PEP carboxylase, or phosphoenolpyruvate carboxylase, is an enzyme involved in the process of carbon dioxide (CO2) fixation in plants. It plays a crucial role in both C3 and C4 plants, but it is especially important in C4 plants due to their unique carbon fixation pathway.

1. CO2 Fixation in C3 and C4 Plants:
- C3 plants are the majority of plant species and include crops like wheat, rice, and soybeans. They fix CO2 directly into a three-carbon compound called 3-phosphoglycerate (3-PGA) using an enzyme called RuBisCO.
- C4 plants, on the other hand, have an additional carbon fixation pathway that helps them adapt to hot and dry conditions. They first fix CO2 into a four-carbon compound called oxaloacetate in the mesophyll cells using PEP carboxylase. This compound is then transported to bundle sheath cells, where it releases CO2 for fixation by RuBisCO.

2. Catalysis of CO2 Fixation in Bundle Sheath Cells:
- PEP carboxylase catalyzes the reaction that fixes CO2 into pyruvic acid in bundle sheath cells of C4 plants.
- The enzyme adds CO2 to phosphoenolpyruvate (PEP) to form oxaloacetate, a four-carbon compound. This reaction is known as carboxylation.
- Oxaloacetate is subsequently reduced to malate or converted to other four-carbon acids, which are then decarboxylated to release CO2 in the bundle sheath cells where RuBisCO is present.

3. Efficiency at Lower Atmospheric CO2 Concentration:
- PEP carboxylase has a higher affinity for CO2 than RuBisCO. This means that it can fix CO2 more efficiently at lower atmospheric CO2 concentrations.
- In environments where CO2 levels are low, such as hot and dry conditions, C4 plants have an advantage over C3 plants because PEP carboxylase can effectively capture CO2 even when it is scarce.
- This allows C4 plants to minimize water loss through stomata by concentrating CO2 in the bundle sheath cells and reducing the frequency of stomatal opening.

4. Abundance:
- While PEP carboxylase is a crucial enzyme in carbon fixation, it is not the most abundant enzyme in the world.
- The most abundant enzyme in the world is RuBisCO, which is found in all photosynthetic organisms and plays a central role in the Calvin cycle, the main pathway of carbon fixation in C3 plants.

In conclusion, PEP carboxylase is involved in CO2 fixation in both C3 and C4 plants, but it is particularly important in C4 plants due to its role in the initial fixation of CO2 in bundle sheath cells. It is more efficient at fixing CO2 at lower atmospheric concentrations, allowing C4 plants to thrive in hot and dry conditions. However, it is not the most abundant enzyme in the world; that distinction belongs to RuBisCO.