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- 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 of pyrophosphate to orthophosphate (catalyzed by pyrophosphatase), hence the presence of H2O in the formula.
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
This is due to the high amounts of phosphate present in the cell, which shift the equilibrium to the left-hand side of the equation.