Protein folding, stability, and solvation structure in osmolyte solutions

Abstract

An understanding of the impact of the crowded conditions in the cytoplasm on its biomolecules is of clear importance to biochemical, medical, and pharmaceutical science. Our previous work on the use of small biochemical compounds to crowd protein solutions indicates that a quantitative description of their nonideal behavior is possible and straightforward. Here, we show the structural origin of the nonideal solution behavior. We discuss the consequences of these findings regarding protein folding stability and solvation in crowded solutions through a structural analysis of the m-value or the change in free-energy difference of a macromolecule in solution with respect to the concentration of a third component.

FIGURE 1

Volume fraction dependence of the radial distribution functions g_αβ and Kirkwood 𝒢 factors in an example liquid system with Lennard-Jones potential. Functions shown for packing fractions 15, 25, 35, and 45%. (1) At distances closer than the sum of the two radii (contact distance), steric exclusion is operative. (2) At intermediate distances, there are strongly concentration-dependent solvation features, most notably the first solvation shell just outside the region of steric exclusion. (3) At large distances there is no correlation between particles and the pair correlation function approaches unity. The inset shows the Kirkwood-Buff integrals (Eq. 1) as a function of the packing fraction. Note the slope and sign changes with respect to packing fraction.

FIGURE 2

Change in solvation preference : dependence on osmolyte concentration (normalized to solvation preference at 0M). The curves were calculated from Eq. 8 using osmolyte activity coefficient data (22) assuming a constant m-value. In the case of the denaturing osmolyte urea the solvation preference deviates plus or minus a few percent. In the case of the protecting osmolytes (except glycine), it changes two- to fourfold. The curves are labeled at the right end: u = urea. Polyols are: go = glycerol, e = erythritol, ma = mannitol, so = sorbitol. Amino acids are: g = glycine, a = alanine, p = proline, sa = sarcosine, gb = glycine betaine. Saccharides are: x = xylose, f = fucose, gl = glucose, ga = galactose, rh = rhamnose, mo = mannose, mt = maltose, su = sucrose, ra = raffinose.