The effect of volume occupancy upon the thermodynamic activity of proteins: some biochemical consequences.

The thermodynamic activity of proteins in solution is substantially altered by the addition of unreactive or 'inert' macromolecules occupying more than a few percent of total solution volume. Approximate theoretical models of this effect have been formulated using a simplified geometrical representation of molecular shapes. These models predict that under certain conditions, the structure and function of proteins in physiological media with a high total macromolecular content may be qualitatively different than in dilute solution. Experimental studies of the effect of 'inert' macromolecules on protein structure and/or function are reviewed, and it is found that under favorable circumstances the simplified models can provide a satisfactory semiquantitative description of the data.