Protein Unfolding and Chemical Reactions Under Force: Complexity Versus Simplicity

Abstract

Elucidating the dynamics of reactant molecules along the particular pathway that connects them with the resulting products is a cornerstone of modern chemistry. In general, for small systems composed of only a few molecules, the valley connecting reactants with products is narrow and the position of the transition state is highly localized. By contrast, in the case of more complex reactions involving bigger molecular systems such as proteins, the potential energy surfaces become rougher, which typically results in heterogeneous reaction pathways with multiple transition state structures. In both cases, detailed information on the energy landscapes has been mainly restricted to the results obtained using computational methods. Single molecule force-clamp spectroscopy can be now used to directly monitor the individual unfolding pathways of single proteins and the reactive trajectories corresponding to the chemical reduction of a single disulfide bond embedded within the core of a protein. Herein, we compare the effect of a mechanical constant force on the energy landscape of two different reactions; the unfolding of a single protein and the reduction of a single disulfide bond, both occurring within the same single protein. Our results demonstrate that the native state of the 27th immunoglobulin-like domain of titin is rough and probably composed of a set of similar structures with slightly different energy. Interestingly, shortening of the I27 protein by the presence of disulfide bonds has a strong effect on the measured heterogeneity in the unfolding pathways. Our results are quantitatively discussed within the framework of the static disorder theory. In sharp contrast with these results, the energy landscape of an SN2 chemical reaction, occurring within a much shorter length-scale, is smooth, implying a high degree of homogeneity within the individual reactive pathways connecting the reactants with the reaction products.