How osmolytes counteract pressure denaturation on a molecular scale

Life in the deep sea exposes enzymes to high hydrostatic pressure which decreases their stability. For survival, deep sea organisms tend to accumulate various osmolytes, most notably trimethylamine N-oxide (TMAO) used by fish, to counteract pressure denaturation. Yet, exactly how they work still remains unclear. Here, we use a rigorous statistical thermodynamics approach to clarify the mechanism of osmoprotection. We show that the weak, non-specific, and dynamic interactions of water and osmolytes with proteins can be characterized only statistically, and that the competition between protein-osmolyte and protein-water interactions is crucial in determining conformational stability. Osmoprotection is driven by a stronger exclusion of osmolytes from the denatured protein than from the native conformation, and the water distribution has no significant effect on these changes for low osmolyte concentrations.