Cori Cycle (Lactic Acid Cycle)

Cori Cycle (Lactic Acid Cycle)

• Lactate is produced in strongly contracting skeletal muscles during anaerobic oxidation of bloodborne glucose.
• The produced lactate cannot be further metabolized in skeletal muscles and diffuses out of the muscles, entering blood circulation.
• Lactate is carried to the liver, where it is converted to glucose via the gluconeogenic pathway and released back into blood circulation.
• The reformed glucose becomes available for utilization in skeletal muscles.

Glycogenolysis

• Glycogenolysis is the breakdown of glycogen stored in liver and muscles.
• It maintains blood glucose levels between meals and provides energy for muscular activities.

Steps of Glycogenolysis

• Step 1: Glycogen undergoes phosphorolysis to yield glucose-1-phosphate and limit dextrin in the presence of enzyme glycogen phosphorylase.
• Step 2: The branches of limit dextrins are removed with the help of enzyme glucosyl transferase.
• Step 3: The branching point with a single glucose residue is cleaved by debranching enzyme, releasing the free glucose residue.
• Steps 4 and 5: Glucose-1-phosphate is converted to glucose-6-phosphate in the presence of enzyme phosphoglucomutase.

Fate of Glucose-6-Phosphate

• In liver: Glucose-6-phosphate is dephosphorylated to free glucose, which diffuses out of hepatic cells and enters blood circulation.
• In muscles: Glucose-6-phosphate is utilized in glycolysis as a source of energy for muscular activities.

Glycogenesis

• Glycogen synthesis occurs by a pathway distinctly different from the reversal of glycogen breakdown.
• Steps of glycogenesis:
  • Activation of glucose: UDP-glucose is formed from glucose-1-phosphate and UTP by the catalytic activity of UDP-glucose pyrophosphorylase.
  • Glycogen synthesis: Activated glucose units are sequentially added by the enzyme glycogen synthase.
  • Branching enzyme: Creates alpha-1,6 linkages, making the molecule more globular.