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. 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 (By similarity) (PubMed:32451445). Forms complexes of many stoichiometries with Raf1 with and without RbcS (PubMed:32451445). RuBisCO interacts with the C-terminus of CcmM (By similarity). This cyanobacterium makes beta-type carboxysomes (PubMed:24907906). In the carboxysome RuBisCO is organized into a paracrystalline array (By similarity). Part of the rbcL-rbcS operon, transcribed in light and constitutively during growth on fructose. 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. RuBisCO folding and assembly commences when the nascent large subunit folds with the help of chaperonin GroEL-GroES. Both RbcX and Raf1 help folded RbcL release from the chaperonin and dimerize; dimeric Raf1 binds to RbcL(2) leading to an RbcL8-Raf1(8) complex. RbcS displaces Raf1, resulting in holoenzyme formation. 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.