Substrate-Level Phosphorylation
Substrate-level phosphorylation 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 phosphorylation:
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 phosphorylation:
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
Substrate-level phosphorylation 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.
