Glycogenesis (Glycogen Synthesis)

1. 1

   Glucose + ATP → glucose-6-phosphate + ADP

   Hexokinase / Glucokinase 2.7.1.1 / 2.7.1.2 ST-0029 Irreversible ATP Mg2+
   In Glucose ATP

   Out Glucose-6-phosphate ADP
   › Notes

   Hexokinase in muscle and most tissues or glucokinase in liver phosphorylates glucose using ATP and Mg2+. This irreversible trapping step channels glucose toward intracellular metabolism, including glycogen synthesis.

2. 2

   Glucose-6-phosphate ↔ glucose-1-phosphate

   Phosphoglucomutase ST-0030 Reversible
   In Glucose-6-phosphate

   Out Glucose-1-phosphate
   › Notes

   Phosphoglucomutase reversibly transfers the phosphate through a glucose-1,6-bisphosphate intermediate; a catalytic phosphoserine is primed by glucose-1,6-bisphosphate. This reaction directs glucose carbon toward activated-sugar formation.

3. 3

   Glucose-1-phosphate + UTP → UDP-glucose + PPi

   UDP-glucose pyrophosphorylase 2.7.7.9 ST-0031 Irreversible UTP Mg2+ PPi
   In Glucose-1-phosphate UTP

   Out UDP-glucose PPi
   › Notes

   UDP-glucose pyrophosphorylase requires UTP and Mg2+ to activate glucose as UDP-glucose. Subsequent hydrolysis of pyrophosphate by ubiquitous inorganic pyrophosphatase (PPi + H2O → 2 Pi) drives the overall activation forward; the activation sequence is effectively irreversible.

4. 4

   UDP-glucose + glycogenin–Tyr-OH → glycogenin–Tyr-O-glucose + UDP; repeated additions form a short alpha(1→4)-glucosyl primer

   ↪ glycogenin–Tyr-O-glucose + UDP; repeated additions form a short alpha(1 → 4)-glucosyl primer

   Glycogenin (autoglucosylation) 2.4.1.186 ST-0032 Irreversible/directional Mg2+
   In UDP-glucose Glycogenin-Tyr-OH

   Out Glycogenin-Tyr-O-glucose UDP Repeated additions form a short alpha(1→4)-glucosyl primer
   › Notes

   Glycogenin is an autoglycosylating protein that uses UDP-glucose and divalent cation (typically Mg2+). It creates the required primer, usually about 8 residues long, because glycogen synthase cannot initiate a new chain de novo.

5. 5

   UDP-glucose + glycogen(n) → glycogen(n+1) + UDP

   Glycogen synthase 2.4.1.11 ST-0033 Irreversible/directional rate-limiting
   In UDP-glucose Glycogen

   Out Glycogen(n+1) UDP
   › Notes

   Glycogen synthase transfers glucose from UDP-glucose to a nonreducing end, forming an alpha(1→4) glycosidic bond. This is the principal rate-limiting and committed elongation step; it requires a pre-existing primer and is activated allosterically by glucose-6-phosphate.

6. 6

   An alpha(1→4)-linked chain segment → alpha(1→6)-branched glycogen

   ↪ 4)-linked chain segment → alpha(1 → 6)-branched glycogen

   Glycogen branching enzyme (GBE1; 4:6 transferase) 2.4.1.18 ST-0034 Irreversible/directional
   In An alpha(1→4)-linked chain segment

   Out Alpha(1→6)-branched glycogen
   › Notes

   The glycogen branching enzyme (amylo-alpha(1,4)→alpha(1,6)-transglycosylase; 4:6 transferase) transfers a terminal block of roughly 6–7 residues from a sufficiently long chain to the C6 hydroxyl of an internal glucose residue. It creates an alpha(1→6) branch at least several residues away from another branch, increasing solubility and the number of nonreducing ends for future synthesis and degradation.

Showing all 6 steps.

Overview

Glycogenesis stores excess glucose as glycogen, a highly branched alpha-glucan that provides a rapidly mobilizable, osmotically economical glucose reserve. Liver glycogen buffers blood glucose between meals, whereas skeletal-muscle glycogen supplies local fuel for contraction. Synthesis occurs when glucose and energy are abundant, particularly after a carbohydrate-containing meal.

Cellular location

Glycogen synthesis occurs in the cytosol on glycogen particles, which contain enzymes and regulatory proteins organized around a glycogenin core. It is most active in hepatocytes and skeletal myofibers; cardiac muscle, kidney, adipose tissue, glia, and other cells also contain smaller stores. The relevant glucose-6-phosphate and glucose-1-phosphate reactions take place in the cytosol.

Net energetics

For each glucosyl residue added from free glucose, phosphorylation costs 1 ATP, and UDP-glucose formation consumes 1 UTP (energetically equivalent to ATP because UTP is regenerated from UDP by nucleoside diphosphate kinase at ATP expense). Thus the cost is 2 ATP equivalents per residue stored from free glucose; no NADH, NADPH, FADH2, or GTP is formed. For glucose-6-phosphate entering directly, only the UTP-equivalent cost is required.

Clinical significance

Branching permits compact storage while allowing simultaneous access by many glycogen synthase or glycogen phosphorylase molecules at nonreducing ends. Insulin-stimulated glycogenesis in liver limits postprandial hyperglycemia, while muscle glycogenesis replenishes exercise-depleted local stores. Glycogen synthase deficiency causes glycogen storage disease (GSD) type 0, characterized by low hepatic glycogen, fasting ketotic hypoglycemia, and postprandial hyperglycemia; branching enzyme deficiency (GSD IV, Andersen disease) forms poorly branched polyglucosan and can cause progressive liver disease.