Carboxysome
The resulting concentration of carbon dioxide near RuBisCO decreases the proportion of ribulose-1,5-bisphosphate oxygenation and thereby avoids costly photorespiratory reactions.
Carbon is primarily stored in the form of HCO3− which cannot re-cross the lipid membrane, as opposed to neutral CO2 which can easily escape the cell.
This stockpiles carbon in the cell, creating a disequilibrium between the intracellular and extracellular environments of about 30x the Ci concentration in water.
Carboxysomes are the best studied example of bacterial microcompartments, the term for functionally diverse organelles that are alike in having a protein shell.
[11] These were later observed in other cyanobacteria[12] and in some chemotrophic bacteria that fix carbon dioxide—many of them are sulfur oxidizers or nitrogen fixers (for example, Halothiobacillus, Acidithiobacillus, Nitrobacter and Nitrococcus; all belonging to Pseudomonadota).
[14] The authors proposed that since these appeared to be organelles involved in carbon dioxide fixation, they should be called carboxysomes.
Electron cryo-tomography studies[15][16][17] have confirmed the approximately icosahedral geometry of the carboxysome, and have imaged Rubisco proteins inside arranged in a few concentric layers or fibril-like structures.
[15][17][18] The non-icosahedral faceted shapes of some carboxysomes can naturally be explained within the elastic theory of heterogeneous thin shells.
Such an air-lock, in contrast to BMC-H proteins with constitutively open pores, has been suggested to serve as a route for larger substrates (ribulose-1,5-bisphosphate) and products (3-phosphoglycerate) that must cross the shell.
CsoS2 has also been shown to bind to shell proteins via its 7 Middle Region (MR) repeats and C-terminal domain (CTD).
CsoS4A/B were the first BMC-P proteins to be experimentally demonstrated as minor components of the BMC shell[4] (only 12 pentamers are required to cap the vertices of an icosahedron).
CsoS1D is the first BMC-T which has been structurally characterized; it is also the first example of dimerization of two BMC building blocks in a face-to-face fashion to create a tiny cage.
[55][56] Once the procarboxysome (the carboxysome core) is formed, the N-terminus of the adapter protein CcmN interacts with the N-terminus of CcmM, while the C-terminus of CcmN recruits the shell proteins CcmK (BMC-H) and CcmO (BMC-T), utilizing a 15-20 amino acids long peptide.
[51][35] The final step is the addition of the vertices formed by the BMC-P protein CcmL, which then cap the enzymatic core and facets.
[57] As is the case with other BMCs, the carboxysome is attracting significant attention by researchers for applications in plant synthetic biology.
[61] The introduction of carboxysomes into plant chloroplasts as part of a CO2 concentrating mechanism [62][63] such as that found in cyanobacteria is predicted to significantly improve net CO2 fixation and yield.
[64][65] Expression of beta-carboxysomal shell proteins [66] and Form IB Rubisco-CcmM complexes in tobacco chloroplasts has been achieved,[67] but did not result in compartments containing RuBisCO.
A further advance has been the construction of minimal alpha-carboxysomes containing Form IA Rubisco and the CsoS1A and CsoS2 proteins from the cyanobacterium Cyanobium PCC7001 in tobacco chloroplasts.
