Phosphoenolpyruvate carboxylase

Overview

Phosphoenolpyruvate carboxylase (PEPC) is an enzyme in the family of carboxy-lyases found in plants and some bacteria. It catalyzes the addition of bicarbonate (HCO3-) to phosphoenolpyruvate (PEP) to form the four-carbon compound oxaloacetate and inorganic phosphate. This is a key step in the process of photosynthesis.

Structure

PEPC is a homotetramer, meaning it is composed of four identical subunits. Each subunit has a molecular weight of approximately 110 kDa and contains a single active site. The enzyme is typically found in the cytosol of plant cells, where it plays a crucial role in carbon fixation.

Function

The primary function of PEPC is in the initial process of carbon fixation in the C4 photosynthetic pathway. This pathway is named for the four-carbon compound oxaloacetate that is first formed in the process. The enzyme also plays a role in the Crassulacean Acid Metabolism (CAM) pathway, which is a water-conserving, night-time CO2 fixation process used by some plants.

Regulation

PEPC is regulated by several mechanisms. It is activated by glucose-6-phosphate and inhibited by malate. The enzyme is also regulated by phosphorylation; a serine residue near the N-terminus of the enzyme can be phosphorylated by PEPC-kinase, which increases the enzyme's activity.

Clinical Significance

In humans, PEPC is not normally expressed. However, it has been found to be upregulated in some cancers, including liver, lung, and breast cancers. This suggests that the enzyme may play a role in the metabolic reprogramming that occurs in cancer cells.

See Also

• C4 photosynthesis
• Crassulacean Acid Metabolism
• Enzyme Kinetics