Difference, Similarity of Liver Glycogen and Muscle Glycogen

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Glycogen is a highly branched polysaccharide similar to amylopectin and sometimes referred to as animal starch. It is found in almost all animals, microorganisms and fungi, excluding plants. It is primarily stored in skeletal muscles and liver in animals. Although their structure and function are similar, skeletal muscle glycogen and liver glycogen differ significantly, especially in their mobilization rates.


Both muscle glycogen and liver glycogen are spherical glucose polymers with a core protein called glycogenin. Short glucose chains are attached to this central protein and extend outward to form glycogen β-particles with 1-12 tiers. Each β-particle has branched internal chains, unbranched outer chains and surface proteins involved in glycogen metabolism. Multiple β-particles assemble into larger one called glycogen α-particle.

β-particles are primarily distributed in skeletal muscles and liver, with small amounts found in nerve cells, kidneys and heart. Glycogen α-particles are abundant in liver, though they are also detected in small quantities in other cells.

Synthesis, Degradation: Glycogen Branches

The branches synthesis and degradation are identical in liver and muscle glycogen. Their breakdown involves phosphorolysis rather than hydrolysis. Phosphorylase cleaves one glucose from the non-reducing ends and add a phosphate group to form glucose-1-phosphate. It’s then converted into glucose-6-phosphate that enters glycolysis pathway, or it released into blood as glucose. Branch points are hydrolyzed by debranching enzymes rather than phosphorylated. If a branch is too short to be catalyzed by phosphorylase, debranching enzyme transfers the entire short branch to a neighboring chain and hydrolyzes 1,6-glycosidic bond. Because debranching enzyme works slowly, it’s considered as rate-limiting enzyme.

During glycogen synthesis, the free glucose is sequentially activated to glucose-6-phosphate, glucose-1-phosphate, and UDP-glucose (consuming one UTP). UDP-glucose is added to a smaller glycogen primer. If a branch becomes too long, the branching enzyme transfers a segment on another chain to form a new branch.


Synthesis, breakdown: Glycogen Granules in Liver

Liver glycogen exists in two forms. β-particles has a diameter of 20-40 nanometers. The 200 nanometers α-particle contains 20-40 β-particles. Their primary task is to store energy and maintain stable blood sugar level, so being mobilized in a very short time isn’t required, Your liver contains about 100 grams glycogen for half a day energy consuming.

Glycogen in liver undergoes continuous cycles of synthesis and degradation during one day. After eating, high blood glucose levels prompt liver to replenish its glycogen stores. If α-particles are present in liver during this process, they are broken down into β-particles. The newly formed and pre-existing β-particles receive free glucose together. Once these β-particles is big sufficiently, they reassemble into α-particles. Consequently, liver glycogen is composed of a few growing β-particles, a large number of mature β and α-particles.

Liver glycogen is degraded to supply glucose for blood during hunger. Smaller glycogen β-particles are mobilized more rapidly, while α-particles must be broken down into β-particles before providing glucose. Typically, only the outer four tiers in β-particles are consumed during degradation, leaving the intact inner tiers that accounts for about 5% volume. The glucose release and fluctuation in blood sugar levels is prevented by glycogen α-particles, as they are bigger and have small surface area to volume ratio. In diabetic patients, the more fragile glycogen α-particles result in a spike blood sugar level.

It's interesting that liver glycogen isn’t completely degraded but broken down into smaller β-particles instead, even after several days starvation. This partial decomposition saves considerable energy and time, as reassembling large glycogen from smaller ones is more efficient than starting from scratch.

Synthesis, Decomposition: Glycogen Granules in Muscle

There are two types of glycogen in skeletal muscle: smaller β-particles with 7-8 tiers or fewer, known as proglycogen (PG), and larger β-particles with more tiers, known as macroglycogen (MG). Muscle glycogen is distributed near muscle fibers and mitochondria. The smaller size allows for rapid mobilization during anaerobic glycolysis in intense exercise. A typical adult has 300 to 700 grams of muscle glycogen that is sufficient for moderate-intensity exercise within 2 hours.

PG is broken down into glucose more readily due to its larger surface area to volume ratio. During low or moderate intensity exercise (>70% VO₂ max), energy derived from PG and MG is approximately equal. PG provides more energy in a short-term high-intensity exercise. The breakdown of MG is significantly suppressed in repeated exercise, particularly high-intensity interval training. MG is only mobilized under extreme conditions, such as long-term starvation and prolonged high-intensity exercise.

Frequently Asked Questions

Why doesn't muscle glycogen turn into blood sugar directly? Cori cycle

It’s well known that liver glycogen and blood sugar can be converted into each other. In muscle cells, glucose-6-phosphate can neither cross cell membrane nor converted into glucose due to the lack of glucose-6-phosphatase. Nonetheless, the indirect conversion still occurs because of intermediate lactic acid. When lots of energy is needed in a short time, muscle glycogen is broken down into glucose for anaerobic glycolysis pathway where lactic acid is produced. Then lactic acid diffuses from cell membrane into blood and transported to liver to become glucose again. It's called Cory cycle or lactic acid cycle that links blood sugar, liver and muscle glycogen. It prevents the accumulated cytotoxic lactic acid and reuses energy and materials.

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