Computer modeling of force-induced titin domain unfolding.

Titin, a 1 micron long protein found in striated muscle myofibrils, possesses unique elastic and extensibility properties, and is largely composed of a PEVK region and beta-sandwich immunoglobulin (Ig) and fibronectin type III (FnIII) domains. The extensibility behavior of titin has been shown in atomic force microscope and optical tweezer experiments to partially depend on the reversible unfolding of individual Ig and FnIII domains. We performed steered molecular dynamics simulations to stretch single titin Ig domains in solution with pulling speeds of 0.1-1.0 A/ps, and FnIII domains with a pulling speed of 0.5 A/ps. Resulting force-extension profiles exhibit a single dominant peak for each domain unfolding, consistent with the experimentally observed sequential, as opposed to concerted, unfolding of Ig and FnIII domains under external stretching forces. The force peaks can be attributed to an initial burst of a set of backbone hydrogen bonds connected to the domains' terminal beta-strands. Constant force stretching simulations, applying 500-1000 pN of force, were performed on Ig domains. The resulting domain extensions are halted at an initial extension of 10 A until the set of all six hydrogen bonds connecting terminal beta-strands break simultaneously. This behavior is accounted for by a barrier separating folded and unfolded states, the shape of which is consistent with AFM and chemical denaturation data.