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. The CO(2)/O(2) specificity factor (tau) is 39. Heterohexadecamer of 8 large chains and 8 small chains (PubMed:9882445, PubMed:17574029, PubMed:7922027); disulfide-linked (PubMed:8245022). The disulfide link is formed within the large subunit homodimers (PubMed:8245022, PubMed:7922027, PubMed:17574029) (Probable). The exposed C-terminus binds in a cleft in the RbcX2 (shown with endogenous and Anabaena strain CA protein) (PubMed:20075914, PubMed:21765418). RbcX2 is displaced by RbcS; as RbcX2 is removed RbcS mediates the ordering of an internal RbcL loop (Thr-64-Leu-70) in a catalytically active conformation (PubMed:17574029, PubMed:21765418) (Probable). In the carboxysome RuBisCO is organized into a paracrystalline array (By similarity). This cyanobacterium makes beta-type carboxysomes (Probable). Binding of RbcS probably induces a conformation change that displaces RbcX2 and triggers the final mature conformation. 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. Monomers require chaperone RbcX2 to assemble into RbcL8 octamers yielding RbcL8-(RbcX2)8 (PubMed:17574029) (Probable). RbcX2 is displaced by RbcS; as RbcX2 is removed RbcS mediates the ordering of an internal RbcL loop (Thr-64-Leu-70) in a catalytically active conformation (PubMed:17574029, PubMed:21765418) (Probable). 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.