Glycogen is a highly branched polymer of glucose that serves as a major storage form of carbohydrate in animals and also occurs in many microorganisms. Its synthesis and breakdown allow cells to buffer changes in glucose availability and energy demand. In mammals, the principal stores are in the liver and skeletal muscle, where they serve different physiological purposes. (pubmed.ncbi.nlm.nih.gov)
Molecular structure
Glycogen consists predominantly of α-D-glucose residues joined by glycosidic bonds. Within each chain, α(1→4) bonds connect successive residues; α(1→6) bonds form branch points. Branches occur approximately every 8–12 residues, producing a compact structure with numerous nonreducing ends—the chain ends at which most synthesis and degradation occur. (ncbi.nlm.nih.gov)
Glycogen resembles amylopectin, the branched component of starch, but generally has shorter chains and more frequent branching. These differences help keep glycogen accessible to metabolic enzymes rather than packed into the semicrystalline structures characteristic of starch. Glycogen particles also contain associated proteins involved in their synthesis, degradation, and regulation. Smaller particles, conventionally called β particles, can associate into larger α particles, particularly in the liver. (pmc.ncbi.nlm.nih.gov)
The many chain ends permit multiple enzymes to act simultaneously, enabling rapid storage or mobilization of glucose residues. Glycogen is therefore not simply an inert deposit: its architecture helps determine how efficiently its carbohydrate reserve can be used. (ncbi.nlm.nih.gov)
Distribution and physiological functions
Liver and skeletal muscle
Liver glycogen supports blood glucose homeostasis, particularly between meals and during the early stages of fasting. Skeletal muscle glycogen primarily supplies fuel for contraction. The liver has a higher glycogen concentration per unit tissue, whereas skeletal muscle generally holds the larger total reserve because of its greater mass. (ncbi.nlm.nih.gov)
The distinction depends partly on glucose-6-phosphatase. Liver cells can convert glycogen-derived glucose 6-phosphate into free glucose for release into the bloodstream. Skeletal muscle lacks the glucose-6-phosphatase activity required for this role; instead, it uses glucose 6-phosphate locally, particularly through glycolysis, to generate adenosine triphosphate (ATP). (pmc.ncbi.nlm.nih.gov)
Other tissues and organisms
Smaller glycogen stores occur in the heart, kidneys, and brain. Brain glycogen is concentrated mainly in astrocytes, a type of glial cell, and contributes to local metabolic buffering. Its distribution and regulation differ from those of the much larger liver and muscle stores. (ncbi.nlm.nih.gov)
Glycogen metabolism is not restricted to animals. It also provides a carbohydrate reserve in many bacteria. An important biochemical difference is that bacterial glycogen synthesis generally uses ADP-glucose as the activated glucose donor, whereas mammalian synthesis uses UDP-glucose. (pubmed.ncbi.nlm.nih.gov)
Synthesis
Glycogen synthesis, or glycogenesis, involves several coordinated reactions:
- Glucose is phosphorylated to glucose 6-phosphate.
- Phosphoglucomutase converts glucose 6-phosphate to glucose 1-phosphate.
- UDP-glucose pyrophosphorylase produces UDP-glucose, the activated glucose donor.
- Glycogen synthase transfers glucose residues from UDP-glucose to existing chains, forming α(1→4) bonds.
- A branching enzyme transfers a segment of a chain to create an α(1→6) branch point. (pmc.ncbi.nlm.nih.gov)
In the conventional initiation pathway, glycogenin attaches glucose residues to itself and constructs a short primer that glycogen synthase can extend. Glycogen synthesis is therefore more than the repeated addition of free glucose: it requires activated substrates, chain extension, and branching. (pmc.ncbi.nlm.nih.gov)
Glycogenin is not universally indispensable, however. Experiments in glycogenin-deficient mice demonstrate that glycogen can still form, including liver α particles. These findings distinguish glycogenin’s established priming activity from an absolute requirement for all glycogen synthesis. (sciencedirect.com)
Breakdown
Glycogen breakdown, or glycogenolysis, occurs principally through the coordinated action of glycogen phosphorylase and the glycogen debranching enzyme. Phosphorylase uses inorganic phosphate to remove residues from α(1→4)-linked chains as glucose 1-phosphate. It cannot pass branch points, which require debranching activity; cleavage of the α(1→6) bond releases free glucose. (pmc.ncbi.nlm.nih.gov)
Glucose 1-phosphate is converted to glucose 6-phosphate, connecting glycogen breakdown with other pathways of metabolism. Because most residues emerge already phosphorylated, their entry into glycolysis bypasses the ATP-consuming phosphorylation required for free glucose. This is particularly relevant to muscle energy metabolism. (pmc.ncbi.nlm.nih.gov)
A separate pathway operates within lysosomes, where acid α-glucosidase degrades glycogen. This pathway is biochemically distinct from cytoplasmic phosphorylase-mediated breakdown. (pmc.ncbi.nlm.nih.gov)
Regulation
Glycogen metabolism responds to both hormonal signals and conditions within the cell. Insulin promotes storage, favoring glycogen synthase activity and suppressing glycogen breakdown. Glucagon stimulates glycogen breakdown in the liver, while epinephrine can stimulate breakdown in both liver and skeletal muscle. (ncbi.nlm.nih.gov)
Regulation includes reversible phosphorylation of enzymes and allosteric effects, in which binding of a metabolite changes enzyme activity. Glucose 6-phosphate activates glycogen synthase, while AMP can activate muscle glycogen phosphorylase. These mechanisms connect the balance between synthesis and breakdown to substrate availability and cellular energy demand. (pmc.ncbi.nlm.nih.gov)
Disorders of glycogen metabolism
Inherited defects in glycogen synthesis, degradation, or associated glucose handling cause glycogen storage diseases. Their consequences depend on the affected enzyme and tissue; the term does not imply that every disorder produces excessive glycogen accumulation. (pubmed.ncbi.nlm.nih.gov)
Representative examples include:
- Type I, or von Gierke disease: defects in the glucose-6-phosphatase system impair the production of free glucose, causing fasting hypoglycemia and accumulation of glycogen and fat, particularly in the liver and kidneys. (medlineplus.gov)
- Type II, or Pompe disease: deficiency of lysosomal acid α-glucosidase causes glycogen to accumulate within lysosomes. (ptacts.uspto.gov)
- Type V, or McArdle disease: deficiency of muscle glycogen phosphorylase impairs the use of muscle glycogen and commonly causes exercise intolerance, muscle pain, and cramps. (medlineplus.gov)
Disease can also involve glycogen structure rather than simply its quantity. Abnormally organized glucose polymers may become poorly soluble and form deposits, emphasizing the importance of appropriate branching and molecular architecture. (pmc.ncbi.nlm.nih.gov)
History
Claude Bernard reported the isolation of glycogen from liver tissue in March 1857. His investigations established that the liver contained a glucose-producing reserve, helping transform understanding of animal carbohydrate metabolism. (pmc.ncbi.nlm.nih.gov)
In the twentieth century, Carl and Gerty Cori elucidated major steps in glycogen metabolism, including the production of glucose 1-phosphate and enzymatic glycogen conversion. They received shares of the 1947 Nobel Prize in Physiology or Medicine for their discovery of the course of the catalytic conversion of glycogen. (nobelprize.org)
References
- Glycogen metabolism and structure: A reviewpubmed.ncbi.nlm.nih.gov
- Glycogen metabolism in humanspmc.ncbi.nlm.nih.gov
- Biochemistry, Glycogenncbi.nlm.nih.gov
- Glycogen and its metabolism: some new developments and old themespmc.ncbi.nlm.nih.gov
- Carbohydrate Metabolismpmc.ncbi.nlm.nih.gov
- Brain glycogen structure and its associated proteins: past, present and futurepmc.ncbi.nlm.nih.gov
- Glycogen storage disease type I: MedlinePlus Geneticsmedlineplus.gov
- Glycogen storage disease type IIptacts.uspto.gov
- Glycogen storage disease type V: MedlinePlus Geneticsmedlineplus.gov
- Claude Bernard’s route to the isolation of glycogen: the journey that changed scientific views on the physiological role of the liver and animal metabolismpmc.ncbi.nlm.nih.gov
- Gerty Cori – Factsnobelprize.org