Question

Why is glucose 6 phosphate rearranged to fructose 6 phosphate?

Answer 1

The first step of glycolysis is not the committed step as the glucose-6-phosphate produced can be shuttled to other metabolic pathways such as pentose phosphate pathway. The committed step in glycolysis is the conversion of fructose- 6-phosphate into fructose-1,6-bisphosphate by PFK-1 (phosphofructokinase-1). This step requires that glucose-6-phosphate gets first converted into fructose-6-phosphate. As per the name, fructose-1,6-bisphosphate has 2 phosphate groups attached to it- one at C-1 & another at C-6. In the structure of glucose, C-1 is an aldehyde group that can't be phosphorylated, it is first reduced to an alcoholic group by molecular interconversion with C-2 which gets converted into a ketone. The ∆G° value is very small for this reaction & therefore this reaction occurs readily in eituer direction. Now both C-1 & C-6 have hydroxyl group that can be phosphorylated hence fructose-1,6-bisphosphate is formed.

Also, step 4 of glycolysis which is the breakdown of fructose-1,6-bisphosphate into glyceraldehyde-6-phosphate & dihydroxyacetone phosphate requires the presence of a ketonic group at C-2( not present in glucose but fructose). Hence, the interconversion of glucose into fructose during glycolysis is a necessary prelude to subsequent reactions.