Microcompartment

Microcompartments are protein-based, non-membrane-bound organelles found in many bacteria that compartmentalize specific metabolic pathways by enclosing enzymes and substrates within a selectively permeable protein shell. These structures, often called bacterial microcompartments (BMCs), create defined spaces that concentrate reactants and limit exposure to other cellular components.

The shells are built from multiple paralogous proteins containing the BMC domain. They assemble into hexameric, pentameric, and fused tandem modules that form polyhedral shells surrounding a central lumen. Typical microcompartment diameters range from about 50 to 150 nanometers. Shell pores allow diffusion of small metabolites and cofactors while restricting larger or reactive intermediates, thereby shaping the interior chemistry.

Several well-studied types illustrate the diversity and function of microcompartments. Carboxysomes enclose RuBisCO and carbonic anhydrase to enhance carbon fixation in cyanobacteria and some chemoautotrophs by increasing local CO2 concentration. Metabolosomes such as Pdu (propanediol utilization) and Eut (ethanolamine utilization) sequester enzymes involved in processing reactive aldehydes and other intermediates, reducing cytosolic toxicity and promoting substrate channeling.

Genetically, BMCs are organized in operons that encode shell proteins along with the cargo enzymes. Assembly is coordinated so that the shell encapsulates the enzymatic core during or after expression.

Microcompartments represent a distinct bacterial strategy for metabolic efficiency and protection. They are of interest for biotechnology and synthetic biology as potential platforms for engineered nanoreactors and biocatalysis.