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. Both reactions occur simultaneously and in competition at the same active site (By similarity). Replacing the endogenous type I ccbLS genes in H.neapolitanus with this carboxysomally targeted enzyme reconstitutes RuBisCO with about 25% of normal activity; the active enzyme is targeted to carboxysomes (PubMed:18974784). 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. kcat is 0.27 sec(-1) for holoenzyme. Optimum pH is 8.0. Heterohexadecamer of 8 large chains and 8 small chains. Forms a CsoS2-CsoS1-RuBisCO complex (By similarity). This bacterium makes alpha-type carboxysomes. Induced by growth in low levels of dissolved inorganic carbon (at protein level). Encoded in a cso-type operon. 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.