Substrate-level Phosphorylation

In living cells, it occurs during glycolysis (in the cytoplasm) and the Krebs cycle (in the mitochondria). Thus, substrate-level phosphorylation takes place both in the presence and absence of oxygen during glycolysis. However, in the Krebs cycle, it happens strictly in an aerobic environment.

Here, substrate-level phosphorylation happens in two separate steps:

Step 1: First, a phosphoryl group gets transferred from 1, 3-bisphosphoglycerate to ADP, forming 3-phosphoglycerate (3PG) and ATP. This step is catalyzed by phosphoglycerate kinase (PGK), giving off 2 ATPs.

2 (1,3-bisphosphoglycerate) + 2 ADP → 2 (3-phosphoglycerate) + 2 ATP

Step 2: Second, a phosphoryl group is transferred from two phosphoenolpyruvate (PEP) molecules to ADP, forming two pyruvate molecules and ATPs. The enzyme pyruvate kinase catalyzes the reaction.

2 (Phosphoenolpyruvate) + 2 ADP → 2 Pyruvate + 2 ATP

Thus, in glycolysis, 4 ATPs are produced from 1 molecule of glucose.

The only step where substrate-level phosphorylation happens in the Krebs cycle is:

A phosphoryl group is transferred from succinyl-CoA to GDP, forming succinate and GTP. This step is catalyzed by the enzyme succinyl-CoA synthetase, producing 2 ATPs per glucose molecule (since the cycle runs twice and two pyruvate molecules enter the Krebs cycle).

• 2 Succinyl-CoA + 2 GDP → 2 Succinate +2 GTP

• Unlike oxidative phosphorylation, substrate-level phosphorylation and oxidation do not go hand in hand. It is a much quicker and more efficient source of ATP and GTP to the cell than oxidative phosphorylation during cellular respiration.
• As the process can occur under aerobic and anaerobic conditions, it is a vital energy-yielding process. It helps in the respiration of oxygen-depleted muscle cells, human RBCs that lack mitochondria, and even during the fermentative growth of microorganisms.