Substratnivåfosforylering

Substratnivåfosforylering, or substrate-level phosphorylation in English, is a metabolic process that forms ATP (or GTP) directly by transferring a phosphate group from a high-energy substrate to ADP. This pathway does not rely on the electron transport chain or a proton motive force; instead, it yields ATP through direct phosphate transfer in specific enzymatic steps.

In glycolysis, two enzymes catalyze substrate-level phosphorylation. Phosphoglycerate kinase converts 1,3-bisphosphoglycerate to 3-phosphoglycerate, and pyruvate kinase converts phosphoenolpyruvate to pyruvate, each transferring a phosphate to ADP and producing ATP. In the mitochondrial-associated citric acid cycle, succinyl-CoA synthetase catalyzes the conversion of succinyl-CoA to succinate, generating GTP (which can be readily converted to ATP). These reactions provide ATP without the involvement of the mitochondrial membrane potential or electron transport.

The process contrasts with oxidative phosphorylation, which depends on a proton gradient established by the electron transport chain to drive ATP synthase. Substratnivåfosforylering can be especially important under anaerobic conditions or when rapid ATP production is needed, such as during high glycolytic flux, where oxidative phosphorylation is limited or unavailable. It also occurs in certain bacteria and plants, contributing to their energy economy during specific metabolic states.

Overall, substrate-level phosphorylation represents a direct and rapid means of ATP generation tied to specific catabolic steps, complementing the broader energy yield achieved through oxidative phosphorylation in cellular respiration.