Thermodynamic Mechanism of Protein Stabilization: Crowders vs. Osmolytes

Abstract

Solutes preferentially excluded from macromolecules can drive depletion attractions in important biological association processes. The established Asakura-Oosawa theory relates depletion forces to the excluded volume reduction and the ensuing entropy gain upon macromolecular compaction(1, 2). Accordingly, cosolute-induced protein stabilization is often described in terms of entropically driven “molecular crowding”(3). In agreement, many experiments of protein folding and other macromolecular processes suggest that depletion forces are predominantly entropic for some cosolutes, such as polyethylene glycol polymers. However, for other cosolutes, such as polyol osmolytes, the effect is enthalpically dominated, while the entropic change can even be unfavorable(4). Using the Kirkwood-Buff theory we demonstrate that depletion forces can be quantified using the effective interaction between cosolute and macromolecule. (5) Specifically, by incorporating interactions beyond hard-core, the depletion force attains considerable enthalpic contributions. This analytic theory, along with Monte-Carlo simulations, trace the origins of enthalpically dominated depletion forces to “soft” cosolute-macromolecule repulsions(6). Moreover, these depletion forces can be entropically disfavoured if the effective cosolute-macromolecule interaction consistes of an entropic attractive component and an enthalpic repulsive component(5). Finally, a theoretical mean-field model based on the Flory-Huggins solution theory allows to further trace this effective cosolute-macromolecule interaction to the underlying pairwise interactions in solution(7).1. Asakura, S., and F. Oosawa. 1954. J. Chem. Phys. 22: 1255-1256.2. Asakura, S., and F. Oosawa. 1958. J. Polym. Sci. 33: 183-192.3. Minton, A. 1981. Biopolymers. 20: 2093-2120.4. Sukenik, S., L. Sapir, and D. Harries. 2013. Curr. Opin. Colloid Interface Sci. 18: 495-501.5. Sapir, L., and D. Harries. 2015. Curr. Opin. Colloid Interface Sci. 20: 3-10.6. Sapir, L., and D. Harries. 2014. J. Phys. Chem. Lett. 5: 1061-1065.7. Sapir, L., and D. Harries. 2015. J. Chem. Theory Comput. 11: 3478-3490.