Transaldolases and Transketolases

It should be mentioned that most natural aldolase enzymes can also be assayed using enzyme-coupled systems relaying the reaction to a redox process with NAD. The formation of NADH by active microbial colonies in expression libraries of mutant enzymes was detected colorimetrically in agar plates using phenazine methosulfate and nitroblue tetrazolium, which forms an insoluble precipitate. The assay was used by Williams et al. [14] and Woodhall et al. [15] for evolving sialic acid aldolases to accept non-natural aldehyde acceptors. 

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Two NADPH Molecules Are Generated by the Pentose Phosphate Pathway Transaldolase and Transketolase Catalyze the Interconversion of Many Phosphorylated Sugars Production of Ribose-5-phosphate and Xylulose-5-phosphate... 

Transaldolase and Transketolase Catalyze the Interconversion of Many Phosphorylated Sugars... 

The key enzymes involved in these conversions are transaldolase and transketolase. The two enzymes are similar in their substrate specificities. Both require a ketose as a donor and an aldose as an acceptor. The steric requirements at positions C-1 through C-4 are the same as the requirements of aldolase in the glycolytic pathway, except that aldolase requires phosphorylation at C-1, and both transaldolase and transketolase require a free hydroxyl group at C-1. 

Both transaldolase and transketolase require a ketose phosphate as the donor molecule and an aldose phosphate as the acceptor. Furthermore, both enzymes require the same steric configuration at carbons 3 and 4 as is found in glucose and fructose. Ribulose-5-phosphate, the first pentose phosphate to be formed in the pentose phosphate pathway, does not have the correct configuration to serve as a substrate for either transaldolase or transketolase. However, both a suitable donor ketose and an acceptor aldose can be made by isomerizations of ribulose-5-phosphate, and enzymes that... 

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. 

Class 2. Transferases transfer chemical groups from one molecule to another, or within a single molecule. They include amino, acyl, methyl, glucosyl, and phosphoryl transferases, kinases, phosphomutases, transaldolase, and transketolase. 

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. 

Mode 1. Much more ribose 5-phosphate than NADPH is required. For example, rapidly dividing cells need ribose 5-phosphate for the synthesis of nucleotide precursors of DNA. Most of the glucose 6-phosphate is converted into fructose 6-phosphate and glyceraldehyde 3-phosphate by the glycolytic pathway. Transaldolase and transketolase then convert two molecules of fructose 6-phosphate and one molecule of glyceraldehyde 3-phosphate into three molecules of ribose 5-phosphate by a reversal of the reactions described earlier. The stoichiometry of mode 1 is... 

D. Both transaldolase and transketolase produce glyceraldehyde 3-phosphate, but only transketolase requires thiamine pyrophosphate. 

Explain how the pentose phosphate pathway and the glycolytic pathway are linked through reactions catalyzed by transaldolase and transketolase. 

Transaldolase and transketolase have which of the following similarities ... 

The metabolism of fructose-6-P by the nonoxidative branch could in theory be initiated by transketolase, with triose phosphate as an acceptor, or by transaldolase, with erythrose-4-P as an acceptor. Dische (6) and Bonsignore et al. (7) found, however, that fructose-6-P was rapidly converted to sedoheptulose-7-P in cell extracts in the absence of added acceptor, and this observation was confirmed with a mixture of purified transaldolase and transketolase the presence of both enzymes was required 8). It appeared that each enzyme had reacted with fructose-6-P to form the acceptor for the other enz3rme, as shown in Fig. 6-2. 

That either the oxidative or the nonoxidative branch alone is able to provide sufficient pentose phosphates to support growth has been demonstrated by studies of various microorganisms. Thus, Candida (Torxda) ulilis apparently uses the oxidative pathway only, even though it contains transaldolase and transketolase (16, 17). In contrast, Alcaligenes faecalis and Pseudomonas saccharophilia lack the oxidative branch (18,19). In some cases still different pathways of pentose phosphate synthesis may occur in bacteria. Human erythrocytes deficient in glucose-6-P dehydrogenase can synthesize pentose phosphates adequately via the nonoxidative branch (80). 

Chapter I. Methyl, hydroxyl and formyl transfer by G. L. Cantoni (Bethesda, Md.) Chapter II. Transaldolase and transketolase by B. L. Horecker (New York, N.Y.) Chapter III. Transfer of acyl groups (CoA structure, function) by R.Vagelos (Bethesda,... 

A rapid, high yield synthesis of C-enriched intermediates of the pentose-phosphate pathway has been developed based on a combination of chemical and enzymic reactions. [l- C]Ribose and [l- C]arbinose 5-phospha,tes, available by the classical Kiliani method, were converted to a variety of specifically labelled 5-, 6-, 7-, and 8-carbon sugar phosphates (e.g.. D-erythro-pentulosc 5-phosphate, sedoheptulose mono- and di-phosphates) with the help of aldolase, transaldolase, and transketolase. ... 

Zw, glucoso-6-phosphate will be mainly metabolized in glycolysis. This, in turn, will lead to the anaerobic biosynthesis of pentoses from glycolytic metabolites with the participation of transaldolase and transketolase enzymes. It is also possible that the lethal effect on the Pgd" mutation is aggravated because the accumulation of 6-phosphogluconate is inhibitory to the first steps of glycolysis. Similar data were obtained by Hughes and Lucchesi (1977). 

D- rythrose 4-phosphate is a normal intermediate in plant and animal metabolism and is formed by the action of transaldolase and transketolase. An enzyme from acetobacter forms acetyl phosphate and D-erythrose phosphate from fructose 6-phosphate and inorganic phosphate . Phosphorylation of D-erythrose with a bacterial enzyme and ATP or with Jteferenees p. 140... 

Subsequent research has shown that the pentose phosphate so produced is further metabolized in reactions utilizing the enzymes transaldolase and transketolase and involving as intermediates pentose, heptulose, tetrose and triose (5, 7, 4 and 3 carbon sugars) phosphates which are also involved in the photosynthetic carbon reduction cycle. A version of the pentose phosphate pathway is shown in Fig. 4.7. 

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