Understanding the dependence on the pulling speed of the unfolding pathway of proteins

TL;DR

This paper models protein unfolding under force, revealing how the unfolding pathway depends on pulling speed, with a critical velocity determining whether the weakest or the pulled unit unfolds first.

Contribution

It introduces a simple one-dimensional chain model to analyze how pulling speed influences protein unfolding pathways, identifying a critical velocity that switches the unfolding sequence.

Findings

Unfolding pathway depends on pulling speed.

Existence of a critical pulling speed $v_c$.

Analytical expression for $v_c$ derived.

Abstract

The dependence of the unfolding pathway of proteins on the pulling speed is investigated. This is done by introducing a simple one-dimensional chain comprising $N$ units, with different characteristic bistable free energies. These units represent either each of the modules in a modular protein or each of the intermediate "unfoldons" in a protein domain, which can be either folded or unfolded. The system is pulled by applying a force to the last unit of the chain, and the units unravel following a preferred sequence. We show that the unfolding sequence strongly depends on the pulling velocity $v_{p}$. In the simplest situation, there appears a critical pulling speed $v_{c}$: for pulling speeds $v_{p}<v_{c}$, the weakest unit unfolds first, whereas for $v_{p}>v_{c}$ it is the pulled unit that unfolds first. By means of a perturbative expansion, we find quite an accurate expression for this critical velocity.