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 compounds:
These two products, glyceraldehyde-3-phosphate and dihydroxyacetone phosphate, are rapidly interconverted by
triose phosphate isomerase.
The Keq 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:
In shorthand:
The Scope of Biochemistry
Glycolysis
