FIGURE 8.

Molecular mechanics simulations of the scallop S2 atomic model with differential stretching of the chains. (A,B) Stretching just the D-chain at both ends. (C,D) Stretching only one end of each chain with the opposite end free. (A,C) The simulated force spectra of incremental extension of the S2 by constrained energy minimizations. E1 configuration showing the stretching mode (inset). Note its similarity to the unique shape of the experimental force spectra from the myosin coiled-coil. Also note that the simulated force spectrum for this mode is different in shape from the other simulated curves and experimental force spectra. (B,D) Images of the S2 atomic model during incremental stretching. The coiled-coil unravels at nearly random locations along its length and twists during the extension. The position of the 853–863 region at the different extension lengths is marked by black arrowheads. In panel B, the weakly hydrogen-bonded region between residues 853 and 863 unfolds in the D-chain at E3 and then in the C-chain at E5. In panel D, the coils in both chains at the end where the restraints are applied unfold first. The coils at the opposite end of the chains remain intact for most of the simulation. The chains are shown as tubes colored as the type of residue: red, acidic; blue, basic; yellow, hydrophobic; green, polar. The E0 configuration is the same as that in Fig. 3. E1–E9 represent structures from 1- to 9-nm extension.