Metabolism

• Glycogen synthesis is the process by which glycogen is synthesized from glucose.
• Glycogen is synthesized from glucose through the Cori cycle.
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• Glycogen synthesis involves the enzymes glycogen phosphorylase, transferase, and debranching enzyme.
• Hormones involved in glycogen synthesis include epinephrine/glucagon and tissues such as liver and muscle.
• The reactions catalyzed by debranching enzyme include glycogen breakdown near an (α1→6) branch point.
• Following sequential removal of terminal glucose residues by glycogen phosphorylase, glucose residues near a branch are removed in a two-step process.
• The transferase activity of the enzyme shifts a block of three glucose residues from the branch to a nearby nonreducing end, to which the segment is reattached in (α1→4) linkage.
• The single glucose residue remaining at the branch point, in (α1→6) linkage, is then released as free glucose by the (α1→6) debranching enzyme.
• Glycogen degradation (glycogen phosphorylase) yields glucose 1-phosphate.
• Gluconeogenesis is the process where glucose can be synthesized from non-carbohydrate sources.
• Glycogen depletion is not an actual cause of fatigue.
• French cyclist Tony Gallopin is an example of an athlete who has experienced glycogen depletion.
• Entry points for amino acids and glycerol are present in gluconeogenesis.
• Gluconeogenesis is not a direct reversal of glycolysis, but shares several of the same steps and bypasses three irreversible reactions of glycolysis: glucose to glucose 6 P, fructose to F 1,6 BP, and PEP to pyruvate.
• In gluconeogenesis, the three irreversible reactions are: Pyruvate to PEP (a two-step reaction), F 1,6 BP to fructose, and G 6 P to G.
• Oxaloacetate and mitochondria involvement in pyruvate to PEP conversion is a key feature of gluconeogenesis.
• Biotin and HCO3 are involved in gluconeogenesis.
• ATP, GTP, and NADH are involved in gluconeogenesis.
• Gluconeogenesis and glycolysis are reciprocally regulated, meaning that within a cell, one pathway is relatively inactive while the other is highly active.
• The interconversion of fructose 1,6-bisphosphate and fructose 6-phosphate is a key regulatory site in the reciprocal regulation of glycolysis and gluconeogenesis.
• Glycolysis and gluconeogenesis are reciprocally regulated at the interconversion of phosphoenolpyruvate and pyruvate.
• If ATP is needed, glycolysis predominates.
• If glucose is needed, gluconeogenesis is favored.
• In the liver, the rates of glycolysis and gluconeogenesis are adjusted to maintain blood-glucose levels.
• Fructose 2,6-bisphosphate stimulates phosphofructokinase-1 (PFK-1) and inhibits fructose 1,6-bisphosphatase (FBPase-1) in the liver.
• When blood glucose is high, insulin is secreted, stimulating glycolysis and inhibiting gluconeogenesis.
• Glycogen phosphorylase is responsive to hormones such as epinephrine, glucagon and insulin.
• Glycogen is the major animal storage polysaccharide, a polymer of glucose where the glucosyl residues are linked by α (1  4) glycosidic bonds, with branches ( α (1  6) glycosidic bonds) approximately every 8-15 glucosyl residues.
• In type 2 diabetes, insulin fails to act, a condition called insulin resistance (type-2-diabetes).
• A transferase shifts a small oligosaccharide near the branch point to a nearby chain, thereby making the glucose moieties accessible to the phosphorylase.
• There are important differences between liver and muscle for glycogen degradation: liver maintains glucose homeostasis for the entire body, while muscle uses glucose to produce energy for itself.
• The key regulatory enzyme for glycogen degradation is glycogen phosphorylase.
• Glycogen phosphorylase degrades glycogen from the nonreducing ends of the glycogen molecule.
• Glycogen phosphorylase cannot cleave near branch points and can only cleave α -1,4-glycosidic bonds.
• Glycogen is linked to a protein, glycogenin.
• Liver removes the lactate and converts it into glucose, which can be released into the blood.
• A debranching enzyme ( α -1,6-glucosidase) then cleaves the α -1,6 bond at the branch point, releasing a free glucose.
• The onset of fatigue coincides with the depletion of glycogen reserves.
• Glucose 1-phosphate is converted into glucose 6 - phosphate by phosphoglucomutase.