Transketolase, pentose

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

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-... 

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. 

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. 

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) ... 

The transketolase of the pentose cycle, for instance, is a good example of such limitation. This enzyme catalyses two different reactions (see Fig. 11.1), which also operate in Calvin s cycle. 

TK =transketolase TA = transaldolase AL = aldolase Simplest solution to the problem of pentoses (only reactions of carbon transfer are indicated). 

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

FIGURE 14-22 Nonoxidative reactions of the pentose phosphate pathway, (a) These reactions convert pentose phosphates to hexose phosphates, allowing the oxidative reactions (see Fig. 14-21) to continue. The enzymes transketolase and transaldolase are specific to this pathway the other enzymes also serve in the glycolytic or gluconeogenic pathways, (b) A schematic diagram showing the pathway... 

In the second phase, transaldolase (with TPP as cofactor) and transketolase catalyze the interconversion of three-, four-, five-, six-, and seven-carbon sugars, with the reversible conversion of six pentose phosphates to five hexose phosphates. In the carbon-assimilating reactions of photosynthesis, the same enzymes catalyze the reverse process, called the reductive pentose phosphate pathway conversion of five hexose phosphates to six pentose phosphates. 

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). 

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. 

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. 

Stromal enzymes, including transketolase and transaldolase, rearrange the carbon skeletons of triose phosphates, generating intermediates of three, four, five, six, and seven carbons and eventually yielding pentose phosphates. 

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). 

Ketols can also be formed enzymatically by cleavage of an aldehyde (step a, Fig. 14-3) followed by condensation with a second aldehyde (step c, in reverse). An enzyme utilizing these steps is transketolase (Eq. 17-15),132b which is essential in the pentose phosphate pathways of metabolism and in photosynthesis. a-Diketones can be cleaved (step d) to a carboxylic acid plus active aldehyde, which can react either via a or c in reverse. These and other combinations of steps are often observed as side reactions of such enzymes as pyruvate decarboxylase. A related thiamin-dependent reaction is that of pyruvate and acetyl-CoA to give the a-diketone, diacetyl, CH3COCOCH3.133 The reaction can be viewed as a displacement of the CoA anion from acetyl-CoA by attack of thiamin-bound active acetaldehyde derived from pyruvate (reverse of step d, Fig. 14-3 with release of CoA). 

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