is a polysaccharide
(Glc) which functions as the secondary short term energy storage in animal
cells. It is made primarily by the liver
and the muscles
, but can also be made by the brain
, and the vagina
. Glycogen is the analogue of starch
, a less branched glucose polymer in plants
, and is commonly referred to as animal starch
, having a similar structure to amylopectin
. Glycogen is found in the form of granules in the cytosol
in many cell
types, and plays an important role in the glucose cycle
. Glycogen forms an energy
reserve that can be quickly mobilized to meet a sudden need for glucose, but one that is less compact than the energy reserves of triglycerides
(fat). In the liver hepatocytes
, glycogen can compose up to 8% of the fresh weight (100–120 g in an adult) soon after a meal. Only the glycogen stored in the liver can be made accessible to other organs. In the muscles
, glycogen is found in a much lower concentration (1% of the muscle mass), but the total amount exceeds that in liver. Small amounts of glycogen are found in the kidneys
, and even smaller amounts in certain glial
cells in the brain
and white blood cells
. The uterus also stores glycogen during pregnancy to nourish the embryo.
Structure and biochemistry
Glycogen is a highly branched polymer that is better described as a dendrimer of about 60,000 glucose residues and has a molecular weight between 106 and 107 daltons (~4.8 million). Most of Glc units are linked by α-1,4 glycosidic bonds, approximately 1 in 12 Glc residues also makes alpha-1,6 glycosidic bond with a second Glc, which results in the creation of a branch. Glycogen does not possess a reducing end: the 'reducing end' glucose residue is not free but is covalently bound to a protein termed glycogenin as a beta-linkage to a surface tyrosine residue. Glycogenin is a glycosyltransferase and occurs as a dimer in the core of glycogen. The glycogen granules contain both glycogen and the enzymes of glycogen synthesis (glycogenesis) and degradation (glycogenolysis). The enzymes are nested between the outer branches of the glycogen molecules and act on the non-reducing ends. Therefore, the many non-reducing end-branches of glycogen facilitate its rapid synthesis and catabolism.
Function and regulation of liver glycogen
As a meal containing carbohydrates
is eaten and digested, blood glucose
levels rise, and the pancreas
. Glucose from the hepatic portal vein
enters the liver
). Insulin acts on the hepatocytes to stimulate the action of several enzymes
, including glycogen synthase
. Glucose molecules are added to the chains of glycogen as long as both insulin and glucose remain plentiful. In this postprandial
or "fed" state, the liver takes in more glucose from the blood than it releases.
After a meal has been digested and glucose levels begin to fall, insulin secretion is reduced, and glycogen synthesis stops. About four hours after a meal, glycogen begins to be broken down and converted again to glucose. Glycogen phosphorylase is the primary enzyme of glycogen breakdown. For the next 8–12 hours, glucose derived from liver glycogen will be the primary source of blood glucose to be used by the rest of the body for fuel.
Glucagon is another hormone produced by the pancreas, which in many respects serves as a counter-signal to insulin. When the blood sugar begins to fall below normal, glucagon is secreted in increasing amounts. It stimulates glycogen breakdown into glucose even when insulin levels are abnormally high.
In muscle and other cells
Muscle cell glycogen appears to function as an immediate reserve source of available glucose for muscle cells. Other cells that contain small amounts use it locally as well. Muscle cells lack glucose-6-phosphatase enzyme, so they lack the ability to pass glucose into the blood, so the glycogen they store internally is destined for internal use and is not shared with other cells, unlike liver cells.
Glycogen debt and endurance exercise
Due to the body's inability to hold more than around 2,000 kcal
of glycogen, long-distance athletes such as marathon
runners, cross-country skiers
, and cyclists
go into glycogen debt, where almost all of the athlete's glycogen stores are depleted after long periods of exertion without enough energy consumption. This phenomenon is referred to as "hitting the wall
". In marathon runners it normally happens around the 20 mile (32 km) point of a marathon, where around 100 kcal are spent per mile, depending on the size of the runner and the race course. However, it can be delayed by a carbohydrate loading
before the task.
When experiencing glycogen debt, athletes often experience extreme fatigue to the point that it is difficult to move.
A study published in the Journal of Applied Physiology (online May 8, 2008) suggests that when athletes ingest both carbohydrate and caffeine following exhaustive exercise, their glycogen is replenished more rapidly.
Disorders of glycogen metabolism
The most common disease in which glycogen metabolism
becomes abnormal is diabetes
, in which, because of abnormal amounts of insulin, liver glycogen can be abnormally accumulated or depleted. Restoration of normal glucose metabolism usually normalizes glycogen metabolism as well.
In hypoglycemia caused by excessive insulin, liver glycogen levels are high, but the high insulin level prevents the glycogenolysis necessary to maintain normal blood sugar levels. Glucagon is a common treatment for this type of hypoglycemia.
Various inborn errors of metabolism are caused by deficiencies of enzymes necessary for glycogen synthesis or breakdown. These are collectively referred to as glycogen storage diseases.
Glycogen synthesis differs from glycogen breakdown. Unlike breakdown, synthesis is endergonic
, meaning that glycogen is not synthesized without the input of energy. Energy for glycogen synthesis comes from UTP
, which reacts with glucose-1-phosphate
, forming UDP-glucose
, in reaction catalysed by UDP-glucose pyrophosphorylase
. Glycogen is synthesized from monomers of UDP-glucose
by the enzyme Glycogen synthase
, which progressively lengthens the glycogen chain. As glycogen synthase can only lengthen an existing chain, the protein glycogenin
is needed to initiate the synthesis of glycogen.
Glycogen is cleaved from the nonreducing ends of the chain by the enzyme glycogen phosphorylase
to produce monomers of glucose-1-phosphate that is then converted to Glucose 6-phosphate
. A special debranching enzyme
is needed to remove the alpha(1-6) branches in branched glycogen and reshape the chain into linear polymer. The G6P monomers produced have three possible fates:
it is not gluconeogenesis it is glucogenesis in the breakdown section of glycogen. No amino acids or proteins are being broken down to create this glucose, therefore it is glucogenesis.