Question

Sucrose is a disaccharide consisting of glucose and fructose. Following cleavage by sucrase glucose is readily metabolized through glycolysis. Explain how fructose serves as an alternative glycolytic intermediate?

Answer 1

Glycolysis is a catabolic pathway in which glucose is broken down into 2 molecules of pyruvate to produce 2 ATP.

The first step involves the conversion of glucose to glucose-6-phosphate which can be shuttled toother metabolic pathways and hence is not a committed step. The committed step in glycolysis involves the conversion of fructose- 6-phosphate to fructose-1,6-bisphosphate catalysed by the enzyme phosphofructokinase. For this reaction first the glucose-6-phosphate is converted to fructose-6-phosphate as the glucose molecule consists of an aldehyde functional group that cannot be phosphorylated directly instead it is first reduced to an alcoholic group by molecular interconversion with C-2 which gets converted into a ketone and give rise to fructose-6-phosphate which can be phosphorylated to producr fructose-1,6-bisphosphate.

Moreover, the breakdown of fructose-1,6-bisphosphate into glyceraldehyde-6-phosphate & dihydroxyacetone phosphate requires the presence of a ketonic group at C-2 which is not present in glucose but in fructose. Hence, the interconversion of glucose into fructose during glycolysis is a necessary for glycolysis to progress.