What is phosphoenolpyruvate (PEP) and its role as the primary CO₂ acceptor?

Phosphoenolpyruvate (PEP) is a three-carbon compound that acts as the primary CO₂ acceptor in the C4 pathway of photosynthesis. In C4 plants like maize and sugarcane, PEP is present in the mesophyll cells and plays a crucial role in efficient carbon fixation.

When CO₂ enters the mesophyll cells, the enzyme PEP carboxylase catalyzes the reaction between PEP and CO₂, forming oxaloacetate (OAA), a four-carbon compound. This step is the first stage of the C4 cycle (also called the Hatch and Slack pathway). The OAA is then converted into other four-carbon acids, such as malic acid or aspartic acid, which are transported to bundle sheath cells for further processing in the Calvin cycle.

Unlike C3 plants, where ribulose bisphosphate (RuBP) is the primary CO₂ acceptor, C4 plants use PEP to capture CO₂ even when its concentration is low, making photosynthesis more efficient under conditions of high light, temperature, and limited water. This adaptation helps C4 plants minimize photorespiration and thrive in hot, dry environments.