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Transketolase- is an enzyme of Pentose phosphate pathway.

The pentose phosphate pathway (also called Phospho Gluconate pathway or hexose mono phosphate shunt [HMP shunt]) is an alternative route for the metabolism of glucose. It does not lead to formation of ATP but has two major functions: (1) The formation of NADPH for synthesis of fatty acids and steroids, and (2) the synthesis of ribose for nucleotide and nucleic acid formation.

This pathway consists of two phases: the oxidative generation of NADPH and the non oxidative interconversion of sugars (Figure 1). In the oxidative phase, NADPH is generated when glucose 6-phosphate is oxidized to ribose 5-phosphate. This five-carbon sugar and its derivatives are components of RNA and DNA, as well as ATP, NADH, FAD, and coenzyme A.

In the nonoxidative phase, the pathway catalyzes the interconversion of three-, four-, five-, six-, and seven-carbon sugars in a series of nonoxidative reactions that can result in the synthesis of five-carbon sugars for nucleotide biosynthesis or the conversion of excess five-carbon sugars into intermediates of the glycolytic pathway. All these reactions take place in the cytosol.


Figure 1- showing an overview of HMP pathway.(GAP-Glyceraldehyde-3-Phosphate)

Transketolase transfers the two-carbon unit comprising carbons 1 and 2 of a ketose on to the aldehyde carbon of an aldosesugar. It therefore effects the conversion of a ketose sugar into an aldose with two carbons less and an aldose sugar into a ketose with two carbons more.The reaction requires Mg2+ and thiamine diphosphate (vitamin B1)as coenzyme. The two-carbon moiety transferred is probably glycoaldehyde boundto thiamine diphosphate. Thus, Transketolase catalyzes the transfer of the two-carbon unit from Xylulose 5-phosphate to ribose 5-phosphate, producing the seven-carbon ketose sedoheptulose 7-phosphate and the aldose glyceraldehyde 3-phosphate. These two products then undergo transaldolation.(Figure- 2)


Figure 2- showing the reaction catalyzedby Transketolase

Transaldolase catalyzes the transfer of a three-carbon Dihydroxyacetone moiety (carbons 1–3) from the ketose sedoheptulose 7-phosphate onto the aldose glyceraldehyde 3-phosphate to form the ketose fructose 6-phosphate and the four-carbon aldose Erythrose4-phosphate. (Figure-3)


Figure 3-showing the reaction catalyzed by Transaldolase

In a further reaction catalyzed by Transketolase, Xylulose5-phosphate serves as a donor of glycoaldehyde. In this case Erythrose4-phosphate is the acceptor, and the products of the reaction are fructose6-phosphate and glyceraldehyde 3-phosphate. (Figure 4)






Figure 4- showing the rearrangement of sugars to form glycolytic intermediates catalyzed by Transketolase

Clinical Significance– Since Transketolase requires the presence of Thiamine pyro phosphate(TPP) as a coenzyme, in the deficiency of TPP, Transketolase activity is grossly affected. Measurement of red blood cell Transketolase activity is an index for the determination of underlying Thiamine deficiency.

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