RuBisCO is an enzyme that acts both as a carboxylase and oxygenase. Why do you think RuBisCO carries out more carboxylation in C4 plants?

RuBisCO (Ribulose-1,5-bisphosphate carboxylase/oxygenase) is a key enzyme involved in carbon fixation during photosynthesis. It catalyzes two competing reactions: carboxylation, where it adds carbon dioxide (CO2) to ribulose-1,5-bisphosphate (RuBP) to initiate the Calvin cycle, and oxygenation, where it adds oxygen (O2) to RuBP, leading to a process called photorespiration.

C4 plants have evolved an additional biochemical pathway to concentrate CO2 around RuBisCO, thereby minimizing the oxygenation reaction and enhancing the carboxylation reaction. This pathway involves spatial separation of initial CO2 fixation (carboxylation) and the Calvin cycle. In C4 plants, initial CO2 fixation occurs in mesophyll cells, where phosphoenolpyruvate carboxylase (PEP carboxylase) catalyzes the formation of oxaloacetate (a 4-carbon compound) from phosphoenolpyruvate and CO2. Oxaloacetate is then converted to malate or aspartate, which are transported to bundle sheath cells surrounding the leaf veins.

In bundle sheath cells, malate or aspartate is decarboxylated to release CO2, which is then used by RuBisCO for the Calvin cycle. Because CO2 concentration in the bundle sheath cells is much higher due to this concentration mechanism, RuBisCO has a higher chance of carboxylating RuBP rather than oxygenating it.

This spatial separation of initial CO2 fixation and Calvin cycle reactions, along with the CO2 concentration mechanism, allows RuBisCO in C4 plants to predominantly carry out carboxylation rather than oxygenation, leading to higher photosynthetic efficiency, especially in environments with high temperatures and/or low CO2 concentrations.