Glycogen

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The synthesis and breakdown of glycogen allows steady state for blood glucose and provides energy for the body.


Glycogen is a multibranched polysaccharide of glucose that serves as a form of energy storage in animals, fungi, and bacteria.


Its polysaccharide structure represents the main storage form of glucose in the body.


Glycogen functions as one of two forms of energy reserves.


Glycogen is for short-term use.


The other form is triglyceride stores in adipose tissue for long-term storage. 


Glycogen is made and stored primarily in the cells of the liver and skeletal muscle.


Glycogen in the liver can make up 5–6% of the organ’s weight.


The liver of an adult weighing 1.5 kg can store roughly 100–120 grams of glycogen.


In skeletal muscle, glycogen is found in 1–2% of the muscle mass.


The skeletal muscle of an adult weighing 70 kg stores roughly 400 grams of glycogen.


Glycogen storage, specially within muscles and liver. depends on physical conditioning, basal metabolic rate, and dietary activity.


Small amounts of glycogen are present in other tissues and cells; kidneys, red blood cells, white blood cells, and glial cells in the brain.


 During pregnancy the uterus contains glycogen that helps nourish the embryo.


To maintain the 4 g of glucose present in the blood at all times, glycogen stores in the liver and skeletal muscle are utilized.


Glycogen stores in skeletal muscle serve as a form of energy storage for the muscle itself.


The breakdown of muscle glycogen impedes muscle glucose uptake from the blood, thereby increasing the amount of blood glucose available for use in other tissues.


Liver glycogen stores is a store of glucose for use throughout the body, particularly the central nervous system.


The human brain consumes approximately 60% of blood glucose in fasted, sedentary individuals.


It is the analogue of starch, a glucose polymer that functions as energy storage in plants. 


It is found in the form of granules in the cytoplasm in many cell types.


It plays an important role in the glucose cycle. 


It is an energy reserve that can be quickly mobilized to meet a sudden need for glucose.


It is less compact than the energy reserves of triglycerides.


It is a branched biopolymer consisting of linear chains of glucose residues with an average chain length of approximately 8–12 glucose units.


Glucose units are linked together linearly by α(1→4) glycosidic bonds from one glucose to the next. 


Glycogen in muscle, liver, and fat cells is stored in a hydrated form.


Glycogen in muscle, liver, and fat cells is composed of three or four parts of water per part of glycogen associated with 0.45 millimoles (18 mg) of potassium per gram of glycogen.


Glucose is an osmotic molecule, and can have profound effects on osmotic pressure in high concentrations possibly leading to cell damage or death if stored in the cell without being modified.


Glycogen is a non-osmotic molecule.


It can be used as a solution to storing glucose in the cell without disrupting osmotic pressure.


Glycogen physiology: 


A carbohydrate or protein meal is consumed and digested, resulting in blood glucose levels rising.


As glucose levels rise the pancreas secretes insulin. 


Blood glucose from the portal vein enters liver cells, and Insulin acts on the those cells to stimulate the action of several enzymes, including glycogen synthase. 


Glucose molecules are added to glycogen chains as long as both insulin and glucose remain at adequate levels. 


Postprandially the liver takes in more glucose from the blood than it releases.


Following ingestion and digestion glucose levels begin to fall, insulin secretion is reduced, and glycogen synthesis stops. 


If the body requires energy,  glycogen is broken down and converted again to glucose. 


Glycogen phosphorylase is the primary enzyme of glycogen breakdown. 


After eating, the next 8–12 hours the glucose derived from liver glycogen is the primary source of blood glucose used by the rest of the body for fuel.


Glucagon, in many respects serves as a countersignal to insulin. 


When insulin levels fall  below normal,and when blood levels of glucose begin to fall below the normal range, glucagon is secreted in increasing amounts


Glucagon  stimulates both glycogenolysis and gluconeogenesis.


Muscle cell glycogen is an immediate reserve source of available glucose for muscle cells. 


Muscle cells lack glucose-6-phosphatase, which is required to pass glucose into the blood.


Glycogen storeage in muscle  is available solely for internal use


Liver cells readily break down their stored glycogen into glucose and send it through the blood stream as fuel for other organs.


Glycogen synthesis requires the input of energy. which comes from uridine triphosphate (UTP), which reacts with glucose-1-phosphate, forming UDP-glucose, in a reaction catalysed by UTP—glucose-1-phosphate uridylyltransferase. 


Glycogen is synthesized from monomers of UDP-glucose initially by the protein glycogenin.


Glycogenin has two tyrosine anchors for the reducing end of glycogen.


When about eight glucose molecules ate added to a tyrosine residue, the enzyme glycogen synthase progressively lengthens the glycogen chain using UDP-glucose.


It adds  α(1→4)-bonded glucose to the reducing end of the glycogen chain.


Glycogen is cleaved from the ends of the chain by the enzyme glycogen phosphorylase to produce monomers of glucose-1-phosphate


Phosphorolysis proceeds in the direction of glycogen breakdown because the ratio of phosphate and glucose-1-phosphate is usually greater than 100.


Glucose-1-phosphate is then converted to glucose 6-phosphate (G6P) by phosphoglucomutase. 


G6P can continue on the glycolysis pathway and be used as fuel, enter the pentose phosphate pathway via the enzyme glucose-6-phosphate dehydrogenase to produce NADPH and 5-carbon sugars.


In the liver and kidney, G6P can be dephosphorylated back to glucose by the enzyme glucose 6-phosphatase. 


Glycogen metabolism becomes abnormal with diabetes.


With diabetes abnormal amounts of insulin, liver glycogen can be abnormally accumulated or depleted. 


Restoration of normal glucose metabolism usually normalizes glycogen metabolism.


With hypoglycemia caused by excessive insulin, liver glycogen levels are high, but the high insulin levels prevent the glycogenolysis necessary to maintain normal blood sugar levels. 


Therefore, glucagon is a common treatment for this type of hypoglycemia.


Inborn errors of metabolism are caused by deficiencies of enzymes necessary for glycogen synthesis or breakdown, and  are referred to as glycogen storage diseases.


Long-distance athletes, such as marathon runners, cross-country skiers, and cyclists, often experience glycogen depletion.


Glycogen depletion occurs when almost all glycogen stores are depleted after long periods of exertion without sufficient carbohydrate consumption. 


Glycogen depletion occurrence is referred to as hitting the wall.


Glycogen depletion can be forestalled by carbohydrate loading. 


Muscular insulin sensitivity is increased as a result of temporary glycogen depletion.


Glycogen debt is  often associated with extreme fatigue to the point that it is difficult to move. 


The ingestion of both carbohydrate and caffeine following exhaustive exercise, replenishes glycogen stores more rapidly.



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