One of the steps in the pentose phosphate pathway for glucose catabolism is the reaction of sedoheptulose 7-phosphate with glyceraldehydes 3-phosphate in the presence of a transaldolase to yield erythrose 4-phosphate and fructose 6-phosphate.

(a) The first part of the reaction is formation of a protonated Schiff base of sedoheptulose 7-phosphate with a lysine residue in the enzyme followed by a retro-aldol cleavage to give an enamine plus erythrose 4-phosphate. Show the structure of the enamine and the mechanism by which it is formed.

(b) The second part of the reaction is nucleophilic addition of the enamine to glyceraldehydes 3-phosphate followed by hydrolysis of the Schiff base to give fructose 6-phosphate. Show themechanism.

Transcribed Image Text:

ҫH2он сH-он но H- OH но H- -он -HO- Н H- -HO- Н. HO- сн-OPО3 H- CH2OPO,- O- -OH Н. CH2OPO, CH2OPO,2- Sedoheptulose 7-phosphate Glyceraldehyde Erythrose Fructose 3-phosphate 6-phosphate 4-phosphate