Revealing the nature of the native state ensemble through cold denaturation.

Recent advances in NMR methodology have enabled the structural analysis of proteins at temperatures far below the freezing point of water, thus opening a window to the cold denaturation process. Although the phenomenon of cold denaturation has been known since the mid-1970s, the freezing point of water has prevented detailed and structurally resolved studies without application of additional significant perturbations of the protein ensemble. As a result, the cold-denatured state and the process of cold denaturation have gone largely unstudied. Here, the structural and thermodynamic basis of cold denaturation is explored with emphasis placed on the insights that are uniquely ascertained from low-temperature studies. It is shown that the noncooperative cold-induced unfolding of protein results in the population of partially folded states that cannot be accessed by other techniques. The structurally resolved view of the cold denaturation process therefore can provide direct access to the cooperative substructures within the protein molecule and provide an unprecedented structurally resolved picture of the states that comprise the native state ensemble.