Plants that are classified as C4 are able to fix carbon in an alternate mechanism that produces a 4-carbon molecule. When carbon dioxide is added to phosphoenolpyruvate [fohs-foh-uh-nol-py-ROO-vayt] (PEP) to form the 4-C product oxaloacetate [ok-suh-loh-AS-uh-tayt], what is the name of the enzyme that catalyzes this reaction?