Glycogenesis
The fed state banks glucose as branched glycogen.
Costs 1 UTP (≈1 ATP) per glucose stored from G6P
Glucose is activated as UDP-glucose, then added to a growing chain that only glycogen synthase can extend and only onto a glycogenin primer. Insulin turns synthase on by dephosphorylating it, the exact residue glucagon’s PKA uses to turn it off.
The same reversible enzyme runs the reverse direction during glycogenolysis. G6P arrives from hexokinase/glucokinase; glycolysis step 1 is the shared front door.
Near-equilibrium on paper, but pyrophosphatase instantly hydrolyzes the PPᵢ, pulling the reaction irreversibly forward, the standard trick for activating a sugar as a nucleotide donor.
Rate-limiting. Adds α-1,4 links to the nonreducing end. Activated by insulin (dephosphorylation via protein phosphatase 1) and by G6P; switched off directly by PKA, the same signal that, one tier down through phosphorylase kinase, turns phosphorylase on. Cannot start a chain from nothing: glycogenin self-primes the first ~8 residues.
Transfers a block of ~7 residues onto C6 of a nearby chain, making an α-1,6 branch. Branches mean more nonreducing ends, so both synthesis and breakdown run fast, and the granule stays soluble. Deficiency is Andersen disease (GSD IV).