option to include a xylose (glucose) isomerase (Xyll E.C. 5.3.1.5), which has similar stereochemical specificity, for ketose to aldose equilibration (Scheme 2.2.5.13). Starting from racemic lactaldehyde 32a, the transketolase forms 5-deoxy-D-xylulose 35a, which indeed was accepted by the Xyll in situ for diastereospecific conversion into 5-deoxy-D-xylose 36a. The latter again proved to be a substrate of transketolase which completed a tandem operation to furnish 7-deoxy-sedoheptulose 37a as the sole bisadduct in 24% overall yield and in enantio- and diastereomerically pure quality [35, 36]. All four stereocenters of the resulting product are completely controlled by the enzymes during this one-pot operation. The procedure profits from the limited tolerance of the isomerase... 

Various thiamine diphosphate (ThDP)-dependent a-keto acid decarboxylases have been described as catalyzing C-C bond formation and/or cleavage [48]. Extensive work has already been conducted on transketolase (TK) and pyruvate decarboxylase (PDC) from different sources [49]. Here attention should be drawn to some concepts based on the investigation of reactions catalyzed by the enzymes... 

FIGURE 20-10 Third stage of C02 assimilation. This schematic diagram shows the interconversions of triose phosphates and pentose phosphates. Black dots represent the number of carbons in each compound. The starting materials are glyceraldehyde 3-phosphate and dihydroxyacetone phosphate. Reactions catalyzed by transaldolase ( and ) and transketolase ((3) and ) produce pentose phosphates that are converted to ribulose 1,5-bisphosphate—ribose... 

FIGURE 20-11 Transketolase-catalyzed reactions of the Calvin cycle, (a) General reaction catalyzed by transketolase the transfer of a two-carbon group, carried temporarily on enzyme-bound TPP, from a ketose donor to an aldose acceptor, (b) Conversion of a hexose and a triose to a four-carbon and a five-carbon sugar (step of Fig. 20-10). (c) Conversion of seven-carbon and three-carbon sugars to two pentoses (step of Fig. 20-10). 

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. 

An example of an a-ketol formation that does not involve decarboxylation is provided by the reaction catalyzed by transketolase, an enzyme that plays an essential role in the pentose phosphate pathway and in photosynthesis (equation 21) (B-77MI11001). The mechanism of the reaction of equation (21) is similar to that of acetolactate synthesis (equation 20). The addition of (39) to the carbonyl group of (44) is followed by aldol cleavage to give a TPP-stabilized carbanion (analogous to (41)). The condensation of this carbanionic intermediate with the second substrate, followed by the elimination of (39), accounts for the observed products (B-7IMIHOO1). 

By 1998, X-ray structures had been determined for four thiamin diphosphate-dependent enzymes (1) a bacterial pyruvate oxidase,119120 (2) yeast and bacterial pyruvate decarboxylases,121 122c (3) transketolase,110123124 and (4) benzoylformate decarboxylase.1243 Tire reactions catalyzed by these enzymes are all quite different, as are the sequences of the proteins. However, the thiamin diphosphate is bound in a similar way in all of them. 

The reaction catalyzed by transketolase superficially resembles the transaldolase reaction in that the substrate... 

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). 

Transketolases are characterized by their ability to transfer a two-carbon unit from a ketose to an aldehyde. The C3 and C7 sugar-phosphates can subsequently be converted to a C4 and a Csugar-phosphate, erythrose 4-phosphate (3.17) and fructose 6-phosphate (3.2), respectively. This reaction is catalyzed by a transaldolase, which transfers a three-carbon glyceraldehyde unit from an aldose to a ketose. Erythrose-4-phosphate (3.17) can be used in the shikimate pathway (see Section 6). A second transketolase reaction can generate a second fructose-6-phosphate (3.2) and glyceraldehyde-3-phosphate (3.4) residue from erythrose-4-phosphate (3.17) and xylulose-5-phosphate (3.15). Hexose-phosphate isomerase converts the... 

Understand the physiologic importance of the hexose monophosphate shunt and understand reactions catalyzed by glu-cose-6-phosphate and 6-phosphogluconate dehydrogenases, transaldolase, and transketolase discuss the importance of the hexose monophosphate shunt in red cell physiology. 

The reactions catalyzed by transketolase and transaldolase are distinct yet similar in many ways. One difference is that transketolase transfers a two-carbon unit, whereas transaldolase transfers a three-carbon unit. Each of these units is transiently attached to the enzyme in the course of the reaction. In transketolase, the site of addition of the unit is the... 

Transaldolase catalyzes the transfer of a C3 unit. The reaction occurs via an aldol cleavage similar to that seen with aldolase there is a schiff base intermediate formed with an active site lysine. The difference between aldolase and transaldolase is in the acceptor groups in aldolase the acceptor is a proton, in transaldolase it is another sugar. This reaction yields a F-6-P, which can go to Glycolysis, and an E-4-P which reacts with Xu-5-P catalyzed by the same transketolase seen above. This second transketolase reaction yields F-6-P and Ga-3-P, both intermediates of Glycolysis and the end products of the Pentose-P pathway. 

E. In the first three reactions of the pentose phosphate pathway, glucose is converted to ribulose 5-phosphate and C02, with the production of NADPH. These reactions are not reversible. Ribose 5-phosphate and xylulose 5-phosphate may be formed from ribulose 5-phos-phate. A series of reactions catalyzed by transketolase and transaldolase produce the glycolytic intermediates fructose 6-phosphate and glyceraldehyde 3-phosphate. 

Figure 15-3. The reaction catalyzed by the enzyme transketolase, which transfers a glycoaldehyde group from a ketose to an aldose.

In the Calvin cycle, transketolase also catalyzes the reversals of these reactions above. 

Xylulose-5-phosphate is an intermediate of the pentose phosphate pathway produced from ribulose-5-phosphate in the reaction catalyzed by phospho pentose epimerase or from glyceraldehyde-3-phosphate and sedoheptulose-7-phosphate in the reaction catalyzed by transketolase. 

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