glycogen synthesis

Meaning: Glycogenesis stores glucose after a meal, mainly in liver and skeletal muscle. Glucose is phosphorylated to glucose 6-phosphate, converted to glucose 1-phosphate, then reacts with UTP to form UDP-glucose and pyrophosphate. Hydrolysis of the pyrophosphate helps drive this step forward. Glycogen synthase transfers glucose from UDP-glucose to the nonreducing end of a chain, forming α-1,4 glycosidic bonds. It extends a short primer made by glycogenin. A branching enzyme then moves a segment of the chain to create an α-1,6 branch.

Equation: glycogen(n) + UDP-glucose → glycogen(n+1) + UDP, where n is the number of glucose residues (a pure count, no unit) and UDP-glucose is the activated glucose donor (mol).

On the MCAT: Expect questions on hormonal control. Insulin leads to dephosphorylation and activation of glycogen synthase. Glucagon (mainly acting on liver) and epinephrine lead to PKA-mediated phosphorylation, which inactivates it. Other items to know are which bond each enzyme makes and what happens to branching if an enzyme is missing.

Common confusion: Glycogen synthesis is not simply glycogenolysis run backward. Degradation uses glycogen phosphorylase, which uses inorganic phosphate to release glucose 1-phosphate. Synthesis uses UDP-glucose and glycogen synthase. Phosphorylation also has opposite effects: it activates phosphorylase but inactivates synthase.