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Enzymes transketolase

In Nature, realms of complex biochemical reactions are catalyzed by enzymes, many which lack metals in their active site. Among them, nucleophilic acylation reactions are catalyzed by transketolase enzymes in the presence of coenzyme thiamine (78, vitamin Bi), a natural thia-... [Pg.82]

An X-ray structure of a thiamine dependent transketolase enzyme was determined by Schneider et al. after isolation from Saccharomyces cerevisiae in the 1990s and is shown in Fig. 10 (Sundstrom et al. 1993 Nilsson et al. 1997). The thiamine cofactor is embedded in a narrow channel in the centre of the enzyme. From the complex surrounding of the heart of this enzyme it seems to be obvious that chemical reactions at the catalytically active site in this channel proceed inevitably with high selectivities. [Pg.83]

Fig. 10. Structure of the transketolase enzyme isolated from Saccharomyces cerevisiae... Fig. 10. Structure of the transketolase enzyme isolated from Saccharomyces cerevisiae...
As there are no reports of adverse effects from consumption of excess thiamine from food and supplements (supplements of 50 mg/day are widely available without prescription), and the data are inadequate for a quantitative risk assessment, no UL has been defined for thiamine. However, as stimulators of transketolase enzyme synthesis such as thiamine support a high rate of nucleic acid ribose synthesis necessary for tumor cell survival, chemotherapy resistance, and proliferation, some concern has been expressed that thiamine supplementation of common food products may contribute to increased cancer rates in the Western world. There is, however, littie evidence to support this assumption. Rarely, individuals given high-dose intravenous thiamine in treatment of beriberi have developed anaphylaxis, the frequency being about 1 100,000. [Pg.1092]

The transketolase enzyme of Racker el obtained in crystalline form from baker s yeast, catalyzes the cleavage of ribulose-5-phosphate, with the formation of D-glyceraldehyde-3-phosphate upon the addition of an acceptor aldehyde, such as ribose-5-phosphate or glycolaldehyde. The reaction of hydroxypyruvate with D-glyceraldehyde-3-phosphate as acceptor aldehyde leads to the decarboxylation of the hydroxypyruvate with the formation of ribulose-5-phosphate. The transketolase enzyme was demonstrated to have a requirement for thiamine pyrophosphate. ... [Pg.167]

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). [Pg.63]

Gahnan JL, Steadman D, Bacon S, Morris P, Smith MEB, Ward JM, Dalby PA, Hailes HC. a,a -Dihydroxyketone formation using aromatic and heteroaromatic aldehydes with evolved transketolase enzymes. Chem. Commun. 2010 46 7608-7610. [Pg.856]

Cazares, A, Galman, JL, Cragoa, LG, Smith, MEB, Strafford, J, Rios-Solis, L, Lye, GJ, Dalby, PA, Hailes, HC. Non-a-hydroxylated aldehydes with evolved transketolase enzymes. Org. Biomol. Chem. 2010 8 1301 1309. [Pg.856]

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. [Pg.672]

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. [Pg.675]

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. [Pg.1288]

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... [Pg.277]

Ribulose 5-phosphate is the substrate for two enzymes. Ribulose 5-phosphate 3-epimerase alters the configuration about carbon 3, forming another ketopentose, xylulose 5-phosphate. Ribose 5-phosphate ketoisom-erase converts ribulose 5-phosphate to the corresponding aldopentose, ribose 5-phosphate, which is the precursor of the ribose required for nucleotide and nucleic acid synthesis. Transketolase transfers the two-carbon... [Pg.163]

The activation of apo-transketolase(the enzyme protein) in erythrocyte lysate by thiamin diphosphate added in vitro has become the accepted index of thiamin nutritional status. [Pg.489]

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. [Pg.360]

Sprenger, G.A. et al.. Identification of a thiamin-dependent synthase in Escherichia coli required for the formation of the 1-deoxy-D-xylulose 5-phosphate precursor to isoprenoids, thiamin, and pyridoxol, Proc. Natl. Acad Sci. USA 94, 12857, 1997. Lange, B.M. et al., A family of transketolases that directs isoprenoid biosynthesis via a mevalonate-independent pathway, Proc. Natl. Acad Sci. USA 95, 2100, 1998. Lois, L.M. et al., Cloning and characterization of a gene from Escherichia coli encoding a transketolase-like enzyme that catalyzes the synthesis of D-1- deoxyxylulose 5-phosphate, a common precursor for isoprenoid, thiamin, and pyridoxol biosynthesis, Proc. Natl. Acad. Sci. USA 95, 2105, 1998. [Pg.389]

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 ... [Pg.183]

Subsequent studies196 on crystalline transketolase have revealed the presence of a contaminating enzyme termed pentulose 5-phosphate waldenase (or epimerase) the presence of which had led to the erroneous conclusion that d-erythro-pentulose 5-phosphate was the substrate for transketolase. d-erythro-Pentulose 5-phosphate is virtually unattacked by transketolase prepared from spinach or liver. In subsequent discussions of experiments involving the use of transketolase, in this article, the enzymic reactions must be viewed as the result of action of transketolase and pentulose 5-phosphate waldenase (epimerase). [Pg.223]

Studies202 on the substrate specificity of transketolase suggest that this enzyme will only attack ketoses with the threo configuration at C3 and C4. [Pg.225]

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... [Pg.230]

The hypE proteins are 302-376 residues long and appear to consist of three domains. Domain 1 shows sequence identity to a domain from phosphoribosyl-aminoimida-zole synthetase which is involved in the fifth step in de novo purine biosynthesis and to a domain in thiamine phosphate kinase which is involved in the synthesis of the cofactor thiamine diphosphate (TDP). TDP is required by enzymes which cleave the bond adjacent to carbonyl groups, e.g. phosphoketolase, transketolase or pyruvate decarboxylase. Domain 2 also shows identity to a domain found in thiamine phosphate kinase. Domain 3 appears to be unique to the HypF proteins. [Pg.82]

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. [Pg.297]

The transketolase specificity could not be restricted further because, as it can be mathematically demonstrated, if a certain enzyme catalyses one of the two reactions, it must necessarily catalyse the other one. There cannot exist any enzyme which catalyses one of them without being able at the same time to catalyse the other one. [Pg.297]


See other pages where Enzymes transketolase is mentioned: [Pg.88]    [Pg.766]    [Pg.608]    [Pg.88]    [Pg.232]    [Pg.1093]    [Pg.9]    [Pg.22]    [Pg.88]    [Pg.766]    [Pg.608]    [Pg.88]    [Pg.232]    [Pg.1093]    [Pg.9]    [Pg.22]    [Pg.672]    [Pg.289]    [Pg.302]    [Pg.302]    [Pg.163]    [Pg.497]    [Pg.12]    [Pg.189]    [Pg.223]    [Pg.224]    [Pg.229]    [Pg.251]    [Pg.541]    [Pg.600]    [Pg.151]    [Pg.140]    [Pg.225]   
See also in sourсe #XX -- [ Pg.302 ]




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Enzymic methods transketolase

Transketolase

Transketolase and Related Enzymes

Transketolase mechanism enzyme

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