Transketolase reaction

We see this type of cleavage in the transketolase reaction described in Chapter 23.)... 

FIGURE 23.32 The transketolase reaction of step 6 in the pentose phosphate pathway. 

FIGURE 23.34 The tnechanistn dependent transketolase reaction. Ii the group transferred in the transk( don might best be described as an t whereas the transferred group in th dolase reaction is actually a ketol. D irony, these names persist for histor... 

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. 

The mechanistic chemistry of the acetolactate synthase and phosphoketolase reactions (shown below) is similar to that of the transketolase reaction (Figure 23.34). Write suitable mechanisms for these reactions. 

The high stereoselectivity of the transketolase reaction also enables the resolution of racemic a-hydroxyaldehydes23,26. Treatment of racemic 2-hydroxyaldehydes and hydroxypyruvic acid with transketolase, gave the corresponding L-2-hydroxyaldehydes that are not substrates for the enzyme and, therefore, remained unreacted. The corresponding D-enantiomers were consumed and gave the condensation products. 

At the beginning of the MEP pathway, the glycolytic products, pyruvate and D-glyceraldehyde (GAP), are condensed in a transketolase reaction to deoxy-xylulose phosphate (DXP) by the deoxy-xylulose phosphate synthase (DXS) enzyme. DXP is the precursor for other pathways leading to pyridoxal and thiamine. 

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) ... 

A ribose 5-phosphate molecule and one of the two xylulose 5-phosphate molecules are used during the first transketolase reaction. The other xylulose 5-phosphate molecule is consumed later, in the second transketolase reaction. 

Transketolase is a dimer with a molecular mass of 140000. Its coenzyme is thiamine bisphosphate. Mgt+ ions are required for the reaction. Both transketolase reaction products are used as substrates at the next step of the cycle. 

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 ... 

A somewhat more trivial thing to remember about the HMP pathway is that this is one of the places you ve seen the vitamin thiamin pyrophosphate. This cofactor is necessary for the transketolase reaction that is in the middle of the HMP pathway. The transketolase reaction converts two C-5 sugars to a C-7 and a C-3. The other place you ve seen thiamin pyrophosphate as a cofactor is in the pyruvate dehydrogenase and a-ketoglutarate dehydrogenase reactions. 

In the well known "transketolase reaction" [9] for instance, the transfer of the fragment H0CH2-C=0 from a hexose to a triose takes place via the "active glycoaldehyde" (Scheme 5.8) ... 

A slightly different acyl anion equivalent is transferred in transketolase reactions, and this anion is then used in a subsequent aldol reaction. TVansketolase removes a two-carbon fragment from keto sugars... 

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... 

The pentose phosphates formed in the transketolase reactions—ribose 5-phosphate and xylulose 5-phos-phate—are converted to ribulose 5-phosphate (steps (7) and (3)), which in the final step ( ) of the cycle is phosphorylated to ribulose 1,5-bisphosphate by ribulose 5-phosphate kinase (Fig. 20-13). This is the third very exergonic reaction of the pathway, as the phosphate anhydride bond in ATP is swapped for a phosphate ester in ribulose 1,5-bisphosphate. 

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. 

Transketolase reactions leading via the pentose or hexose monophosphate shunt pathway of glucose oxidation to the eventual production of pentoses for RNA/DNA synthesis and NADPH for the biosynthesis of fatty acids... 

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). 

Because the transaldolase and transketolase reactions are symmetrical with respect to types of bonds cleaved and formed, their equilibrium constants are near 1. The pool of sugar phosphates is thus near equilibrium in cells that contain these enzymes. In a water-flow analogy, the sugar phosphates and the reactions that interconvert them resemble a large swamp, with ill-defined flows along many interconnecting channels. Water may be fed in from any direction and may leave the swamp in any direction. 

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... 

Scheme 5.55. Using hydroxypyruvic acid (HPA) as the ketol donor renders the transketolase reaction irreversible, making it of synthetic utility. Only D-hydroxyaldehyes are accepted by transketolase.

The donor of the two-carbon unit in this reaction is xylulose 5-phosphate, an epimer of ribulose 5-phosphate. A ketose is a substrate of transketolase only if its hydroxyl group at C-3 has the configuration of xylulose rather than ribulose. Ribulose 5-phosphate is converted into the appropriate epimer for the transketolase reaction by phosphopentose epimerase (see Figure 20.11) in the reverse reaction of that which occurs in the Calvin cycle. 

The role of TPP as a coenzyme in the transketolase reaction is very similar to that of oxidative decarboxylation. The carbanion of TPP combines with the carbonyl carbon of xylulose 5P. Carbon 1 and 2 of xylulose 5P are retained to form hydroxyethyl derivative of TPP. Then it is transferred to the carbonyl carbon of ribose 5P to form sedoheptulose TP. 

TPP is also required for transketolase reaction of the pentose phosphate pathway. 

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. 

Figure 8.27. Coenzyme role of thiamine p3rrophosphate in the transketolase reaction.

Transketolase Transketolase reaction N on-phosphory-lated donors Few enzymes... 

Transketolase Reaction. Transketolase contains a tightly bound thiamine pyrophosphate as its prosthetic group. The enzyme transfers a two-carbon glycoaldehyde from a ketose donor to an aldose acceptor. The site of the addition of the two-carbon unit is the thiazole ring of TPR Transketolase is homologous to the Ej subunit of the pyruvate dehydrogenase complex (p. 478) and the reaction mechanism is similar (Figure 20.21). 

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. 

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