Critical phenomena in the temperature-pressure-crowding phase diagram of a protein

TL;DR

This study models and experimentally verifies a critical point in the temperature-pressure-crowding phase diagram of a protein, revealing conditions where large conformational fluctuations occur near phase merging.

Contribution

It introduces a comprehensive T-P-$$ phase diagram for a protein, demonstrating a critical transition influenced by crowding effects, which was not previously characterized.

Findings

Identification of a critical point at 305 K and 170 MPa.

Crowding shifts the critical temperature to lower values.

Large structural fluctuations occur near the critical line.

Abstract

In the cell, proteins fold and perform complex functions through global structural rearrangements. Function requires a protein to be at the brink of stability to be susceptible to small environmental fluctuations, yet stable enough to maintain structural integrity. These apparently conflicting behaviors are exhibited by systems near a critical point, where distinct phases merge $-$ a concept beyond previous studies indicating proteins have a well-defined folded/unfolded phase boundary in the pressure-temperature plane. Here, by modeling the protein phosphoglycerate kinase (PGK) on the temperature (T), pressure (P), and crowding volume-fraction ($\phi$) phase diagram, we demonstrate a critical transition where phases merge, and PGK exhibits large structural fluctuations. Above the critical temperature (Tc), the difference between the intermediate and unfolded phases disappears. When $\phi$ increases, the Tc moves to a lower T. We verify the calculations with experiments mapping the T-P-$\phi$ space, which likewise reveal a critical point at 305 K and 170 MPa that moves to a lower T as $\phi$ increases. Crowding places PGK near a critical line in its natural parameter space, where large conformational changes can occur without costly free energy barriers. Specific structures are proposed for each phase based on simulation.