A kinetic molecular model of the reversible unfolding and refolding of titin under force extension

Molecular elasticity is a physicomechanical property that is associated wit h a select number of polypeptides and proteins, such as the giant muscle pr otein, titin, and the extracellular matrix protein, tenascin. Both proteins have been the subject of atomic force microscopy (AFRA), laser tweezer, an d other in vitro methods for examining the effects of force extension on th e globular (FNIII/lg-like) domains that comprise each protein. In this repo rt we present a time-dependent method far simulating AFM force extension an d its effect on FNIII/lg domain unfolding and refolding. This method treats the unfolding and refolding process as a standard three-state protein fold ing model (U T F, where U is the unfolded state, T is the transition or int ermediate state, and F is the fully folded state), and integrates this appr oach within the wormlike chain (WLG) concept. We simulated the effect of AF M tip extension on a hypothetical titin molecule comprised of 30 globular d omains (Ig or FNIII) and 25% Pro-Glu-Val-Lys (PEVK) content, and analyzed t he unfolding and refolding processes as a function of AFM tip extension, ex tension rate, and variation in PEVK content. In general, we find that the u se of a three-state protein-folding kinetic-based model and the implicit in clusion of PEVK domains can accurately reproduce the experimental force-ext ension curves observed for both titin and tenascin proteins. Furthermore, o ur simulation data indicate that PEVK domains exhibit extensibility behavio r, assist in the unfolding and refolding of FNIII/lg domains in the titin m olecule, and act as a force "buffer" for the FNIII/lg domains, particularly at low and moderate extension forces.