Review
Advances in the bacterial organelles for CO2 fixation

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Highlights

  • CO2-concentrating mechanisms (CCMs) provide a means for accumulating CO2 around Rubisco to overcome the inherent limitations of Rubisco and enhance CO2 fixation.

  • Carboxysomes are proteinaceous organelles in cyanobacteria and some proteobacteria which serve as the central CO2-fixing factory of CCMs.

  • Carboxysomes sequester the cellular Rubisco and carbonic anhydrase from the cytoplasm, using a selectively permeable shell that structurally resembles virus capsids.

  • Great efforts have been made recently to advance our understanding of the molecular mechanisms underlying carboxysome structure, assembly, biogenesis, and physiology.

  • Advances in fundamental knowledge about carboxysome assembly and function has stimulated rational design and engineering of the protein organelles for improving CO2 fixation and new functions.

Carboxysomes are a family of bacterial microcompartments (BMCs), present in all cyanobacteria and some proteobacteria, which encapsulate the primary CO2-fixing enzyme, Rubisco, within a virus-like polyhedral protein shell. Carboxysomes provide significantly elevated levels of CO2 around Rubisco to maximize carboxylation and reduce wasteful photorespiration, thus functioning as the central CO2-fixation organelles of bacterial CO2-concentration mechanisms. Their intriguing architectural features allow carboxysomes to make a vast contribution to carbon assimilation on a global scale. In this review, we discuss recent research progress that provides new insights into the mechanisms of how carboxysomes are assembled and functionally maintained in bacteria and recent advances in synthetic biology to repurpose the metabolic module in diverse applications.

Section snippets

Bacterial CCMs

The cyanobacterial CCM system is typically composed of three bicarbonate transporters (BicA, SbtA, BCT1) in the plasma membrane, two CO2-uptake complexes (NDH-I3, NDH-I4) in thylakoid membranes, as well as the CO2-fixing organelles distributed in the cytoplasm, known as the carboxysomes (Figure 1) [5]. The bicarbonate transporters pump external HCO3 into the cell and CO2-uptake complexes convert intracellular CO2 to HCO3 that is less membrane permeable than CO2, collectively generating an

Carboxysomes – the CO2-fixing BMCs

Organelle formation and compartmentalization provide the structural basis for the physiological optimization and regulation of metabolic functions and efficiency in cells [16., 17., 18.]. Over the past decade, numerous studies have characterized the specific organelles widespread in the bacterial kingdom, BMCs, which play important roles in CO2 fixation, pathogenesis, and microbial ecology [6,19., 20., 21., 22.]. BMCs are membrane-free organelles composed purely of proteins and form a

Carboxysome biogenesis and positioning

Carboxysome biogenesis, structure, and function are actively regulated by the metabolism of their native hosts. The biogenesis pathways of α- and β-carboxysomes likely differ (Figure 4). Recent studies documented that de novo assembly of β-carboxysomes exploits the 'inside-out' model [87,89]. Rubisco enzymes first condense, mediated by CcmM, to form a liquid-like Rubisco core [60,61]. The recruitment protein CcmN interacts with CcmM by its N terminus and shell proteins through its C-terminal

Repurposing carboxysomes for improving carbon fixation and new functions

The intrinsic self-assembly and modular characteristics of carboxysomes and their significance in enhancing CO2 fixation make them a promising engineering objective. To date, great efforts have been made to engineer α- and β-carboxysome structures in various heterologous organisms. The α-carboxysome-like structures have been reconstituted in E. coli and a Gram-positive bacterium by expressing the cso operon from H. neapolitanus [101., 102., 103.]. Moreover, expressing the multiple operons

Concluding remarks

Systematic studies on carboxysome biosynthesis, structure, and function offer mechanistic insights into the building principles and evolutionary diversity of bacterial organelles for CO2 fixation and microbial ecology. Advanced knowledge about the self-assembly and modularity of carboxysomes and the development of synthetic biology techniques provide a framework for sustainably engineering these carbon-fixation factories or pathways in other organisms, and will foster further biotechnological

Acknowledgments

We acknowledge that we could not include and cite many other important papers in this review due to space constraints. This work was supported by the Royal Society (URF\R\180030, RGF\EA\181061, RGF\EA\180233), the Biotechnology and Biological Sciences Research Council Grant (BB/V009729/1, BB/M024202/1, BB/R003890/1), the National Natural Science Foundation of China (32070109), and the Leverhulme Trust (RPG-2021-286).

Declaration of interests

There are no interests to declare.

Glossary

Anoxygenic phototrophs
organisms that can grow using energy from solar light and perform photosynthesis without evolving oxygen.
Bacterial microcompartments (BMCs)
proteinaceous megadalton-complexes that encapsulate metabolic pathways within the subcellular 'microfactories' using a virus-like protein shell to enhance enzymatic functions.
Carboxylation
a chemical process that is catalysed by Rubisco to fix CO2 to the five-carbon compound ribulose-1,5-bisphosphate (RuBP) and the splitting of the

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