Which enzyme is utilized by C4 plants for carbon fixation?

C4 plants use PEP carboxylase for carbon fixation as their primary mechanism to capture carbon dioxide. This enzyme has a higher affinity for carbon dioxide than Rubisco and operates in the mesophyll cells of C4 plants. When CO2 is fixed by PEP carboxylase, it forms oxaloacetate, which is then converted to malate. This approach allows C4 plants to efficiently minimize photorespiration and maximize sugar production, especially in conditions where light intensity is high and CO2 concentrations can fluctuate.

In contrast, Rubisco is primarily associated with the Calvin cycle in C3 plants, while malate dehydrogenase plays a role in the conversion of malate back to pyruvate during the C4 pathway. Carbonic anhydrase facilitates the conversion of carbon dioxide to bicarbonate, but it does not play a direct role in the initial carbon fixation in the context of C4 photosynthesis. Thus, PEP carboxylase is specifically essential for the initial step of carbon fixation in C4 plants, making it the correct choice.