Phosphoenolpyruvat

Phosphoenolpyruvat (PEP) is a high-energy, three-carbon organophosphate that functions as a key intermediate in central carbon metabolism. It contains a phosphate group attached to the enol form of pyruvate, giving rise to a highly energetic phosphoenol bond. In glycolysis, PEP is produced from 2-phosphoglycerate by the enzyme enolase and is converted to pyruvat by pyruvat kinase, generating one molecule of ATP per PEP. In gluconeogenesis, PEP can be formed from oxaloacetate by PEP carboxykinase, a reaction that uses a nucleoside triphosphate and releases CO2. Some organisms, such as C4 plants, also generate PEP via pyruvate phosphate dikinase in alternative reactions.

Phosphoenolvat participates in several other metabolic processes. It serves as the phosphate donor in the bacterial and plant phosphotransferase system (PTS), where sugars are transported and phosphorylated during uptake. It also feeds the shikimate pathway, the biosynthetic route to aromatic amino acids, through condensation with erythrose-4-phosphate to form DAHP. In many plants and microbes, PEP carboxylase fixes CO2 by converting PEP and CO2 to oxaloacetate, a reaction important in C4 and CAM photosynthesis and in maintaining the oxaloacetate pool for the TCA cycle.

Chemically, PEP is reactive due to its enol phosphate structure and tends to hydrolyze or isomerize under physiological conditions. Its cellular concentration is governed by flux through glycolysis, gluconeogenesis, and anaplerotic CO2 fixation, with its utilization by PK and by alternative pathways shifting in response to cellular energy status and metabolic demand.