Macromolecular crowding

The phenomenon of macromolecular crowding alters the properties of molecules in a solution when high concentrations of macromolecules such as proteins are present. Such conditions occur routinely in living cells; for instance, the cytosol of Escherichia coli contains about 300–400mg/ml of macromolecules. Crowding occurs since these high concentrations of macromolecules reduce the volume of solvent available for other molecules in the solution, which has the result of increasing their effective concentrations. Crowding can promote formation of a biomolecular condensate by colloidal phase separation. This crowding effect can make molecules in cells behave in radically different ways than in test-tube assays. Consequently, measurements of the properties of enzymes or processes in metabolism that are made in the laboratory (in vitro) in dilute solutions may be different by many orders of magnitude from the true values seen in living cells (in vivo). The study of biochemical processes under realistically crowded conditions is very important, since these conditions are a ubiquitous property of all cells and crowding may be essential for the efficient operation of metabolism. Indeed, in vitro studies have shown that crowding greatly influences binding stability of proteins to DNA. The interior of cells is a crowded environment. For example, an Escherichia coli cell is only about 2 micrometres (μm) long and 0.5 μm in diameter, with a cell volume of 0.6 - 0.7 μm3. However, E. coli can contain up to 4,288 different types of proteins, and about 1,000 of these types are produced at a high enough level to be easily detected. Added to this mix are various forms of RNA and the cell's DNA chromosome, giving a total concentration of macromolecules of between 300 and 400 mg/ml. In eukaryotes the cell's interior is further crowded by the protein filaments that make up the cytoskeleton, this meshwork divides the cytosol into a network of narrow pores.