PEP carboxylase or PEPcase (phosphoenolpyruvate carboxylase) is an enzyme involved in the C4 and CAM pathways of photosynthesis. It catalyses the first step of carbon fixation in C4 and CAM plants. PEP carboxylase catalyses the carboxylation of phosphoenolpyruvate (PEP), which is the primary acceptor of CO2 in the C4 pathway.
PEPcase is present in the mesophyll cells. Phosphoenolpyruvate (PEP) is a 3-carbon compound present in mesophyll cells. It acts as a primary carbon dioxide acceptor and is converted into OAA by the action of PEP carboxylase. The enzyme RuBisCO is absent in the mesophyll cells of C4 plants.
Structure and Function
PEP carboxylase is present in plants and certain bacteria. It consists of four subunits present as a dimer of dimers. Two identical subunits join together to form a dimer that is joined to another dimer.
The main function of PEP carboxylase is in photosynthesis during carbon fixation in C4 and CAM pathways. PEP carboxylase also takes part in non-photosynthetic metabolic reactions. It also replenishes OAA in the citric acid cycle or Krebs Cycle. Some of the PEP produced in the glycolysis is directly converted to OAA by PEP carboxylase in certain bacteria and plants. PEP carboxylase is also involved in the functioning stomatal guard cells.
The reaction catalysed by PEP carboxylase is as follows:
PEP + HCO3− → oxaloacetate (OAA) + Pi
Role of PEP Carboxylase in C4 Plants
The C4 pathway of carbon fixation is seen in plants adapted to dry and tropical regions. Here, phosphoenolpyruvate or PEP is the primary acceptor of CO2 and oxaloacetic acid (OAA) a C4 acid is the first carbon fixation product. The reaction is catalysed by PEP carboxylase. This occurs in the mesophyll cells. RuBisCO is absent in the mesophyll cells.
The C4 acid is then transported to bundle sheath cells, where it is decarboxylated to release CO2. The CO2 thus released enters the Calvin cycle in the bundle sheath cell. The bundle sheath cells are rich in RuBisCO, the primary Calvin cycle enzyme. Bundle sheath cells lack PEP carboxylase.
This ensures an increase in CO2 concentration around RuBisCO to stimulate its carboxylase activity and reduce oxygenase activity. Therefore, photorespiration does not occur in C4 plants.
Frequently Asked Questions
What is the role of PEP carboxylase in C4 and CAM plants?
PEP carboxylase is essential for carbon fixation in C4 and CAM plants. It is present in the mesophyll cells. It catalyses the initial carbon fixation where PEP acts as an acceptor for CO2, and produces OAA.
What type of enzyme is PEP carboxylase?
PEP carboxylase is an enzyme belonging to the lyases class of the enzyme. Lyases catalyse the removal of a group by other than the hydrolysis process, forming double bonds. PEP carboxylase is a carboxy-lyase, which catalyses addition or removal of a carboxyl group.
Is PEP carboxylase found in C3 plants?
PEP carboxylase is present in C3 plants but not in abundance. It is involved in replenishing of citric acid cycle intermediates, as it produces OAA from PEP. It is also involved in the functioning of guard cells of stomata. It is not involved in the photosynthesis process in C3 plants unlike C4 and CAM plants.
Does PEP carboxylase require ATP?
The formation of OAA by PEP carboxylase is an exergonic process, hence it does not require ATP. The △G° for reaction is -30 kJmol−1.
In which cells is PEP carboxylase active?
In C4 plants, PEP carboxylase is active in mesophyll cells, where it catalyses the synthesis of OAA from PEP during photosynthesis.
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