Molecular Crowding in the Cells
The main rules of protein folding have
been deduced from a considerable body of
in vitro and in silico studies. It has been
accepted that the same mechanisms are
involved in in vitro refolding and in the
folding of a nascent polypeptide chain in
the cell. However, the intracellular environment
differs markedly from that of
the test tube where low protein concentrations
are used. The interior of a cell is
highly crowded withmacromolecules. The
concentration is so high that a significant
proportion of the volume is occupied. As
mentioned by Ellis, in general, 20 to 30%
volume of the interior of the cells are occupied
by macromolecules; for example,
the concentration of total protein inside
cells ranges from 200 to 300 g L−1. The
total concentration of proteins and RNA
inside Escherichia coli ranges from 300 to
400 g L−1 depending on the growth phase.
Polysaccharides also contribute to the
crowding. It can be predicted practically
that diffusion coefficients will be reduced
by factors up to 10-fold due to crowding.
Since the average time for a molecule to
move a certain distance varies by D−2,
D being the diffusion coefficient, it will
take 100 times longer to move this distance
in the cell as would be necessary
under low concentration conditions. Another
prediction indicates that equilibrium
constants for macromolecular associations
may be increased by two to three orders
of magnitude.
Molecular crowding inside cells also has
consequences for protein folding, favoring
the association of partly folded polypeptide
chains into aggregates. This could explain
why cells contain molecular chaperones,
even though most denatured proteins
refold spontaneously in the test tube.