Unfolding proteins by external forces and temperature: The importance of topology and energetics

Unfolding of proteins by forced stretching with atomic force microscopy or laser tweezer experiments complements more classical techniques using chemical denaturants or temperature. Forced unfolding is of particular interest for proteins that are under mechanical stress in their biological function. For β-sandwich proteins (a fibronectin type III and an immunoglobulin domain), both of which appear in the muscle protein titin, the results of stretching simulations show important differences from temperature-induced unfolding, but there are common features that point to the existence of folding cores. Intermediates detected by comparing unfolding with a biasing perturbation and a constant pulling force are not evident in temperature-induced unfolding. For an α-helical domain (α-spectrin), which forms part of the cytoskeleton, there is little commonality in the pathways from unfolding induced by stretching and temperature. Comparison of the forced unfolding of the two β-sandwich proteins and two α-helical proteins (the α-spectrin domain and an acyl-coenzyme A-binding protein) highlights important differences within and between protein classes that are related to the folding topologies and the relative stability of the various structural elements.