Pep Carboxylase

PEP carboxylase (or phosphoenolpyruvate carboxylase) is an enzyme that plays an important role in the C4 and CAM pathways of photosynthesis. It catalyzes the first step of carbon fixation in C4 and CAM plants, which involves the carboxylation of phosphoenolpyruvate (PEP) to accept CO2.

Mesophyll cells of C4 plants contain phosphoenolpyruvate (PEP) as a 3-carbon compound. PEPcase, an enzyme, is present in these cells and is responsible for converting PEP into OAA (oxaloacetate). On the other hand, RuBisCO is absent in the mesophyll cells of C4 plants.

Structure and Function

Two identical dimers, each consisting of two subunits, join together to form PEP carboxylase, which is present in plants and certain bacteria.

The main function of PEP carboxylase is to facilitate photosynthesis during carbon fixation in C4 and CAM pathways. It also plays a role in non-photosynthetic metabolic reactions, such as replenishing OAA in the Krebs Cycle. Furthermore, PEP carboxylase is involved in the conversion of PEP produced in glycolysis to OAA in certain bacteria and plants, and in the functioning of stomatal guard cells.

The reaction catalysed by PEP carboxylase is:

OAA + Pi → PEP + HCO3−

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 (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 and occurs in the mesophyll cells. RuBisCO is absent in the mesophyll cells.

The C4 acid is transported to bundle sheath cells, where it is decarboxylated to release CO2. This CO2 then enters the Calvin cycle in the bundle sheath cell. These cells are rich in RuBisCO, the primary enzyme in the Calvin cycle. PEP carboxylase, however, is absent in bundle sheath cells.

C4 Pathway or Hatch and Slack Pathway

Therefore, C4 plants do not undergo photorespiration due to increased CO2 concentrations around RuBisCO stimulating its carboxylase activity and decreasing oxygenase activity.

Frequently Asked Questions

The Role of PEP Carboxylase in C4 and CAM Plants

PEP carboxylase plays an important role in C4 and CAM plants by catalyzing the first step of the C4 and CAM pathways. This enzyme helps to fix carbon dioxide from the atmosphere into a four-carbon acid, which is then used in photosynthesis. PEP carboxylase is also important for carbon partitioning in C4 and CAM plants, as it helps to ensure that the four-carbon acid is used for photosynthesis and not respiration.

PEP carboxylase is an essential enzyme for carbon fixation in C4 and CAM plants, as it is present in the mesophyll cells. It catalyses the initial carbon fixation, where PEP acts as an acceptor for CO2, and produces OAA.

PEP carboxylase is a type of carboxylase enzyme.

PEP carboxylase is an enzyme belonging to the lyases class of enzymes. Lyases catalyse the removal of a group by mechanisms other than hydrolysis, resulting in the formation of a double bond. PEP carboxylase is a carboxy-lyase, which catalyses the addition or removal of a carboxyl group.

Yes, PEP carboxylase is found in C3 plants.

PEP carboxylase is present in C3 plants, but not in large amounts. It is responsible for restoring citric acid cycle intermediates, as it produces OAA from PEP. Additionally, it plays a role in the functioning of guard cells of stomata. However, it is not involved in the photosynthesis process in C3 plants, unlike C4 and CAM plants.

Yes, PEP carboxylase does require ATP.

The formation of OAA by PEP carboxylase is an exergonic process, hence it does not require ATP. The ΔG° for the reaction is -30 kJ/mol.

Where is PEP carboxylase active?

In C4 plants, PEP carboxylase is active in mesophyll cells, catalyzing the synthesis of OAA from PEP during photosynthesis.

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