The structural changes accompanying stretch-induced early unfolding events
were investigated for the four type III fibronectin (FN-III) modules, FN-II
I7, FN-IIIg, FN-III9, and FN-III10 by using steered molecular dynamics. Sim
ulations revealed that two main energy barriers, I and II, have to be overc
ome 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 pa
thway proceed from this intermediate state. A second energy barrier has to
be overcome to break the first major cluster of hydrogen bonds between adja
cent beta -strands. Simulations revealed that the height of barrier I varie
d significantly among the four modules studied, being largest for FN-III7 a
nd lowest for FN-III10. whereas the height of barrier II showed little vari
ation. Key residues affecting the mechanical stability of FN-III modules we
re identified. These results suggest that FN-III modules can be prestretche
d into an intermediate state with only minor changes to their tertiary stru
ctures. 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 th
e elasticity of fibrillar