Bacterial microcompartment

Bacterial microcompartments (BMCs) are protein-based organelles found in a wide range of bacteria. They serve as specialized compartments for various metabolic processes, encapsulating specific enzymes and substrates to enhance reaction efficiency and protect the cell from toxic intermediates. BMCs are notable for their icosahedral or quasi-icosahedral symmetry and are composed of thousands of protein subunits. They are a key example of the complexity and adaptability of bacterial cells, illustrating how prokaryotes can compartmentalize functions in a manner somewhat analogous to the organelles of eukaryotic cells.

Structure and Function

The structure of BMCs is defined by a protein shell that encapsulates enzymes involved in a specific metabolic pathway. The shell is primarily made up of hexameric and pentameric proteins that form a facetted surface, with pores that allow the selective passage of substrates, products, and cofactors. The most well-studied BMCs are those involved in carbon fixation, the anabolism of ethanolamine, and the degradation of propanediol.

Carbon Fixation BMCs

In cyanobacteria and some chemolithoautotrophs, BMCs encapsulate enzymes of the Calvin cycle, enhancing the efficiency of carbon fixation by concentrating CO2 and excluding oxygen, which can interfere with the enzyme RuBisCO.

Ethanolamine Utilization

Certain pathogenic bacteria, such as Salmonella and Escherichia coli, contain BMCs that metabolize ethanolamine, using it as a nitrogen and carbon source. This process is crucial for their survival in the gut, where ethanolamine is available from the breakdown of phospholipids.

Propanediol Utilization

BMCs involved in the degradation of 1,2-propanediol are found in several bacterial species. This pathway produces propionate and propanol, which can be used as carbon and energy sources. The encapsulation of this pathway helps to sequester the toxic intermediate, propionaldehyde.

Biotechnological Applications

The modular nature of BMCs and their ability to encapsulate specific metabolic pathways have made them a target for biotechnology. Efforts are underway to engineer BMCs to improve biofuel production, carbon capture, and the synthesis of valuable chemicals. By introducing synthetic BMCs into industrial microorganisms, researchers aim to create more efficient and robust production strains.

Evolution

The evolutionary origin of BMCs is a subject of ongoing research. It is believed that they evolved through the duplication and divergence of a primordial protein, leading to the diverse array of BMCs observed today. Their widespread presence in bacteria suggests an ancient origin and a significant evolutionary advantage in certain environments.

See Also

• Cell
• Organelle
• Protein
• Metabolism

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Bacterial_microcompartment

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  Carboxysomes EM
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  Synechococcus elongatus PCC 7942 electron micrograph showing carboxysomes