Glycogen Metabolism

The red blood cells and the brain have an absolute requirement for glucose for energy metabolism. Glucose is absorbed from the intestines only for 2 or 3 h after a meal; therefore, there must be another source of glucose to maintain a constant blood glucose level. When blood glucose levels increase after a meal, the liver can uptake large amounts of glucose, where it is converted to glucose-6-phosphate that can be used to synthesize glycogen (glycogenesis). When glycogen stores are full, glucose-6-phosphate can enter glycolysis or be used to synthesize glycerol for the formation of fat. When blood glucose levels decrease, during fasting between meals, glycogen is broken down in the liver and glucose is released (glycogenolysis). In the fasting state, glycogen is broken down by the removal of glucose units as glucose-1-phosphate from the many ends of the molecule. This is then isomerised to glucose-6-phos-phate. Only the liver can release free glucose because muscle tissue lacks glucose-6-phosphatase. The free glucose released by the liver is used by the brain and red blood cells.

Glucose-6-phosphate released in the muscle tissue from glycogen can enter directly into glycolysis for energy production by the muscle. Alternatively, it

Table 2 Summary of relative importance of different metabolic pathways in intermediary metabolism in different tissues

Tissue Principal catabolic and anabolic pathways

Brain 25% basal O2 consumption

Metabolizes glucose only, except after prolonged starvation when it can adapt to uptake and metabolize ketones Blood Mature red blood cells have no mitochondria ;

energy from anaerobic glycolysis: glucose ! lactate

Muscle Preferentially metabolize fatty acids and ketones produced from the liver Anaerobic glycolysis of glucose from glycogen stores

Aerobic respiration of glucose from glycogen or fatty acids/ketones Liver Mostly amino acid oxidation for generation of ATP

Most important tissue for maintaining blood glucose by gluconeogenesis from amino acids and lactate (via Cori cycle) and glycerol and also from breakdown of glycogen stores Fatty acid synthesis and synthesis of lipoproteins for transport Production of ketones into circulation Site of the pentose-phosphate pathway for generation of NADPH + H+ Adipose Designed for the storage of fat tissue Can synthesize fat from glucose Kidneys Gluconeogenesis

Amino acid oxidation for ATP generation can be metabolized to pyruvate and then transaminated to alanine that is exported from the muscle to the liver, where it can be used as a substrate for gluconeogenesis. Table 2 shows the relative importance of energy metabolic pathways in different tissues of the body.

See also: Amino Acids: Metabolism. Energy: Balance. Fatty Acids: Metabolism. Glucose: Metabolism and Maintenance of Blood Glucose Level. Protein: Requirements and Role in Diet. Sports Nutrition.

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