8.2: Energy-requiring Steps of Glycolysis
Glycolysis begins with a single glucose molecule.
Using ATP, the enzyme hexokinase transfers a phosphate group to the six-carbon sugar to produce glucose 6-phosphate, which becomes trapped inside the cell due to its negative charge.
Next, the enzyme phosphor-gluco-isomerase catalyzes the conversion of the phosphoglucose into one of its isomers, fructose 6-phosphate. The phosphofructose can now be phosphorylated by a rate-limiting enzyme phosphofructokinase to produce fructose-1,6-bisphosphate.
Finally, with two phosphate groups attached the sugar molecule is cleaved by aldolase into two, three-carbon isomers—glyceraldehyde-3-phosphate, or G3P, and dihydroxyacetone-phosphate, DHAP.
Another catalyzing enzyme, triose phosphate isomerase, converts the DHAP into G3P to yield two molecules. Thus, during this energy-investment phase, a net total of two ATPs are used to split one initial glucose molecule into two, smaller sugars.