szubsztrátfoszforiláció

Substrate-level phosphorylation is a metabolic process that generates adenosine triphosphate (ATP) by the direct transfer of a phosphate group from a phosphorylated substrate to adenosine diphosphate (ADP). This process occurs in the cytoplasm during glycolysis and in the mitochondrial matrix during the citric acid cycle. Unlike oxidative phosphorylation, which involves a series of redox reactions and an electron transport chain, substrate-level phosphorylation is a simpler enzymatic reaction.

In glycolysis, net ATP production occurs through substrate-level phosphorylation at two key steps. The enzyme phosphoglycerate kinase catalyzes the transfer of a phosphate group from 1,3-bisphosphoglycerate to ADP, forming ATP and 3-phosphoglycerate. Later, pyruvate kinase catalyzes the transfer of a phosphate group from phosphoenolpyruvate to ADP, generating ATP and pyruvate. These reactions are crucial for energy production in anaerobic respiration and the initial stages of aerobic respiration.

During the citric acid cycle, substrate-level phosphorylation occurs once per cycle. The enzyme succinyl-CoA synthetase catalyzes the conversion of succinyl-CoA to succinate, with the simultaneous phosphorylation of GDP to GTP. GTP is energetically equivalent to ATP and can be readily converted to ATP by the enzyme nucleoside-diphosphate kinase. This single ATP-generating step within the citric acid cycle contributes to the overall energy yield of cellular respiration. Substrate-level phosphorylation provides a direct and efficient way to produce ATP, particularly when oxygen is limited.