Transketolases aldol transfer

At first two molecules of triose phosphate combine to form hexose diphosphate fructose diphosphate), which is then hydrolyzed by a specific phosphatase to fructose 6-phosphate and phosphate. Hexose phosphate and triose phosphate interact in a transketolase reaction to produce erythrose 4-phosphate and xylulose 5-phosphate, which then rearranges to ribulose 5-phosphate (the first pentose molecule). In a type of aldol condensation, erythrose phosphate and triose phosphate combine to form sedoheptulose diphosphate (C4 - - C3 = C7), which is subsequently dephos-phorylated. The enzyme transketolase then transfers a C2 fragment from heptulose to triose phosphate yielding 2 moles of pentose xylulose 5-phosphate and ribose 5-phosphate) both must be rearranged to ribulose 5-phosphate. Having ribulose 5-phosphate available, the cycle can commence again first, phosphorylation with ATP to the diphosphate, then acceptance of CO2 by the diphosphate, and production of 2 moles of phosphoglyceric acid. 

Transketolase from Different Sources Catalyzed Aldol Transfer of Hydroxyacetyl Moiety from Hydroxypyruvate to Aldehydes... 

A slightly different acyl anion equivalent is transferred in transketolase reactions, and this anion is then used in a subsequent aldol reaction. TVansketolase removes a two-carbon fragment from keto sugars... 

Based on the stereospecific transketolase-catalyzed ketol transfer from hydroxy-pyruvate (20) to D-glyceraldehyde 3-phosphate (18), we have thus developed a practical and efficient one-pot procedure for the preparation of the valuable keto-sugar 19 on a gram scale in 82% overall yield [29]. Retro-aldolization of D-fructose 1,6-bisphosphate (2) in the presence of FruA with enzymatic equilibration of the C3 fragments is used as a convenient in-situ source of the triose phosphate 18 (Scheme 2.2.5.8). Spontaneous release of CO2 from the ketol donor 20 renders the overall synthetic reaction irreversible [29]. 

Like transketolase, transaldolase (TA, E.C. 2.2.1.2) is an enzyme in the oxidative pentose phosphate pathway. TA is a class one lyase that operates through a Schiff-base intermediate and catalyzes the transfer of the C(l)-C(3) aldol unit from D-sedoheptulose 7-phosphate to glyceraldehyde-3-phosphate (G3P) to produce D-Fru 6-P and D-erythrose 4-phosphate (Scheme 5.59). TA from human as well as microbial sources have been cloned.110 111 The crystal structure of the E. coliu and human112 transaldolases have been reported and its similarity to the aldolases is apparent, since it consists of an eight-stranded (o /(3)s or TIM barrel domain as is common to the aldolases. As well, the active site lysine residue that forms a Schiff base with the substrate was identified.14112 Thus, both structurally and mechanistically it is related to the type I class of aldolases. 

Transaldolase catalyzes the transfer of a C3 unit. The reaction occurs via an aldol cleavage similar to that seen with aldolase there is a schiff base intermediate formed with an active site lysine. The difference between aldolase and transaldolase is in the acceptor groups in aldolase the acceptor is a proton, in transaldolase it is another sugar. This reaction yields a F-6-P, which can go to Glycolysis, and an E-4-P which reacts with Xu-5-P catalyzed by the same transketolase seen above. This second transketolase reaction yields F-6-P and Ga-3-P, both intermediates of Glycolysis and the end products of the Pentose-P pathway. 

Briefly, two of the five glyceraldehyde 3-phosphates are isomerized to glycerone phosphate, one of which reacts with a third glyceraldehyde 3-phosphate under the influence of fructose-bisphosphate aldolase (Section 11.2) to yield fructose 1,6-bis-phosphate which is dephosphorylated.to fructose 6-phosphate (Section 11.7). Transketolase catalyses a two-carbon unit transfer between fructose 6-phosphate and a fourth glyceraldehyde 3-phosphate to yield erythrose 4-phosphate and xylulose 5-phosphate. An aldol condensation of erythrose 4-phosphate with the second glycerone phosphate, catalyst by fructose-bisphosphate aldolase, produces sedoheptulose 1,7-bisphosphate which on dephosphorylation yields sedoheptulose 7-phos-phate. A second transketolase reaction utilizes sedoheptulose 7-phosphate and a flfth glyceraldehyde 3-phosphate to produce xylulose 5-phosphate and ribose 5-phosphate. The epimerization of both xylulose 5-phosphates and the isomerization of ribose 5-phosphate (Section 11.9) produces ribulose 5-phosphates which are phosphorylated to regenerate three ribulose 1,5-bisphosphate molecules. 

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