RuBisCO activase enzyme, molecular model. This is RuBisCO activase (Rca) from the tobacco plant (Nicotiana tabacum). RuBisCO stands for ribulose-1,5-bisphosphate carboxylase oxygenase. RuBisCO activase is required in some plants to activate the process by which the RuBisCO enzyme aids carbon fixation (such as photosynthesis). Hence this enzyme also plays an important role in photosynthesis, especially in plants kept in relative darkness. The structure of the activase enzyme is the hexamer shown here, with six subunits.