The enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase, most commonly known by the shorter name RuBisCO or just rubisco is used in the Calvin cycle to catalyze the first major step of carbon fixation.uBisCo has a molecular weight of 490,000 Daltons and is composed of eight large subunits and eight small subunits. The large subunits are coded in the chloroplast itself, while the small units are coded in the nucleus of the cell.