Phosphoenolpyruvatcarboxylase

Phosphoenolpyruvate (PEP) is a high-energy, three-carbon metabolite that functions as an essential intermediate in carbohydrate metabolism. It is the enol phosphate form of pyruvate, consisting of a three-carbon skeleton bearing a phosphate group on the enol moiety. PEP is found in the cytosol of cells and serves as a key energy donor in metabolism.

In glycolysis, PEP is formed from 2-phosphoglycerate by the enzyme enolase, with the elimination of water. Pyruvate kinase then transfers a phosphate group from PEP to ADP, generating ATP and producing pyruvate. The hydrolysis of PEP releases a large amount of free energy, making this step one of the major ATP-generating reactions in glycolysis.

In gluconeogenesis, PEP is produced from oxaloacetate by phosphoenolpyruvate carboxykinase (PEPCK), using GTP as a phosphate donor. This bypasses the pyruvate kinase step, enabling the synthesis of glucose from non-carbohydrate precursors. In plants, PEP is also synthesized by phosphoenolpyruvate carboxylase (PEPC) in C4 and CAM photosynthesis, where it is fixed into oxaloacetate as part of carbon concentration mechanisms.

PEP serves as a substrate for various biosynthetic pathways, and its cellular concentration is tightly regulated by the balance of glycolysis and gluconeogenesis, as well as by the cell’s energy state. Its proper regulation is important for efficient energy production and carbohydrate metabolism across diverse organisms.