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 (PubMed:2928307) (Probable). Both reactions occur simultaneously and in competition at the same active site (Probable). Binds to abscisic acid (ABA) which has weakly inhibits carboxylation and more strongly inhibits enzyme activation (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 (PubMed:2118958, PubMed:8648644, PubMed:9092835). Ca(2+) can substitute but is not catalytically competent (PubMed:14596800, PubMed:9034362). Abscisic acid (ABA) causes weak inhibition of RuBisCO catalytic activity, but more potent inhibition of RuBisCO activation. Heterohexadecamer of 8 large chains and 8 small chains. The disulfide bond which can form between Cys-247 in the large chain dimeric partners within the hexadecamer appears to be associated with oxidative stress and protein turnover (By similarity). The disulfide bonds reported in 1RBO may be the result of oxidation during crystallization. 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. The Plant Kingdom's sloth - Issue 38 of September 2003