Glycogenesis
Glycogenesis is the process of glycogen synthesis, in which glucose molecules are added to chains of glycogen. This process is activated by insulin in response to high glucose levels.
The first step involves the synthesis of UDP-glucose from glucose-1-phosphate and UTP:
- Glucose 1-phosphate + UTP + H2O → UDP-glucose + 2 Pi
This reaction is catalyzed by UDP-glucose pyrophosphorylase. It would be reversible if it were not for the rapid exergonic hydrolysis (hence the need for water) of pyrophosphate to orthophosphate (catalyzed by pyrophosphatase).
In the second reaction, UDP-glucose is transferred to the hydroxyl group of the existing chain, forming an α-1,4-glycosidic link. This reaction is catalyzed by glycogen synthase. Glycogen synthase can only add to a chain that contains at least four units. Therefore, the protein glycogenin is used as a "primer-molecule." α-1,6 Links are created by a branching enzyme. Glycogen synthase is activated by phosphoprotein phoshatase, which is activated by insulin.
Glycogen synthesis cannot proceed through the simple reaction:
Glucose 1-phosphate + Glycogenn → Glycogenn+1 + Pi
due to the high ammounts of phosphate present in the cell, which shift the equilibrium to the left-hand side of the equation.