In: Chemistry
Consider the mechanism of cleavage catalyzed by the enzyme aldolase. What would be the products of the reaction if aldolase used glucose-6-phosphate as the substrate, instead of fructose-1,6-bisphosphate (show the structures of glucose-6-phosphate and its products)? Based on your answer, suggest a possible rationale for why nature chose to include two extra steps prior to the aldolase step in glycolysis.
The step prior to the aldolase step is the isomerisation of glucose-6-phosphate to fructose-6-phosphate. The open-chain form of glucose has an aldehyde group at carbon 1, but the open-chain form of fructose has a keto group at carbon 2. Hence it can be said that the isomerization of glucose 6-phosphate to fructose 6-phosphate is a conversion of an aldose into a ketose. The reaction includes two extra steps prior to the aldolase step because both glucose 6-phosphate and fructose 6-phosphate are present primarily in the cyclic forms. The enzyme first opens the six-membered ring of glucose 6-phosphate, catalyze the isomerization, and then it promotes the formation of the five-membered ring of fructose 6-phosphate.