Molten globule-like transition state of protein barnase measured with calorimetric force spectroscopy

TL;DR

This study uses single-molecule calorimetric force spectroscopy to characterize the molten globule-like transition state of protein barnase, revealing significant thermodynamic changes consistent with the molten globule hypothesis.

Contribution

It provides detailed thermodynamic measurements of the transition state of barnase folding, confirming its molten globule-like properties with high energy and low entropy.

Findings

Transition state has a large heat capacity change ($50 ext{ cal/mol}\u00b7K$).

Significant change in _p (~90%) between unfolded and transition states.

Major enthalpy change (~80%) occurs between transition and native states.

Abstract

Understanding how proteins fold into their native structure is a fundamental problem in biophysics, crucial for protein design. It has been hypothesized that the formation of a molten globule intermediate precedes folding to the native conformation of globular proteins; however, its thermodynamic properties are poorly known. We perform single-molecule pulling experiments of protein barnase in the range of 7$^\circ$C to 37$^\circ$C using a temperature-jump optical trap. We derive the folding free energy, entropy and enthalpy, and heat capacity change ($\Delta C_p = 1050\pm50$ cal/mol$\cdot$K) at low ionic strength conditions. From the measured unfolding and folding kinetic rates, we also determine the thermodynamic properties of the transition state, finding a significant change in $\Delta C_p$ ($\sim$ 90$\%$) between the unfolded and the transition states. In contrast, the major change in enthalpy ($\sim$ 80$\%$) occurs between the transition and native states. These results highlight a transition state of high energy and low configurational entropy structurally similar to the native state, in agreement with the molten globule hypothesis.