What Is Glycogen?
Glycogen is a highly branched polysaccharide, or complex carbohydrate, that functions as the primary energy storage molecule in animals. It is often referred to as 'animal starch' because it serves the same storage purpose as starch does in plants. The molecule is composed of many thousands of glucose units linked together by $\alpha$-(1→4) glycosidic bonds in its linear chains and $\alpha$-(1→6) glycosidic bonds at its numerous branch points. This intricate, branched structure makes it a compact and readily accessible source of glucose for the body.
For humans, glycogen is primarily stored in two locations: the liver and the skeletal muscles. Liver glycogen is used to regulate blood glucose levels and is broken down during periods of fasting to supply energy to the brain and red blood cells. In contrast, muscle glycogen serves as a local fuel source, providing a quick burst of energy for the muscle cells themselves during physical activity.
How Do Humans Digest Glycogen?
Unlike indigestible fibers like cellulose, which have $\beta$-(1→4) glycosidic bonds that humans cannot break, glycogen and starch are digestible due to their $\alpha$-(1→4) and $\alpha$-(1→6) linkages. The process of digesting dietary glycogen begins in the mouth and continues through the gastrointestinal tract, involving several key enzymes.
The Role of Amylase
Digestion of complex carbohydrates like starch and glycogen starts in the mouth with salivary $\alpha$-amylase. This enzyme randomly cleaves the $\alpha$-(1→4) glycosidic bonds of the polysaccharide chains, breaking them down into smaller fragments called dextrins, maltose (a disaccharide), and maltotriose (a trisaccharide). The activity of salivary amylase is short-lived, as it is quickly inactivated by the highly acidic environment of the stomach.
The bulk of carbohydrate digestion occurs in the small intestine, where pancreatic $\alpha$-amylase is released from the pancreas. This enzyme continues the work of breaking down the $\alpha$-(1→4) linkages, further reducing the complex carbohydrates into smaller glucose chains and maltose. However, amylase alone cannot completely break down glycogen because it cannot cleave the $\alpha$-(1→6) branch points.
The Importance of the Debranching Enzyme
To fully digest a branched polysaccharide like glycogen, a specific enzyme known as the glycogen debranching enzyme is required. This enzyme has two functions:
- Glucosyltransferase Activity: It transfers a block of three glucose residues from a side branch to the end of another, longer branch, leaving a single glucose residue at the branch point.
- Glucosidase Activity: It then hydrolyzes the remaining single glucose unit, which is attached by an $\alpha$-(1→6) bond, releasing it as free glucose.
Through the combined action of amylase and the debranching enzyme, dietary glycogen is systematically disassembled into individual glucose units. These monosaccharides are then absorbed through the intestinal wall into the bloodstream to be used for energy or stored.
Dietary Glycogen vs. Stored Glycogen
It's important to distinguish between the digestion of dietary glycogen and the body's internal mobilization of its own glycogen stores, a process called glycogenolysis.
Digestion in the Gastrointestinal Tract
When humans consume animal products, such as meat, that contain glycogen, it enters the digestive system just like any other carbohydrate. As described above, enzymes from the saliva and pancreas break it down into absorbable glucose molecules. This is an external, enzymatic process that happens in the gastrointestinal tract and yields glucose that can be distributed throughout the body via the bloodstream.
Glycogenolysis in the Liver and Muscles
In contrast, glycogenolysis is an internal metabolic process for releasing stored glucose from within the body's cells. It involves different enzymes than external digestion, most notably glycogen phosphorylase, which cleaves the $\alpha$-(1→4) bonds and produces glucose-1-phosphate, and the debranching enzyme for the $\alpha$-(1→6) bonds.
Differences in glycogenolysis exist between the liver and muscle cells. Liver cells can convert glucose-6-phosphate into free glucose using the enzyme glucose-6-phosphatase, allowing it to be released into the bloodstream to raise blood sugar levels. Muscle cells, however, lack this enzyme, so the glucose-6-phosphate is used exclusively for energy production within the muscle cell and cannot be released into the general circulation.
Comparison: Dietary Glycogen Digestion vs. Internal Glycogenolysis
| Feature | Dietary Glycogen Digestion | Internal Glycogenolysis (Liver) |
|---|---|---|
| Location | Gastrointestinal (GI) tract | Hepatocytes (liver cells) |
| Initiating Factor | Consumption of animal products | Low blood glucose (fasting) |
| Primary Enzymes | $\alpha$-Amylase and Glycogen Debranching Enzyme | Glycogen Phosphorylase and Glycogen Debranching Enzyme |
| Primary Product | Free Glucose (absorbed into blood) | Free Glucose (released into blood) |
| Purpose | To extract nutrients from food | To maintain blood glucose homeostasis |
| Speed | Part of normal digestive process | Rapid response to hormonal signals like glucagon |
Conclusion
In summary, humans are fully capable of digesting glycogen consumed as part of an animal-based diet. Our digestive system uses a combination of salivary and pancreatic amylase to break down the linear parts of the glycogen polymer, with the glycogen debranching enzyme handling the complex $\alpha$-(1→6) branch points. This external digestive process releases glucose into the bloodstream, where it is utilized for energy or converted to the body's own storage form of glycogen. This mechanism is distinct from the body's internal process of glycogenolysis, which is controlled by different metabolic pathways to regulate blood sugar levels during fasting or exercise. For more information on complex carbohydrate metabolism, the National Center for Biotechnology Information provides valuable resources on biochemistry topics like glycogen metabolism in humans.(https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4802397/)
