Catalytic Droplets: Enzyme Containing Microcompartments.

Bacterial microcompartments (BMCs) and biomolecular condensates are two different forms of protein compartmentalization with different aims and functional advantages, representing specific designs of sequestering enzymes or segments of metabolic pathways. BMCs represent special proteinaceous organelles that are entirely composed of protein and are typically characterized by polyhedral shapes. By encapsulating and organizing metabolic enzymes with their substrates and cofactors BMCs act as specialized compartments within bacterial cells that promote and enhance specific biochemical pathways. They also serve important protective functions shielding vulnerable enzymes within a defined microenvironment and sequestering toxic or volatile intermediates. On the other hand, biomolecular condensates (also known as membrane-less organelles, MLOs) are dynamic, cell size-dependent, cytoplasmic and nucleoplasmic entities that typically contain both RNA and protein. They have unique morphologies, specific distribution patterns, are characterized by specific set of resident proteins, but their structural integrity is not supported by encapsulation in the membrane. Instead, their biogenesis is driven by liquid-liquid phase separation, and their structure is entirely controlled and mediated by the protein-protein, protein-RNA, and/or protein-DNA interactions. MLOs represent a different liquid state of cytoplasm or nucleoplasm (or mitochondrial matrix or chloroplastic stroma), whose major biophysical properties are rather similar to those of the rest of the intracellular fluid. Often, MLOs emerge in response to some specific environmental cues, being exploited by cells to respond in real time in a smart stimuli-responsive manner. BMCs are more permanent entities with selective transport through the protein shell. In that way and in many respects, they are closer to intracellular membrane-bounded organelles of eukaryotes than to MLOs. This chapter discusses diverse functions of BMCs and considers the ways by which they contribute to metabolic innovation in bacteria. Some functional roles of MLOs are considered as well.