Which enzyme catalyzes the initial CO2 fixation in C4 plants?

Which enzyme catalyzes the initial CO2 fixation in C4 plants?

• A. RuBisCO
• B. NADP+-malate dehydrogenase
• C. PEP carboxylase
• D. Carbonic anhydrase

Answer: (C)

In C4 plants the first CO2 fixation is carried out by an enzyme that fixes bicarbonate (HCO3-) onto phosphoenolpyruvate to form oxaloacetate in the mesophyll cells. This enzyme, PEP carboxylase, is highly efficient at capturing CO2 and, unlike RuBisCO, does not react with O2, so it minimizes photorespiration when CO2 levels are low inside the leaf. The oxaloacetate is then quickly converted to malate and shuttled to the bundle-sheath cells, where it is decarboxylated to release CO2 for RuBisCO to fix in the Calvin cycle. This arrangement concentrates CO2 around RuBisCO and keeps the initial fixation separate from the site of the Calvin cycle, which is why PEP carboxylase is the best answer. Carbonic anhydrase can help supply bicarbonate by interconverting CO2 and HCO3-, supporting the PEP carboxylase step, but it is not the enzyme doing the initial carboxylation itself. NADP+-malate dehydrogenase, meanwhile, converts oxaloacetate to malate, not the CO2 fixation step.

In C4 plants the first CO2 fixation is carried out by an enzyme that fixes bicarbonate (HCO3-) onto phosphoenolpyruvate to form oxaloacetate in the mesophyll cells. This enzyme, PEP carboxylase, is highly efficient at capturing CO2 and, unlike RuBisCO, does not react with O2, so it minimizes photorespiration when CO2 levels are low inside the leaf. The oxaloacetate is then quickly converted to malate and shuttled to the bundle-sheath cells, where it is decarboxylated to release CO2 for RuBisCO to fix in the Calvin cycle. This arrangement concentrates CO2 around RuBisCO and keeps the initial fixation separate from the site of the Calvin cycle, which is why PEP carboxylase is the best answer. Carbonic anhydrase can help supply bicarbonate by interconverting CO2 and HCO3-, supporting the PEP carboxylase step, but it is not the enzyme doing the initial carboxylation itself. NADP+-malate dehydrogenase, meanwhile, converts oxaloacetate to malate, not the CO2 fixation step.