Computational investigation of cold denaturation in the Trp-cage miniprotein

The functional native states of globular proteins become unstable at low temperatures, resulting in cold unfolding and impairment of normal biological function. Fundamental understanding of this phenomenon is essential to rationalizing the evolution of freeze-tolerant organisms and developing improved strategies for long-term preservation of biologicalmaterials. We present fully atomistic simulations of cold denaturation of an alpha-helical protein, the widely studied Trp-cage miniprotein. In contrast to the significant destabilization of the folded structure at high temperatures, Trp-cage cold denatures at 210 K into a compact, partially folded state; major elements of the secondary structure, including the alpha-helix, are conserved, but the salt bridge between aspartic acid and arginine is lost. The stability of Trp-cage's alpha-helix at low temperatures suggests a possible evolutionary explanation for the prevalence of such structures in antifreeze peptides produced by cold-weather species, such as Arctic char. Although the 3(10)-helix is observed at cold conditions, its position is shifted toward Trp-cage's C-terminus. This shift is accompanied by intrusion of water into Trp-cage's interior and the hydration of buried hydrophobic residues. However, our calculations also show that the dominant contribution to the favorable energetics of low-temperature unfolding of Trp-cage comes from the hydration of hydrophilic residues.