Phosphoenolpyruvate carboxylase (also known as PEP carboxylase, PEPCase, or PEPC; EC 126.96.36.199) is an enzyme in the family of carboxy-lyases that catalyzes the addition of bicarbonate to phosphoenolpyruvate (PEP) to form the four-carbon compound oxaloacetate:
PEP + HCO3- → oxaloacetate + Pi
This reaction is used for carbon fixation in so-called "CAM" and "C4" plants where it plays a key role in photosynthesis. The enzyme is also found in some bacteria, but not in animals or fungi.
PEP carboxylase in photosynthesis
After conversion of CO2 to bicarbonate by carbonic anhydrase, PEP carboxylase assimilates the available bicarbonate into a four-carbon compound (oxaloacetate, which is further converted to malate) that can be stored or shuttled between plant cells. This allows for a separation of initial carbon fixation by contact with air and secondary carbon fixation into sugars by RuBisCO during the light-independent reactions of photosynthesis.
In succulent CAM plants adapted for growth in very dry conditions, PEP carboxylase fixes bicarbonate during the night when the plant opens its stomata to allow for gas exchange. During the day time, the plant closes the stomata to preserve water and releases CO2 inside the leaf from the storage compounds produced during the night. This allows the plants to thrive in dry climates by conducting photosynthesis without losing water through open stomata during the day.
In C4 plants, for example maize, PEP carboxylase fixes bicarbonate in the mesophyll cells of the leaf and the resulting four-carbon compound, malate, is shuttled into the bundle sheath cells where it releases CO2 for fixation by RuBisCO. Thus, the two processes are separated spatially, allowing for RuBisCO to operate in a low-oxygen environment to circumvent photorespiration. Photorespiration occurs due to the inherent oxygenase activity of RuBisCO in which the enzyme uses oxygen instead of carbon dioxide without incorporating carbon into sugars or generating ATP. As such, it is a wasteful reaction for the plant. By comparison, C4 carbon fixation via PEP carboxylase is more efficient.