Substrate-level phosphorylation is a metabolic process that generates adenosine triphosphate (ATP) through the direct transfer of a phosphate group to adenosine diphosphate (ADP) from a phosphorylated substrate, without the involvement of a proton gradient or electron transport chain (which is characteristic of oxidative phosphorylation).
The two main central metabolic pathways where substrate-level phosphorylation occurs are:
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Glycolysis: This pathway, which occurs in the cytoplasm of cells, breaks down glucose into pyruvate. During glycolysis, ATP is produced at two key steps:
- The conversion of 1,3-bisphosphoglycerate to 3-phosphoglycerate by phosphoglycerate kinase.
- The conversion of phosphoenolpyruvate (PEP) to pyruvate by pyruvate kinase.
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Citric Acid Cycle (Krebs Cycle): This occurs in the mitochondrial matrix and is responsible for further oxidizing products from glycolysis. Substrate-level phosphorylation occurs during the conversion of succinyl-CoA to succinate, catalyzed by succinyl-CoA synthetase, which directly generates ATP (or GTP, depending on the organism).
In both pathways, the phosphate group is transferred from a high-energy substrate directly to ADP, resulting in the production of ATP, and this is contrasted with oxidative phosphorylation, where ATP is produced through a chemiosmotic coupling process involving the electron transport chain.