RuBisCO catalyzes two reactions: the carboxylation of D-ribulose 1,5-bisphosphate, the primary event in carbon dioxide fixation, as well as the oxidative fragmentation of the pentose substrate in the photorespiration process. Both reactions occur simultaneously and in competition at the same active site (Probable). Binds to abscisic acid (ABA) (PubMed:26197050). 2 (2R)-3-phosphoglycerate + 2 H(+) = CO2 + D-ribulose 1,5-bisphosphate + H2O D-ribulose 1,5-bisphosphate + O2 = (2R)-3-phosphoglycerate + 2-phosphoglycolate + 2 H(+) Binds 1 Mg(2+) ion per subunit. Heterohexadecamer of 8 large chains and 8 small chains; disulfide-linked. The disulfide link is formed within the large subunit homodimers (PubMed:29372894). Interacts with RBCX1 and RBCX1 (PubMed:21922322). An intermediate complex made of eight RbcL subunits interacts with the chaperone BSD2 (PubMed:29217567). The disulfide bond which can form in the large chain dimeric partners within the hexadecamer appears to be associated with oxidative stress and protein turnover. The basic functional RuBisCO is composed of a large chain homodimer in a 'head-to-tail' conformation. In form I RuBisCO this homodimer is arranged in a barrel-like tetramer with the small subunits forming a tetrameric 'cap' on each end of the 'barrel'. Belongs to the RuBisCO large chain family. Type I subfamily.