PEPcarboxylase

PEPcarboxylase, formally known as phosphoenolpyruvate carboxylase (PEPC), is a cytosolic enzyme that catalyzes the carboxylation of phosphoenolpyruvate (PEP) with carbon dioxide to form oxaloacetate and inorganic phosphate. The reaction can be written as PEP + CO2 + H2O → oxaloacetate + Pi, and in many organisms CO2 is supplied by the bicarbonate–carbon dioxide equilibrium. PEPC is a homotetramer in most plants and bacteria, with subunits around 100–110 kDa; plant PEPC genes encode multiple isoforms that are differentially expressed in tissues and developmental stages.

In C4 and CAM plants, PEPC fixes CO2 in the cytosol of mesophyll cells, producing oxaloacetate that is rapidly converted to malate and transported to other cells or compartments. In C4 leaves, malate is decarboxylated to release CO2 for Rubisco in bundle sheath cells, concentrating carbon and reducing photorespiration. In CAM plants, nocturnal PEPC fixes CO2 into malate stored in vacuoles, which is decarboxylated during the day to supply CO2 for the Calvin cycle.

PEPC activity is regulated allosterically and by phosphorylation. Bicarbonate and Mg2+ act as activators; malate and aspartate are common inhibitors. In plants, phosphorylation of a conserved N-terminal serine by PEPC kinase increases catalytic efficiency, lowers sensitivity to inhibitors, and aligns enzyme activity with light conditions.

Beyond photosynthesis, PEPC participates in anaplerotic supply of oxaloacetate for amino acid biosynthesis and other metabolic pathways in bacteria, algae, and non-C4 plants. Its activity contributes to metabolic flexibility under stress and varying CO2 conditions.