Overview

A bacterial microcompartment is a protein-bound structure found within many bacterial cells that encloses enzymes and other proteins to carry out specialized metabolic tasks. These compartments are distinct from membrane-bound organelles of eukaryotes because they are constructed entirely from proteins rather than lipid bilayers. They therefore provide a way for bacteria to organize chemistry in space without using membranes: see bacteria and compare to eukaryotic cell organelles.

Structure and composition

The defining feature of a microcompartment is a polyhedral protein shell that surrounds a concentrated set of enzymes. The shell is made from repeating protein tiles that interlock to form facets and sometimes specialized vertex proteins; these components are often called BMC (bacterial microcompartment) proteins or shell proteins. The internal contents are primarily enzymes required for a pathway, while the shell proteins control passage of substrates and products. Unlike membrane-bound organelles, they do not contain or require lipids.

Size and physical traits

Microcompartments are typically nanometer-scale structures, commonly in the order of about 100–200 nanometres across, forming roughly icosahedral or polyhedral shapes visible by electron microscopy. Their shells are selectively permeable: small metabolites can diffuse through pores in shell proteins, while reactive or volatile intermediates are retained to avoid damage to the rest of the cell.

Types, functions and examples

  • Carboxysomes: enhance CO2 fixation by concentrating carbon-fixing enzymes.
  • Metabolosomes (e.g., Pdu, Eut): compartmentalize pathways that produce toxic aldehydes or other reactive intermediates.
  • Other specialized microcompartments: used in diverse pathways for sugar, amino acid or one-carbon compound metabolism.

Origins, genetics and development

Genes encoding shell proteins and encapsulated enzymes are typically clustered in operons, allowing coordinated expression and self-assembly. Microcompartments were first recognized in the mid-20th century as polyhedral bodies in bacterial cells and have since been shown to assemble via interactions between shell proteins and short encapsulation peptides on enzymes.

Importance and applications

By concentrating enzymes and isolating harmful intermediates, microcompartments improve metabolic efficiency and protect cellular components. Their protein-only architecture makes them attractive templates for synthetic biology and biotechnology, where researchers aim to reprogram or build new compartments to carry out designed reactions. For background on the protein shell and encapsulation mechanisms see protein shell studies and reviews about enzyme encapsulation. Additional general references and resources are available through academic and educational portals (bacteria introductions and cell organelles comparisons) as well as specialized structural databases (lipid-free organelle discussions and nanometre-scale imaging reports).