Abstract

Understanding the effect of mechanical stress on biological systems is of fundamental importance since it is a key parameter always present in Nature. In particular, it is thought that force has played an important role in the evolution of proteins, such that they can perform their function, i.e. fold and unfold, under stress. The development of novel single molecule AFM techniques in combination with protein engineering techniques has paved the path for the study of the mechanical design of modular proteins. The possibility of holding the protein at a constant force permits us for the first time to examine the folding and unfolding trajectory of a protein under a stretching force.

I will present a kinetic analysis of the unfolding times of the protein to measure the diversity of mechanical unfolding pathways. We find a surprisingly broad distribution of unfolding rates that follows a power law. This result is exciting because it suggests that the underlying free energy landscape consists of an exponentially distributed density of states, reminiscent of spin-glasses. Our results predict the existence of a ``glass transition’’ force below which the folded conformations interconvert between local minima on multiple time-scales. These novel techniques open the field of statistical energy landscape theories of protein folding to experimentation.

Questions about the validity of the two-state or funnel models for folding, the folding free energies, the corresponding end-to-end length fluctuations, as well as the cooperative nature of folding multiple protein domains will then lead us into a rich discussion.