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- Glucose 1-phosphate + UTP + H2O → UDP-glucose + 2 Pi
This reaction is catalyzed by UDP-glucose pyrophosphorylase. This reaction 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. This enzyme is activated by insulin.
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