Ribulose-1,5-bisphosphate carboxylase oxygenase, better known as RuBisCO, is an enzyme that catalyzes the first major step of carbon fixation in the Calvin cycle. Carbon fixation is a process by which the atoms of atmospheric carbon dioxide are made available to organisms in the form of energy-rich molecules such as glucose. RuBisCO splits 6-C molecules into two equal parts.