Transketolase reactions catalysed

Transfer of C2 and C3 units in reactions catalysed by transketolase and transaldolase respectively modify the chain length of the sugar. 

TPP involved in reactions catalysed by pyruvate decarboxylase (alcoholic fermentation), pyruvate dehydrogenase a-ketoglutarate dehydrogenase (TCA cycle), transketolase (photosynthesis Calvin cycle) acetolactate synthetase (Val, Leu biosynthesis)... 

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. 

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. 

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. 

Other Carbohydrate-based Methods. N-Hydroxy-l,4-dideoxy-l,4-imino-arabinitol, 77, has been prepared in nine steps from ( )-3-0-benzylglyceral-dehyde. A transketolase mediated reaction was used to establish a pentulose (5-0-benzyl-D-xylulose) with correct absolute stereochemistry, and a 1,2-oxazine was the unexpected product of the acid-catalysed reaction of an aldehydic intermediate with triethylorthoformate (Scheme 15). Reduction of this oxazine with sodium cyanoborohydride in acetic acid, did not effect cleavage of the N-O bond, and yielded the iV-hydroxypyrrolidine as a single diastereoisomer. ... 

An enzyme closely related to the aldolases is transketolase. The enzyme is commercially available (from baker s yeast) and can also be obtained from spinach leaves. Transketolase catalyses the stereospecific synthesis of C—C bonds using aldehydes as the electrophiles, with a suitable 2-carbon ketol donor [e.g. hydroxypyruvate (9)] as the nucleophile (Scheme 5.11). The use of hydroxypyruvate ensures that the reaction goes to completion, since carbon dioxide is evolved as the by-product, and hence the reaction is irreversible. In addition, both magnesium ions and catalytic thiamine pyrophosphate are required as co-factors. 

Transketolase (EC 2.2.1.1) an enzyme that catalyses transketolation, an important process of carbohydrate metabolism, especially in the Pentose phosphate cycle (see) and Calvin cycle (see). T. has been found in a wide variety of cells and tissues, including mammalian liver, green plants and many bacterial species. The enzyme contains divalent metal cations and the coenzyme, thiamin pyrophosphate. Transketolation involves transfer of a C2-unit (often called active glycolaldehyde or a ketol moiety) from a ketose to Cl of an aldose. Only ketoses with L-configuration at C3 and preferably irons configuration on the next carbon (i.e. Cl, 2, 3 and preferably 4 as in fructose) can serve as donors of the C2-unit. The acceptor is always an aldose. Thins-ketolation is reversible. Details of the reaction in which xylulose S-phosphate serves as the donor of... 

The 6-deoxy-L-talose derivative 14 was the major product of osmium mtroxide hydroxylation of the alkene 13, itself prepared from di-O-acetyl-L-rhamnal by allylic rearrangement followed by Mitsunobu inversion at C-4.U 6-Deoxy-D- ructose and 6-deoxy-L-sorbose were obtained as a separable mixture from the transketolase-catalysed reaction of hydroxypyruvate with 2,3-dihydroxybuQialdehyde (mixture of isomers) (Scheme 4). ... 

Transketolase (TK) is involved in anaerobic carbohydrate metabolisms such as the nonoxidative phase of the pentose phosphate pathway. In plants and photosynthetic bacteria, TK is involved in the Calvin-Benson cycle. TK catalyses the transfer of a 2-carbon dihydroxyethyl group from a ketose phosphate (donor substrate such as D-xylulose 5-phosphate) to the Cl position of an aldose phosphate (acceptor substrate such as o-ribose 5-phosphate) (Figure 4.3) (Schneider and Lindqvist 1998). The first product is an aldose phosphate released from the donor (such as glyceraldehyde 3-phosphate) and the second is a ketose phosphate (such as sedoheptulose 7-phosphate), in which the 2-carbon fragment is attached to the acceptor. Examples of the substrates and the products mentioned above are for the first reaction of the pentose phosphate pathway. In the second reaction of the same pathway, the acceptor is D-ery-throse 4-phosphate and the second product is o-fructose 6-phosphate. A snapshot X-ray crystallographic study revealed that an ot-carbanion/enamine a,p-dihydroxyethyl ThDP is formed as a key intermediate (Fiedler et al. 2002). Then, a nucleophilic attack of the a-carbanion intermediate on the acceptor substrate occurs. 

Reactions of transketolase (TK) and phosphoketolase (PK). The first half of the reactions of TK and PK are identical. TK catalyses the transfer of the 2-carbon fragment to an acceptor, whereas PK catalyses dehydration and subsequent nucleophilic attack of phosphate to produce acetyl phosphate. For the first reaction of TK in the pentose pathway, where donor, acceptor, products 1 and 2 are xylulose 5-phosphate, ribose 5-phosphate, glyceraldehyde 3-phosphate and sedoheptulose 7-phosphate, Ri = R2 = -CH0H-CH2-0P03. Pyr and PP represent 2,5-dimethyl-4-amino-pyrimidine and the ethyl diphosphate tail, respectively. 

A transketolase has been isolated from plant tissues and from animal tissues, it catalyses the following reaction ribulose-5-P + ribose-5-P sedoheptulose -7-P + glyceraldehyde-3-P... 

Tissues which are more active in the synthesis of lipids than nucleotides require NADPH rather than ribose moieties. In such tissues, e.g. adipose tissue, the ribose 5-phosphate enters a series of sugar interconversion reactions which connect the pentose phosphate pathway with glycolysis and gluconeogenesis. These interconversion reactions constitute the non-oxidative phase of the pathway (Figure 11.14) and since oxidation is not involved, NADPH is not produced. Two enzymes catalyse the important reactions transketolase which contains thiamin diphosphate (Figure 12.3a) as its prosthetic group and transaldolase. Both enzymes function in the transfer of carbon units transketolase transfers two-carbon units and transaldolase transfers three-carbon units. The transfer always occurs from a ketose donor to an aldose acceptor. The interconversion sequence requires the oxidative phase to operate three times, i.e. three molecules of glucose 6-phosphate yield three molecules of ribulose 5-phosphate. 

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