Solved: One of the steps in the pentose phosphate pathway for glucose catabolism is the

Chapter 29, Problem 29.49

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One of the steps in the pentose phosphate pathway for glucose catabolism is the reaction of xylulose 5-phosphate with ribose 5-phosphate in the presence of a transketolase to give glyceraldehyde 3-phosphate and sedoheptulose 7-phosphate. + Ribose 5-phosphate H OH HO H CH2OPO3 2 C O CH2OH Xylulose 5-phosphate H OH CH2OPO3 2 H OH H OH Glyceraldehyde 3-phosphate O H C CH2OPO3 2 H OH O H C Sedoheptulose 7-phosphate H OH CH2OPO3 2 H OH H OH HO H C O CH2OH + (a) The first part of the reaction is nucleophilic addition of thiamin diphosphate (TPP) ylide to xylulose 5-phosphate, followed by a retro-aldol cleavage to give glyceraldehyde 3-phosphate and a TPP-containing enamine. Show the structure of the enamine and the mechanism by which it is formed. (b) The second part of the reaction is addition of the enamine to ribose 5-phosphate followed by loss of TPP ylide to give sedoheptulose 7-phosphate. Show the mechanism.