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 (PubMed:7922027, PubMed:9882445). The small subunit displaces RbcX2 from the RbcL8-(RbcX2)8 assembly intermediate, and stabilizes an RbcL loop ('Thr-64-Leu-70') in a catalytically competent conformation (PubMed:21765418). The CO(2)/O(2) specificity factor (tau) is 39. Heterohexadecamer of 8 large and 8 small subunits (PubMed:17574029, PubMed:21765418, PubMed:8245022, PubMed:7922027). Displaces RbcX2 from the RbcL8-(RbcX2)8 assembly intermediate (Probable) (PubMed:21765418). In the carboxysome RuBisCO is organized into a paracrystalline array (By similarity). This cyanobacterium makes beta-type carboxysomes (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 small chain family.