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 has a much greater affinity for C02 than for 02. It is the relative concentration of 02 and C02 that determines which of the two will bind to the enzyme.
In C4-plants some Oz does bind to RuBisCO and hence, C02 fixation is decreased. Here the RuBP instead of being converted to two molecules of PGA binds with Oz to form one molecule and phosphoglycolate in a pathway called photorespiration. In the photorespiratory pathway, there is neither synthesis of sugars, nor of ATP, Rather it results in the release of C02 with the utilisation of ATP. In the photorespiratory pathway, there is no synthesis of ATP or NADPH. Therefore, photorespiration is a wasteful process.
In C4-plants, photorespiration does not occur. This is because they have a mechanism that increases the concentration of C02 at the enzyme site. This takes place when the C4 acid from the mesophyll is broken down in the bundle cells to release C02, this results in increasing the intracellular concentration of C02. In turn, this ensures that the RuBisCO functions as a '
carboxylase minimising the oxygenase activity.