Substrate level phosphorylation

• Substrate-level phosphorylation is a type of metabolic reaction in which a phosphate group is directly transferred from a phosphorylated intermediate (a substrate) to ADP (or GDP) to form ATP (or GTP)

Substrate-Level Phosphorylation

• It is a direct method of ATP production, where a phosphate group is transferred from a high-energy substrate molecule to ADP.
• Unlike oxidative phosphorylation, it does not involve the electron transport chain and can occur in both aerobic and anaerobic conditions.
• This process is vital during glycolysis and the citric acid cycle.

Glycolysis and Substrate-Level Phosphorylation

• Glycolysis, occurring in the cytosol, breaks down glucose into two molecules of pyruvate and includes two key steps of substrate-level:

1. 1,3-Bisphosphoglycerate to 3-Phosphoglycerate

   • The enzyme phosphoglycerate kinase transfers a phosphate from 1,3-bisphosphoglycerate to ADP, generating 2 ATP molecules per glucose.

2. Phosphoenolpyruvate (PEP) to Pyruvate

   • Pyruvate kinase transfers a phosphate from PEP to ADP, producing an additional 2 ATP molecules per glucose.

Citric Acid Cycle (TCA Cycle)

• In the mitochondrial matrix, the citric acid cycle includes one key step of substrate-level:

  • Succinyl-CoA to Succinate

    • Succinyl-CoA synthetase transfers a phosphate from succinyl-CoA to GDP, forming GTP, which is then converted to 1 ATP.

Significance of Substrate-level phosphorylation

• It produces less ATP than oxidative phosphorylation but is crucial, especially when oxygen is limited, or the electron transport chain is impaired.
• It ensures ATP production in both aerobic and anaerobic conditions, supporting vital cellular functions.

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