Transaldolase

Although this review has focused on muscle aldolase intensively because it is the most well studied enzyme of this kind, it is only one of a variety of aldolases catalyzing similar reactions using an iminium ion pathway. Many of the mechanistic features of these enzymes, including retention mechanism, borohydride reaction etc., are similar [60-63]. 

Transaldolase is a critical enzyme in the metabolism of carbohydrates directed through the pentose phosphate pathway [63]. It is responsible for transferring a 

3-carbon unit between various acceptor aldehydes so that an equilibrium of various 

No other cofactors are required for enzymatic catalysts of this reaction. Amino acid analysis on isolated isoenzymes I and III indicate that the lower molecular weight monomers from I are not proteolytic degradation products of the monomers from III [68], When dimers of I (a ) are mixed with dimers of III ()8y8) an equilibrium concentration of II (afi) is formed. 

There is substantial evidence indicating the formation of an iminium ion with substrate. The adduct formed between lysine amino group of the enzyme and the substrate is much more stable than the corresponding adduct of aldolase, since the enamine that is formed does not protonate, as it does in aldolase. Horecker and Smymiotis [69] demonstrated, for example, that sedoheptulose does not incorporate labeled dihydroxyacetone when incubated in the presence of transaldolase. The implication is that the dihydroxyacetone adduct formed on the enzyme does not 

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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-... 

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. 

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

Bacellar H, Munoz A et al (1994) Temporal trends in the incidence of HIV-l-related neurologic diseases Multicenter AIDS Cohort Study, 1985-1992. Neurology 44(10) 1892-1900 Banki K, Hutter E et al (1998) Molecular ordering in HIV-induced apoptosis. Oxidative stress, activation of caspases, and cell survival are regulated by transaldolase. . J Biol Chem 273(19) 11944-11953... 

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 is a dimer with a molecular mass of about 70 000. The fructose 6phosphate molecule produced by this reaction enters the glycolysis, while erythrose 4-phosphate is used as a substrate at the subsequent steps of the cycle. 

Transferases transaminase, glycosyltransferase transaldolase transfer of a group from one molecule to another... 

Schurmann, R. and Sprenger, G.A. (2001) Fructose-6-phosphate aldolase is a novel class I aldolase from Escherichia coli and is related to a novel group of bacterial transaldolases. The Journal of Biological Chemistry, 276, 11055-11061. 

I, 7-diphosphate.170 1 (f> This tetrose phosphate is involved with phosphoenol pyruvate in the formation of shikimic acid via 3-deoxy-2-keto-D-ara6ino-heptonic acid 7-phosphate and, hence, of aromatic compounds.170(d) A synthesis of the tetrose phosphate has been described.170 1 Aldolase shows a high affinity for the heptulose diphosphate and, compared with that for D-fructose 1,6-diphosphate, the rate of reaction is about 60 %. The enzyme transaldolase, purified 400-fold from yeast, catalyzes the following reversible reaction by transfer of the dihydroxyacetonyl group.l70(o>... 

The above transketolase and transaldolase reactions were found inadequate to explain the metabolism of D-ribose 5-phosphate, because of the non-accumulation of tetrose phosphate, the 75 % yield of hexose phosphate, and the results of experiments with C14 (the distribution of which differed markedly from the values predicted for such a sequence). 24(b) Thus, with D-ribose-l-C14, using rat-liver enzymes, any hexose formed should have equal radioactivity at Cl and C3, whereas, actually, 74% appeared at Cl. Furthermore, D-ribose-2,3-Cl42 should have given material having equal labels at C2 and C4 in the resultant hexose, whereas, in fact, it had 50% of the activity at C4, C3 was nearly as active as C2, and Cl had little activity. Similar results were obtained with pea-leaf and -root preparations.24 The following reactions, for which there is enzymic evidence,170(b) were proposed, in addition to those involving D-aftro-heptulose, to account for these results.24(b) (o) 200... 

Transketolase and transaldolase appear to play a significant part in photosynthesis.86 Photosynthesizing plants have been exposed to C14C>2 for short periods (up to 15 seconds) and the radioactive products have been... 

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

An additional enzyme that transfers C3 rather than C2 units is called transaldolase, but, in common with aldolase (see Box 10.5), this enzyme utilizes... 

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

Transaldolase, which catalyzes reactions with d-erythrose 4-phosphate and D-fructose 6-phosphate as substrates. As in the case of fructose-1,6-bisphosphate aldolase, this enzyme uses a e-amino side-chain to form a Schiff base intermediate. In this case, however, the triose phosphate moiety is not released but is transferred to the other aldose (in this case, the aldotetrose). 

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