Comparison of the early stages of forced unfolding for fibronectin type III modules

The structural changes accompanying stretch-induced early unfolding events were investigated for the four type III fibronectin (FN-III) modules, FN-III7, FN-IIIg, FN-III9, and FN-III10 by using steered molecular dynamics. Simulations revealed that two main energy barriers, I and II, have to be overcome to initiate unraveling of FN-III's tertiary structure. In crossing the first barrier, the two opposing beta -sheets of FN-III are rotated against each other such that the beta -strands of both beta -sheets align parallel to the force Vector (aligned state). All further events in the unfolding pathway proceed from this intermediate state. A second energy barrier has to be overcome to break the first major cluster of hydrogen bonds between adjacent beta -strands. Simulations revealed that the height of barrier I varied significantly among the four modules studied, being largest for FN-III7 and lowest for FN-III10. whereas the height of barrier II showed little variation. Key residues affecting the mechanical stability of FN-III modules were identified. These results suggest that FN-III modules can be prestretched into an intermediate state with only minor changes to their tertiary structures. FN-III10, for example, extends 12 Angstrom from the native "twisted" to the intermediate aligned state, and an additional 10 Angstrom from the aligned state to further unfolding where the first beta -strand is peeled away. The implications of the existence of intermediate states regarding the elasticity of fibrillar