What enzyme fixes CO₂ in the mesophyll cells of C4 plants?

The enzyme that fixes CO₂ in the mesophyll cells of C4 plants is PEP carboxylase.

PEP carboxylase catalyzes the attachment of bicarbonate, formed from CO₂, to phosphoenolpyruvate (PEP). The initial product is the four-carbon compound oxaloacetate, which gives C4 photosynthesis its name. Oxaloacetate is commonly converted into malate or aspartate before moving to bundle-sheath cells.

In the bundle sheath, the four-carbon compound is decarboxylated, releasing concentrated CO₂ near RuBisCO. RuBisCO then carries out the Calvin-cycle carbon fixation step. This division of labor helps C4 plants reduce photorespiration, especially in hot, bright, or dry environments.

RuBisCO is a tempting alternative, but it is generally responsible for the later fixation of concentrated CO₂ in bundle-sheath cells, not the initial mesophyll fixation. Carbonic anhydrase helps convert CO₂ and bicarbonate, but it does not perform the carbon-fixing reaction itself. The C4 pathway was elucidated in the 1960s by Marshall Hatch and C. Roger Slack.