A carboxy-lyase that plays a key role in photosynthetic carbon assimilation in the CALVIN-BENSON CYCLE by catalyzing the formation of 3-phosphoglycerate from ribulose 1,5-biphosphate and carbon dioxide. It can also utilize oxygen as a substrate to catalyze the synthesis of 2-phosphoglycolate and 3-phosphoglycerate in a process referred to as photorespiration.