Substrate Level Phosphorylation

Substrate level phosphorylation is the direct synthesis of ATP (or GTP) from ADP (or GDP) using energy released from a high-energy substrate without involvement of the electron transport chain.

Examples with real data:

• Glycolysis:

  • Reaction: Phosphoenolpyruvate (PEP) → Pyruvate

  • Enzyme: Pyruvate kinase

  • ATP formed: 1 ATP per reaction (net 2 ATP per glucose from glycolysis)

• TCA Cycle:

  • Reaction: Succinyl‑CoA → Succinate

  • Enzyme: Succinyl‑CoA synthetase

  • GTP formed: 1 GTP per cycle (equivalent to ATP)

🔹 Key point: It occurs in the cytosol (glycolysis) and mitochondrial matrix (TCA cycle) and is independent of oxidative phosphorylation.

Short Answer:

It is the direct formation of ATP (or GTP) from ADP (or GDP) using energy from a high‑energy substrate, without the electron transport chain.
Example: In glycolysis, phosphoenolpyruvate → pyruvate produces ATP.