Role of carboxysomes in cyanobacterial CO₂ assimilation: CO₂ concentrating mechanisms and metabolon implications

Abstract

Many carbon-fixing organisms have evolved CO 2 concentrating mechanisms (CCMs) to enhance the delivery of CO 2 to RuBisCO, while minimizing reactions with the competitive inhibitor, molecular O 2 . These distinct types of CCMs have been extensively studied using genetics, biochemistry, cell imaging, mass spectrometry, and metabolic flux analysis. Highlighted in this paper, the cyanobacterial CCM features a bacterial microcompartment (BMC) called 'carboxysome' in which RuBisCO is co-encapsulated with the enzyme carbonic anhydrase (CA) within a semi-permeable protein shell. The cyanobacterial CCM is capable of increasing CO 2 around RuBisCO, leading to one of the most efficient processes known for fixing ambient CO 2 . The carboxysome life cycle is dynamic and creates a unique subcellular environment that promotes activity of the Calvin-Benson (CB) cycle. The carboxysome may function within a larger cellular metabolon, physical association of functionally coupled proteins, to enhance metabolite channelling and carbon flux. In light of CCMs, synthetic biology approaches have been used to improve enzyme complex for CO 2 fixations. Research on CCMassociated metabolons has also inspired biologists to engineer multi-step pathways by providing anchoring points for enzyme cascades to channel intermediate metabolites towards valuable products.

Adapting from Low to High: An Update to CO2-Concentrating Mechanisms of Cyanobacteria and Microalgae

“…The Rubisco-containing microcompartments in cyanobacteria are called carboxysomes. Carboxysomes are polyhedral protein microbodies, usually of 100–400 nm in diameter, located in the cytoplasm and covered by a single-layer protein shell [ 173 , 174 , 175 , 176 , 177 , 178 ]. On the transmission electron microscopy images, carboxysomes appear as polyhedral electron-dense inclusions in the cytoplasm ( Figure 5 a).…”

The challenge of engineering Rubisco for improving photosynthesis

“…To implement such systems in plants, chloroplasts can serve as an additional strategy to improve Rubisco enzymatic performance. The CCM of cyanobacteria is a microcompartment called the carboxysome, which is filled with Rubisco and is surrounded by a semi‐permeable proteinaceous shell [25]. In algae the CCM microcompartment is called a pyrenoid, in which Rubisco is enclosed by a starch sheath [26].…”

Elucidating the Role of Primary and Secondary Sphere Zn2+Ligands in the Cyanobacterial CO2Uptake Complex NDH-14: The Essentiality of Arginine in Zinc Coordination and Catalysis

“…This led to the continuous dissipation of the HCO3pool due to the rapid equilibration between bicarbonate and CO2 and the consequent high-flux leakage of CO2 from the cell. High cytoplasmic HCO3is used by the other major part of the cyanobacterial CCM, namely the carboxysome [11][12][13][14][15]. The carboxysome contains the entire cellular complement of Rubisco, along with a CA.…”