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) (Ref.3, PubMed:18258595, PubMed:18974784). There are estimated to be 270 RuBisCO heterohexadecamers per carboxysome (Ref.6). Alpha-carboxysomes are able to assemble in the absence of RuBisCO, unlike beta-carboxysomes. The RuBisCO large subunit is required for enzyme integration into carboxysomes; replacing it with the carboxysomally targeted gene (Tcr_0838, AC Q31HD9) of H.crungenus places RuBisCO in the carboxysome, while the non-carboxysomal large subunit of H.crungenus (Tcr_0427, AC Q31IK0) is not incorporated in the carboxysome. 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. For enzyme freed from the carboxysome. Heterohexadecamer of 8 large chains and 8 small chains (By similarity) (Ref.14). Forms a CsoS2-CsoS1-RuBisCO complex (Probable). The N-terminus (residues 1-136) interacts with shell proteins CsoS1A, CsoS1B and CsoS1C (PubMed:30305640). Holo-RuBisCO interacts with the N-terminal repeats of CsoS2; binding is sensitive to ionic strength. A fusion of a single N-terminal repeat to the C-terminus of the large subunit of RuBisCO (cbbL) shows the repeat can lie between a CbbL dimer, making minor contacts to CbbS; thus each RuBisCO holoenzyme could bind 8 repeats (PubMed:32123388). When the major carboxysomal shell protein CsoS1A is disrupted most RuBisCO is found in the soluble (cytoplasmic) fraction (PubMed:16535117). Most protein is found in the carboxysome interior and not associated with the shell (PubMed:14729686). This bacterium makes alpha-type carboxysomes (Ref.6, PubMed:18258595). Produced when grown in air or in air supplemented with 5% CO(2) (at protein level). Part of the cbbL-cbbS operon. The N-terminus (minimally 1-122, fragment 1-136 does not bind any better) interacts with the shell proteins CsoS1A, CsoS1B and CsoS1C. A slightly longer fragment of the N-terminus (residues 1-136) is required for targeting to carboxysomes, and will direct foreign proteins to the carboxysome, with reduced efficiency compared to the whole protein. Does not grow in air, grows poorly in air supplemented with 5% CO(2), a high-CO(2) requiring phenotype (hcr), has apolar effect on expression of downstream cbbS. Expresses type II but not type I RuBisCO, does not form carboxysomes but instead smaller, empty cytoplasmic inclusions (PubMed:9696760). A double ccbL-ccbS deletion is hcr and forms empty but otherwise normal appearing carboxysomes (PubMed:18974784). Required for growth in ambient air (PubMed:31406332). Expression of 10 genes for alpha-carboxysome (Cb) proteins (cbbL-cbbS-csoS2-csoS3-csoS4A-csoS4B-csoS1C-csoS1A-csoS1B-csoS1D) in E.coli generates compartments that resemble Cb, contain RuBisCO and have its catalytic activity, showing it is possible to make artificial, functional Cb using these 10 genes. Cargo proteins can be targeted to these organelles (PubMed:22184212, PubMed:30305640). Alpha-carboxysomes are assembled in the complete absence of this enzyme in this bacteria, suggesting carboxysome-based microcompartments can be designed for applications such as custom chemical reactors or delivery vehicles (PubMed:18974784). When this gene is expressed in S.elongatus PCC 7942 it colocalizes with its beta-carboxysomes, suggesting there a range of modular possibilites for the carboxysome constituent proteins. These experiments open the door to generating carboxysomes in plant cells to increase their photosynthesis and productivity, as well as tailoring bacterial microcompartments to specific metabolic needs and molecule delivery (PubMed:29922315). 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.