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Glycogen

Structure and Function: – Glycogen is a branched biopolymer of glucose with linear chains averaging 8-12 glucose units. – Branches are linked by α(1→6) glycosidic […]

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Structure and Function:
– Glycogen is a branched biopolymer of glucose with linear chains averaging 8-12 glucose units.
– Branches are linked by α(1→6) glycosidic bonds, forming glucose trees with A, B, and C chains.
– It serves as short-term energy storage in animals, fungi, and bacteria, being the main storage form of glucose in the human body.
– Glycogen is stored in liver and skeletal muscle cells, with liver glycogen being a primary source of blood glucose.
– Muscle glycogen provides phosphorylated glucose for muscle cells, crucial for muscle contraction during anaerobic and high-intensity aerobic activities.

Metabolism:
– Glycogen synthesis requires energy input primarily from uridine triphosphate, initiated by glycogenin and elongated by glycogen synthase.
– Branching enzyme transfers glucose residues within the glycogen molecule.
– Glycogen breakdown is catalyzed by glycogen phosphorylase in the direction of glycogen breakdown.
– Glucose-1-phosphate is converted to glucose 6-phosphate, with debranching enzyme reshaping branched glycogen into a linear polymer.
– Glucose 6-phosphate has multiple fates in the body.

Clinical Relevance:
– Disorders of glycogen metabolism can result from conditions like diabetes, hypoglycemia, and inborn errors of carbohydrate metabolism.
– Hypoglycemia with high liver glycogen levels can be treated with glucagon.
– Strategies to prevent glycogen depletion in long-distance athletes include continuous ingestion of high glycemic index carbohydrates.
– Endurance training can improve muscle glycogen storage and fuel use efficiency.
– Carbohydrate loading and post-exercise consumption of carbohydrate and caffeine aid in rapid glycogen replenishment.

Research and Clinical Applications:
– Hormones like insulin and glucagon regulate glycogen metabolism, with enzymes like glycogen synthase and glycogen phosphorylase controlling synthesis and breakdown.
– Clinical guidelines exist for glycogen storage diseases, with research focusing on structure, metabolism, and regulation.
– Studies contribute to understanding sports performance, utilizing techniques like transmission electron microscopy to study glycogen particles.
– Recent developments in glycogen metabolism pathways shed light on its clinical implications and potential therapeutic applications.

Nanomedicine and Advanced Applications:
– Glycogen nanoparticles are explored for potential drug delivery systems.
– Glycogen serves as a building block for advanced biological materials, promoting recovery and muscle function post-exercise.
– Advanced Materials highlight glycogen’s potential in biological applications, with studies exploring its role in cell metabolism and liver health in diabetic conditions.
– Monte Carlo simulations aid in understanding glycogen biosynthesis, with resources like ‘The Ketogenic Diet’ delving into dietary implications on glycogen utilization.
– Wikimedia Commons and external resources provide additional information and tools for accessing glycogen-related research materials.

Glycogen (Wikipedia)

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

Schematic two-dimensional cross-sectional view of glycogen: A core protein of glycogenin is surrounded by branches of glucose units. The entire globular granule may contain around 30,000 glucose units.
A view of the atomic structure of a single branched strand of glucose units in a glycogen molecule.
Glycogen (black granules) in spermatozoa of a flatworm; transmission electron microscopy, scale: 0.3 μm

Glycogen functions as one of three regularly used forms of energy reserves, creatine phosphate being for very short-term, glycogen being for short-term and the triglyceride stores in adipose tissue (i.e., body fat) being for long-term storage. Protein, broken down into amino acids, is seldom used as a main energy source except during starvation and glycolytic crisis (see bioenergetic systems).

In humans, glycogen is made and stored primarily in the cells of the liver and skeletal muscle. In the liver, glycogen can make up 5–6% of the organ's fresh 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 a low concentration (1–2% of the muscle mass): the skeletal muscle of an adult weighing 70 kg stores roughly 400 grams of glycogen. Small amounts of glycogen are also found in other tissues and cells, including the kidneys, red blood cells, white blood cells, and glial cells in the brain. The uterus also stores glycogen during pregnancy to nourish the embryo.

The amount of glycogen stored in the body mostly depends on oxidative type 1 fibres, physical training, basal metabolic rate, and eating habits. Different levels of resting muscle glycogen are reached by changing the number of glycogen particles, rather than increasing the size of existing particles though most glycogen particles at rest are smaller than their theoretical maximum.

Approximately 4 grams of glucose are present in the blood of humans at all times; in fasting individuals, blood glucose is maintained constant at this level at the expense of glycogen stores, primarily from the liver (glycogen in skeletal muscle is mainly used as an immediate source of energy for that muscle rather than being used to maintain physiological blood glucose levels). Glycogen stores in skeletal muscle serve as a form of energy storage for the muscle itself; however, 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 serve as 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.

Glycogen is an analogue of starch, a glucose polymer that functions as energy storage in plants. It has a structure similar to amylopectin (a component of starch), but is more extensively branched and compact than starch. Both are white powders in their dry state. Glycogen is found in the form of granules in the cytosol/cytoplasm 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 (lipids). As such it is also found as storage reserve in many parasitic protozoa.

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