Cooperative equilibrium transitions coupled with a slow annealing step explain the sharpness and hysteresis of collagen folding.

Heat and guanidinium-induced denaturation curves of collagen III and its fragments were fitted by theoretical models to explain the extreme sharpness and the hysteresis between unfolding and refolding. It was shown that a recently proposed kinetic model for collagen denaturation does not account for the observed steepness, with physically reasonable values of activation energy and frequency factors in the Arrhenius equation. The extreme slope, which amounts to 0.38 per centigrade for collagen III at the midpoint of its transition, can only be explained by a highly cooperative equilibrium model. The refolding curve is shifted to lower temperatures by 6 degrees C for collagen III and reversible unfolding matching the initial profile of the native protein is observed only after long-time annealing. A simple formalism is proposed by which experimental denaturation and refolding curves are quantitatively described. The transition proceeds via many cooperative steps with slightly different equilibrium constants for unfolding and refolding. Hysteresis and annealing are caused by very slow steps, which are probably connected with a rearrangement of misfolded regions. These slow steps disappear with decreasing size of collagen fragments and hysteresis is not found for collagen model peptides.