Transketolases

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

P, the seven-carbon sugar serving as the transketolase substrate. Likewise, phosphoribulose kinase carries out the unique plant function of providing RuBP from Ru-5-P (reaction 15). The net conversion accounts for the fixation of six equivalents of carbon dioxide into one hexose at the expense of 18 ATP and 12 NADPH. 

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. 

MORE NADPH THAN RmOSE-5-P IS NEEDED BY THE CELL Large amounts of N/VDPH can be supplied for biosynthesis without concomitant production of ribose-5-P, if ribose-5-P produced in the pentose phosphate pathway is recycled to produce glycolytic intermediates. As shown in Figure 23.39, this alternative involves a complex interplay between the transketolase and transaldolase reac-... 

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. 

One of the steps in the pentose phosphate pathway for glucose catabolism is the reaction of xylulose 5-phosphate with ribose 5-phosphate in the presence of a transketolase to give glyceraldehyde 5-phosphate and sedoheptulose 7-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. 

Table 5. Transketolase Catalyzed Preparative Aldol Additions with Hydroxy-pyruvate...

Restrictions for the substrates of the transketolase-catalyzed reaction only arise from the stereochemical requirements of the enzyme. The acceptor aldehyde must be formaldehyde9,20, glycolaldehydel6,17 or a (R)-2-hydroxyaldehyde10,17. The donor ketose must exhibit a (3(7,4 R) configuration10. The enzyme selectively adds the hydroxyacetyl moiety to the Re-face of the acceptor aldehyde leading to a 3(7 configuration of the products. 

Since the equilibrium reaction mixture contains at least four products, workup can be difficult and therefore, it may be helpful to bring the reaction to completion. For example, in the transketolase-catalyzed reaction of [l-13C]D-ribosc 5-phosphate and [l-l3C]D-i/ /w-2-pen-tulose 5-phosphatc to [l,3-13C]n-a/b,o-2-heptulose 7-phosphate and D-glyceraldehyde 3-phos-... 

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. 

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. 

A number of mechanistically distinct enzymes can likewise be employed for the synthesis of product structures identical to those accessible from aldolase catalysis. Such alternative cofactor-dependent enzymes (e.g. transketolase) are emerging as useful catalysts in organic synthesis. As these operations often extend and/or... 

Flgure10.23 Sialyl Lewis -related selectin inhibitorandfluorogenicscreening compound for transketolase prepared using enzymatic aldolization, and multienzymatic oxidation-aldolization strategy for the synthesis of bicyclic higher carbon sugars. 

Fluorogenic compound (56) for transketolase assays has been prepared making use of FruA specificity [123]. Pendant anionically charged chains have been extended from O- or C-glycosidic aldehydes to furnish low molecular weight mimics of the sialyl Lewis X tetrasaccharide such as (SS) (Figure 10.23) [124], Other higher carbon... 

Figure 10.37 Kinetic resolution by transketolase, and nonequilibrium C—C bond formation by decomposition of hydroxypyruvate.

Figure 10.39 Synthesis of a novel N-hydroxypyrrolidine and a fluorogenic screening substrate for transaldolases based on stereospecific transketolase catalysis.

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