Six‐carbon reactions of glycolysis represent an energy investment of two high‐energy phosphate bonds. Glucose enters glycolysis in a phosphorylated form, as glucose‐6‐phosphate:
When the glucose originates by breakdown of its polymeric forms, starch or glycogen, it is already phosphorylated, as glucose‐1‐ phosphate, and the initial reaction is catalyzed by the enzyme phosphoglucomutase.
When glucose is present in its unphosphorylated form, the first reaction of glycolysis is a phosphorylation. Although the goal of glycolysis is the synthesis of ATP, a high‐energy phosphate must be invested first, catalyzed by the enzyme hexokinase:
The end result of either of these reaction schemes is glucose‐6‐ phosphate, which is now isomerized to fructose‐6‐phosphate by the enzyme phosphoglucose isomerase:
This is a reversible reaction, both in vitro and in vivo. Fructose‐6‐ phosphate is then phosphorylated by the enzyme phosphofructokinase, in a second energy investment involving ATP:
Fructose‐1,6‐bisphosphate, the product of this reaction, is then cleaved by aldolase to two 3‐carbon
These two products, glyceraldehyde‐3‐phosphate and dihydroxyacetone phosphate, are rapidly interconverted by triose phosphate isomerase.
The K eq of the aldolase reaction favors dihydroxyacetone phosphate; however, glyceraldehyde‐3‐phosphate is drained off for the further reactions of glycolysis, while dihydroxyacetone phosphate is not. This means that the concentration of glyceraldehyde‐3‐phosphate is very low during active metabolism and dihydroxyacetone phosphate is converted into glyceraldehyde‐3‐phospate.
The production of the triose phosphates represents the end of the investment. The reactions so far can be summarized as: