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 glyceraldehydes 3-phosphate and sedoheptulose 7-phosphate.

(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 glyceraldehydes 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 themechanism.

Transcribed Image Text:

CH-он CH-он но- H- -HO- C=0 он Н- но HO- -H- HO. CH2OPO, OH H- Н- Н. -он CH2OPO, CH2OPO,- Xylulose 5-phosphate CH2OPO,- Ribose 5-phosphate Glyceraldehyde 3-phosphate Sedoheptulose 7-phosphate