Bacterial microcompartments

Abstract

<p indent="0mm">Traditionally, bacteria were considered primitive organisms that lack intracellular organization. However, this knowledge was overturned by studies that showed bacteria have a high level of organization including research on bacterial microcompartments. Microcompartments are self-assembling organelles composed of a protein shell, and internal enzymes encapsulated by the shell. The overall morphology of bacterial microcompartments is similar to viruses. They are polyhedral in shape and about 100 to <sc>200 nm</sc> in size. The common feature of bacterial microcompartments is that they use a protein shell as a diffusion barrier to help channel toxic or volatile pathway intermediates to the next pathway enzyme. Gene clusters of bacterial microcompartments are widespread, and their specific physiological roles are diverse. 68 types were found in the genome of bacteria across 45 phyla. However, the functions of many types of bacterial microcompartments remain unclear, and only several types have been characterized experimentally. One of the best-studied microcompartments is used for 1,2-propanediol catabolism in <italic>Salmonella typhimurium</italic>. Genes encoding enzymes that are responsible for 1,2-propanediol catabolism and shell proteins form a <italic>pdu</italic> operon. Enzymes and shell proteins of <italic>pdu</italic> operon form a Propanediol utilization (Pdu) microcompartment. 1,2-Propanediol is transported into the lumen of the microcompartment, where it is converted to propionaldehyde by propanediol dehydratase. Propionaldehyde is a toxic chemical and high levels of propionaldehyde inhibit the activity of the cell. The shell of the Pdu microcompartment prevents the propionaldehyde transported into the cytoplasm and reduces its toxicity to the cell. Propionaldehyde is converted to propanol, a less toxic chemical, and finally transported into the cytoplasm. The disruption of the Pdu microcompartment increases propionaldehyde levels in the culture broth, and slows the growth of cells. The carboxysome microcompartment is another type of highly investigated microcompartment. It is used to enhance autotrophic CO₂ fixation and is found in nearly all cyanobacteria and some chemoautotrophs. Bicarbonate crosses the protein shell of the carboxysome and enters the lumen, where bicarbonate is converted to CO₂. The protein shell of the carboxysome helps to maintain a high local concentration of CO₂ by inhibiting its outward diffusion. If the shell of the carboxysome is disrupted, cells are unable to grow autotrophically at atmospheric CO₂. levels. Besides these two types of microcompartments, the ethanolamine utilization (Eut) microcompartment, glycyl-radical propanediol (Grp) microcompartment, choline utilization (Cut) microcompartment, <italic>Planctomycete</italic> and <italic>Verrucomicrobia</italic> (PV) microcompartment, taurine metabolism microcompartment, and ethanol utilization (Eut) microcompartment are microcompartments whose specific function is known. Bacterial microcompartments are interesting structures, and they have potential applications in biomedicine and biotechnology. Some heterologous pathways have been encapsulated into microcompartments and shown potential biotechnology advantages. Research on bacterial microcompartments is in its infancy, but these tiny organelles have big potential.