Distinguishing Signatures of Multipathway Conformational Transitions

The folding and binding of biomolecules into functional conformations are thought to be commonly mediated by multiple pathways rather than a unique route. Yet even in experiments where one can “see” individual conformational transitions, their stochastic nature generally precludes one from determining whether the transitions occurred through one or multiple pathways. We establish model-free, observable signatures in the response of macromolecules to force that unambiguously identify multiple pathways—even when the pathways themselves cannot be resolved. The unified analytical description reveals that, through multiple pathways, the response of molecules to external forces can be shaped in diverse ways, resulting in a rich design space for a tailored biological function already at the single-molecule level.

Figure 1

A fragment of the energy landscape of a biomolecule with multiple reaction pathways connecting distinct conformational states. This Letter addresses the challenge of identifying the presence of multiple pathways and explores the functional advantages of pathway multiplicity.

Figure 2

Fundamental multipathway configurations: distinguishing signatures and functional advantages. Lines: theory. Histograms: simulations. Symbols: simulations, transformed histograms [13, 14]. (a) A pathway switch yields an upturn in the rate and a nonmonotonic trend in the force distributions with loading rate. The transition rate becomes maximized over a broad force range. (b) Switching from a force-resistant to a force-compliant pathway yields a dip in the rate and bimodal force distributions with a zero-force peak. The system rapidly dissociates under strong forces yet increasing resists weak forces. (c) An intermediate leading to a slow transition step under rapid $N-I$ exchange yields a plateau in the rate and bimodal force distributions. Transient insensitivity to perturbation is acquired. (d) An intermediate leading to a fast step yields a sigmoidal rate curve and a nonmonotonic trend in force distributions. Distinct binding modes are acquired. (e),(f) An intermediate irreversibly sequestering population yields bimodal force distributions where the peaks dominate at different loading rates. The transitions become spread over broader time and force ranges. (g) Multiple native states yield a transient unimodality in the force distribution. The rate of transitions leaving the functional state becomes inhibited. (h),(i) For reference: single-pathway transitions with one [15] and multiple [16] barriers with corresponding signatures.