As shown in Figure 2 (left), RuBisCO is one of many enzymes in the Calvin cycle. Substrates. During carbon fixation, the substrate molecules for RuBisCO are ribulose-1,5-bisphosphate and carbon dioxide (distinct from the activating carbon dioxide). RuBisCO also catalyses a reaction between ribulose-1,5-bisphosphate and molecular oxygen (O