PEPCK () is one of three decarboxylation enzymes used in the inorganic carbon concentrating mechanisms of C4 and CAM plants. The others are NADP-malic enzyme and NAD-malic enzyme. In C4 carbon fixation, carbon dioxide is first fixed by combination with phosphoenolpyruvate to form oxaloacetate in the mesophyll. In PEPCK-type C4 plants the oxaloacetate is then converted to aspartate, which travels to the bundle sheath. In the bundle sheath cells, aspartate is converted back to oxaloacetate. PEPCK decarboxylates the bundle sheath oxaloacetate, releasing carbon dioxide, which is then fixed by the enzyme Rubisco.