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 (Probable). In the carboxysome RuBisCO is organized into a paracrystalline array (By similarity). This cyanobacterium makes beta-type carboxysomes (Probable). 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 the chaperonin GroEL-GroES. Requires homodimeric RuBisCO chaperone RbcX2 to assemble into RbcL8 octamers, making RbcL8-(RbcX2)8. The exposed C-terminus of RbcL binds in a cleft in RbcX2 (PubMed:17574029, PubMed:20075914). RbcX2 is displaced by RbcS; as RbcX2 is removed RbcS mediates the ordering of an internal RbcL loop (Thr-63-Leu-69) in a catalytically active conformation (By similarity). Interacts with accumulation factor Raf1; dimeric Raf1 acts after chaperonin GroEL-GroES, binding to RbcL(2) leading to an RbcL8-Raf1(8) complex. RbcS displaces Raf1, resulting in holoenzyme formation (PubMed:26237510). 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. Extended N-terminus.